DISTRIBUTION AND MICROHABITAT SELECTION OF

HEMIGRAPSUS OREGONENSIS (DANA) AND

PACHYGRAPSUS CRASSIPES RANDALL

IN ELKHORN SLOUGH, MONTEREY COUNTY, CALIFORNIA

A Thesis Presented to the Graduate Faculty

of

California State University, Hayward

In Partial Fulfillment

of the Requirements for the Degree

Master of Science in Biology

By

Mark C. Sliger

March 1982

Copyright © 1982

by Mark C. Sliger

ii

ABSTRACT

The vertical distribution and habitat selection of

two species of grapsid crabs Hemigrapsus oregonensis (Dana)

and Pachygrapsus crassipes Randall along the main channel

bank of Elkhorn Slough, Monterey County, California was

investigated. While the vertical distribution of the two

crab species was found to overlap, H. oregonensis typically

occupied burrows in the lower region of the bank and P.

crassipes was usually found in burrows located in the

upper bank or in bank slumps located on the lower mudflat.

Substratum and tidal elevation were found to be the most

important factors influencing crab distribution along the

banks of Elkhorn Slough. Both H. oregonensis and P.

crassipes had similar resistance to desiccation abilities,

however smaller members of each crab species were more

susceptible to desiccation. · Hernigrapsus oregonensis

was found to be able to tolerate silty-clay water while

P. crassipes was highly susceptible to small, unconsoli­

dated mud particles.

iii

DISTRIBUTION AND MICROHABITAT SELECTION OF

HEMIGRAPSUS OREGONENSIS (DANA) AND

PACHYGRAPSUS SIPES RANDALL

IN ELKHORN SLOUGH, MONTEREY COUNTY, CALIFORNIA

By

Nark C. Sliger

Date:

-' .,.( 7. -i"-. ...: ~-- 17 [

iv

ACKNOWLEDGMENTS

There are a great number of people without whose

help this work would not have been completed. Financial

support was provided by Sea Grant #R/CZ-45. I wish to

thank the members of my committee, Drs. James Nybakken,

Pamela Roe and Gregor Caillier for their continued support

through the duration of this research. I am especially

grateful to Dr. Pamela Roe for her contagious vitality,

inspiration and encouragement during times of crisis.

Thanks to Dr. Ann Hurley for providing assistance with

statistical analysis and experimental design during the

critical preliminary phase of the study. I am grateful to

Dr. John Oliver for his stimulating conversations and

cr ical review of the initial draft.

Chris Jong deserves special thanks for providing

friendship, thought-provoking ideas and valuable assistance

in collecting data. Her undaunted spir contributed

greatly to the successful comp tion of this research.

I wish to express my gratitude to numerous iends

at the Moss Landing Marine Laboratories who supported and

encouraged me throughout this study. In particular, I

am indebted to Signe Johnsen for her help with computer

analysis; Sheila Baldridge, the librarian, for locating

needed references; Rosie Stelow for editing the manuscript;

v

. vi

and Lynn McMasters for her exceptional illustrations.

I also greatly appreciate the assistance of Joy Milhaven,

Neal Scanlon and Fred Lauber with the field work. Special

thanks to the "Benthic Bubs" for the camaraderie and help

in unraveling some of the mysteries of Elkhorn Slough.

My sincere appreciation to Valerie Breda, Debbie Fellows

and Melanie Mayer for their advice and emotional support.

Finally, I extend the deepest appreciation to my

family for their understanding, love, and support.

TABLE OF CONTENTS

ABSTRACT o o • •

ACKNOWLEDGMENTS

LIST OF TABLES . .

LIST OF FIGURES

INTRODUCTION . .

METHODS AND MATERIALS

Study Area

Crab Distribution .

Vertical Bank

Bank Slumps

Physical Properties of the Channel Bank

Erosion

Exposure

Field Experiments

Tidal Height Preference

Bank Region Preference

Substratum Transferal

Laboratory Experiments

Desiccation

Tolerance to Silty-Clay Water

RESULTS

vii

iii

v

ix

x·

1

4

4

5

5

6

7

7

10

11

11

13

13

14

14

15

16

Crab Distribution .

Vertical Bank

Bank Slumps

Physical Properties of the Channel Bank

Field Experiments

Tidal Height Preference

Bank Region Preference .

Substratum Transferal

Laboratory Experiments

Desiccation

Tolerance to Silty-Clay Water

DISCUSSION . . . .

SUMMARY

LITERATURE CITED

TABLES .

FIGURES

Page

16

16

17

17

19

19

20

20

21

21

21

22

30

31

35

39

viii

-

Table

1.

2.

LIST OF TABLES

Mean densities and mean differences in densities of Pachygrapsus crassipes and Hemigrapsus oregonensis collected in 0.25 m2 quadrats in Elkhorn Slough from December 1979 to May 1980 . . . . . ..

Summary of stat tical analysis for the physical charac tics of the upper and lower bank regions of the main channel bank of Elkhorn Slough . . . . . . . . .

3. Summary of chi-square tests for tidal height preferences of Pachygrapsus crassipes and Hemigrapsus oregonensis . .

4. Summary of chi-square tests for the bank region preferences of Pachygrapsus crassipes and Hemigrapsus oregonensis

ix

Page

35

36

. . . 37 !

. . . 38

' i

Figure

1.

2.

LIST OF FIGURES

Map of Elkhorn Slough showing position of study site . . . . . . . . . . . . .

Photograph bank slumps at the study site in Elkhorn Slough during low tide

3. Diagram of the device used in the substratum

Page

39

41

erodibility experiment . . . . . . . . . 43

4. Artificial substratum cage against the main channel bank of the study site during low tide . . . . . . . . . . . . . . . . . 45

5. Size frequency diagram of all Hemigrapsus oregonensis collected from mud burrows of the upper bank region . . . . . . . . . . 47

6. Size frequency diagram of all Hemigrapsus oregonensis collected from mud burrows of the lower bank region . . . . . . . . . . 49

7. Size frequency diagram of all Pachygrapsus crassipes collected from mud burrows of the upper bank region . . . . . . . . . . . . 51

8. Size frequency diagram of all Pachygrapsus crassipes collected from mud burrows of the lower bank region . . . . . . . . . . . 53

9. Results of five Vertical Height Preference/ Species Interaction experiments . . . 55

10. Tolerance to desiccation of Hemigrapsus oregonensis and Pachygrapsus crassipes 57

11. Regression of desiccation survival time of Hemigrapsus oregonensis against crab size as determined by carapace width . . . . . . . 59

12. Regression of desiccation survival time of Pachygrapsus crassipes against crab s e as determined by carapace width 61

13. Tolerance to silty-clay water 63

X

INTRODUCTION

While the study of the distribution and habitat

preference of brachyuran crabs has been generally limited

to works which concentrated on the burrowing, depos feed-

ing ocypoid crabs of the genus Uca (for a review, see

Crane, 1975), a number of investigators have focused on

the crabs of the family Grapsidae. For example, Bacon

(1971) studied the distribution of Cyclograpsus insularum

and Cyclograpsus lavauxi and found substratum and tidal

elevation important factors for habitat selection. Sub-

stratum and behavior were suggested by Abele (1973) as

important factors in limiting the distribution in Florida

of six species of closely associated grapsid crabs of

the genus Sesarma. Salinity has been found to affect

the distribution of several species of grapsid crabs

(Snelling, 1959; Jones, 1976; Seiple, 1979) and Kikuchi

et al. (1981) found that the distribution of the pebble

crab (Gaetice depressus was correlat with beach elevation.

Although MacGinitie (1935) f st noted the occurrence

of the grapsid crabs Hemigrapsus oregonensis, Hemigrapsus

nudus and Pachygrapsus crassipes among rocks or in inter-

tidal mud burrows in Elkhorn Slough, California and

additional studies (Knudsen, 1964; Ricketts and Calvin,

1968; Batie, 1974) have shown that Pacific coast populations of

1

i . : .r

i ' I , I ,

i ~

2

the three grapsid crabs generally form distinct distribu-

tional patterns, few authors have explored experimentally

the biological and physical parameters influencing the

distribution and habitat selection of these crabs. Hiatt

(1948) suggested that the observed habitats of H. oregonensis,

H. nudus and P. crassipes along the coast of California

were influenced by substratum and desiccation. He in-

vestigated the relative abilities of the three grapsid

crabs to withstand siccation and found that ~· crassipes

and H. nudus apparently had greater tolerances to desiccation

than H. oregonensis. Low (1970) found that the divergent

habitat preferences of H. oregonensis and H. nudus

were influenced by different physiological tolerances

to muddy water and low oxygen concentrations. Hemigrapsus

oregonensis outlived H. nudus when both spec s of crabs

were placed in flasks filled with low oxygenated, muddy

water. His results were consistent with Hiatt's (1948)

suggestion that morphological differences in the respira-

tory systems of H. oregonensis, H. nudus and P.

affected the ability of the di erent cies to survive

on fine particulate substratum. Willason (1981) found

that the distribution and coexistence of P. crassipes and

H. oregonensis a southern Californian saltrnar were

primarily the result of predation. By preying on H.

P. crassipes restrict that crab spec s to

~

II ~

I . \

lower intertidal areas. He suggested that a possible

release mechanism for H. oregonensis was the inability

3

of small P. crassipes to cope with the stressful estuarine

environment.

Hemigrapsus oregonensis (Dana) and Pachygrapsus

crassipes Randall form an important part of the invertebrate

macrofauna of Elkhorn Slough, Monterey County, California.

Both species of crabs occupy similar habitats, generally

found in burrows located in the intertidal mudflat or

pickleweed Salicornia virginica) marsh. The present study

examined the abundance and distribution of H. oregonensis

and P. crassipes along the intertidal mud banks of this

embayment and sought to answer questions concerning the

limits of their distribution and habitat selection and

how these m~y be influenced by tolerances to environmental

variables and crab agonistic behavior.

METHODS AND MATERIALS

STUDY AREA

Elkhorn Slough is a tidally influenced coastal embay­

ment and seasonal estuary located in Monterey Bay, California.

The main channel, which averages 100 meters in width, is

bordered by extensive tidal mudflats and pickleweed

(Salicornia virginica) marshland. The axial length of the

slough is approximately 10 km and the main channel is well

mixed vertically due to tidal currents (Smith, 1974).

The study area was located along an intertidal bank

bordering the main channel approximately 5 km from the

harbor entrance (Figure 1). In addition to its accessibility,

this particular bank was chosen because of the uniform

vertical height of 1 m. The upper edge of the bank was

located approximately at the +1.4 m tidal level. Two

25 m transects were located along the west bank about 25

meters apart.

The tidal level of the bank was determined by ob­

serving on two occasions wooden stakes placed along the

upper edge of the channel bank during high tide. Stakes

were equipped with stripes of phenaphthazine paper

placed along their lengths and changes in color of the

pH paper indicated the maximum water height attained

during that tidal cycle. The tidal elevation of the

4

5

bank was then estimated from the predicted high tide based

on National Ocean Survey Tables obtained from the U.S.

Department of Commerce. Results are comparable to

Smith's (1974) estimated tidal elevation of +1.45 m for

the upper edge of the Salicornia dominated channel bank.

CRAB DISTRIBUTION

Vertical Bank

Obvious differences in bank morphology and some

preliminary observations on crab distribution in Elkhorn

Slough suggested that systematic sampling of the upper

and lower halves of the main channel bank would yield

the most useful information on the vertical distribution

of Hemigrapsus oregonensis and Pachygraspsus crassipes.

Both H. oregonensis and P. crassipes are nocturnal

(MacGinitie,l935; Hiatt, 1948; pers. obs.) and generally

remain in mud burrows during daylight hours at low tide.

In an earlier study at Goleta Slough, Willason (1981)

found that removing crabs by burrow excavation during

daylight low tides was the most suitable method of

sampling these grapsid crabs.

Individual crabs were collected by placing the upper

2 edge of a 0.25 m quadrat frame along the top of the

main channel bank at a random point along the transects.

All crabs occupying burrow ace within the quadrat were

removed by complete excavation of each burrow. Crabs

were removed from the substratum in a similar manner

in the lower bank after lowering the quadrat SO em.

This method collected 98 percent of the crabs s~mpled,

as determined by comparing results with those obtained

by screening 1 the sediment through a 0.5 mm screen

in six samples.

Sampling was conducted over a six-month period,

from December 1979 to May 1980. Five upper and five

lower bank quadrats were taken monthly. All crabs

collected were identified to species, sexed and measured

to the nearest 0.1 mm. Both Hemigrapsus oregonensis and

Pachygrapsus crass es were measured across the carapace

from the tips of the second anteriolateral spines. Due

to handling difficulties, very small crabs ( < 10 mm) were

measured using a dissecting scope fitted with a disc

micrometer. All other crabs were measured using vernier

calipers.

Bank Slumps

Due to tidal scouring and d ferential erosion

susceptibility of the soil, the main channel bank is

undercut and develops large overhangs. With time, the

overhangs with their resident crab populations break

away from the bank and collapse onto the mudflat and

6

7

form , temporarily stable islands dense root

mat surrounded by unconsolidated mudflat sediment

(Figure 2). In order to determine which crabs occupy bank

slump lands, six bank slumps were randomly selected

and haphazardly sampled with a 0.2S m2 quadrat by burrow

excavation. The six bank slumps were located in the

transect area and situated below the +0.9 m elevation

of the tide.

PHYSICAL PROPERTIES OF THE CHANNEL BANK

Erosion

A consistent pattern of soil erosion occurs along

the banks of the main channel and major tidal creeks

of Elkhorn Slough. As indicated earlier, the banks are

generally undercut, forming overhangs that eventually

collapse onto the tidal mudflat. To tigate the

relative susceptibility of the channel bank of Elkhorn

Slough to erosion by water movement, the bank was

divided into two vertical levels. The upper bank was

that region of the channel bank extending from the

uppermost edge to a dep of SO ern. The bank region

from this midway point to the mudflat below, a distance

of SO ern, was called the lower bank.

The susceptibility of the substratum of the bank

to scouring by tidal currents was explor using a

8

simple laboratory apparatus (Figure 3). The device

allowed an equal and constant flow of water to percolate

upwards and past sediment cores collected from the

upper and lower bank levels. After wet weighing the

cores, pairs of cores were randomly placed the

cylinders of the apparatus and subjected to water move­

ment for 120 minutes. At the completion of the experiment,

cores were removed, drained for three minutes and

weighed. Since the cores varied in size, standardization

was determined by calculating the percentage of

weight lost by the equation:

weight of core (before) - weight of core (after) % loss = X 100

weight of core (before)

In his study of the erosional patterns of San

Francisco Bay marshlands, Pestrong (1965) suggested that

the soils underneath alicornia showed increased resistance

to erosion because their extensive root system and dense

organic material bound soil together and their high

elevation made them drier.

Root mat material, composed of Salicornia roots and

detritus, was observed underneath the Salicornia in Elkhorn

Slough. In order to determine quantitative differences

root mat density between the upper and lower halves of

9

the channel bank, cores (7.5 em X 7.5 em X 15 em) were

collected randomly from each bank level along the two

transect sites. All samples were taken to the laboratory,

immediately wet weighed and wet sieved on a 0.5 mm screen.

The material retained on the sieve was oven dried at

70°C and weighed. Since each sample varied in size,

the ratio of dry root material to wet weight of the core

from which it had been extracted was determined, utilizing

the following equations:

weight dry root material

root mat material

weight wet core

Moisture conten~ of the sediment may also affect the

erosion susceptibility or the soils of the channel bank.

Therefore, the amount of water contained sediment cores

obtained from each bank level was determined. Sediment

cores (3.5 em X 15 em) were collected from the channel

bank at low tide on two occasions. Cores were returned

to the laboratory, immediatelywetweighed then oven

dried at 70°C. Moisture content was determined by the

following relationships:

wet wt. of soil - dry wt. of soil

% moisture --------------------------------- X 100 wet wt. of so

. I i

10

The grain composition of marsh sediment helps

determine the water retention characteristics of the

substratum and -affects the substratum's erosional

behavior (Pestrong, 1965). Therefore, standard pipette

analysis (for technique, see Krumbein and Pettijohn, 1938)

was utilized in sediment s determination of the two

bank levels. Sediment cores (3.5 em X 15 em) were

randomly collected from the channel bank, returned to

the laboratory and immediately frozen. After six days,

the samples were thawed, processed and analyzed (for

methods on particle s e determination, see Folk and

Ward, 1957).

Exposure

Due to the mixed, semi-diurnal tides of Monterey

Bay, crab burrows in the main channel bank of Elkhorn

Slough which differ in vertical elevation of only a few

centimeters may have significantly different periods of

exposure during the year. To determine differences in

inundation rate for the upper and lower bank levels, a

time series analysis of the total number of hours during

the year each bank region is fully exposed was performed.

The percentage time exposed was determined from the

predicted tidal curve for the year 1980 using a computer

program that involved the ten most significant tidal

constituents (Broenkow, pers. comm.).

FIELD EXPERIMENTS

Tidal Height Preference

In a previous study, Willason (1981) utilized an

artificial burrow experiment to determine the preference

11

of Pachygrapsus crassipes for different bank levels in an

estuarine tidal creek. Pachygrapsus crassipes consistently

preferred those burrows found in the higher tidal level

of the enclosure cage. A series of similar experiments

utilizing artificial substrate cages was performed to

investigate the tidal height preference of . crassipes

and Hemigrapsus oregonensis along a section of the main

channel bank of Elkhorn Slough.

Two styrofoam-backed cages (1 m in he and 54 em

in width) enclos with 2 mm wire mesh were placed

vertically against the bank at the study area with the

upper edge of the cage flush with the upper edge of the

bank and the lower edge resting on the mudflat (Figure 4).

Cages were oval shaped in order to reduce the attraction

of crabs to sharp corners (pers. obs.). One artificial

substrate cage was designed for large Pachygrapsus

crassipes (carapace width range 17-36 mm) and had 40

cylindrical holes with diameters of 54 mm and drilled

15 em o the styrofoam. Experiments with smaller P.

(carapace width range 7-15 mm) and all

Hemigrapsus oregonensis (c ace width range 13-23 mm)

......

utilized a second cage with 40 cylindrical holes drilled

15 em into the styrofoam but with diameters of 32 mm.

For both cages, 20 holes were randomly drilled in the

upper half and 20 in the lower half of the cage. For

each replicate experiment, ten marked crabs of one

species (for marking technique, see Kuris, 1971) were

introduced into the cage at low tide during daylight

hours, five placed randomly into holes of the upper

region and five placed into holes in the lower area.

The position of each crab was recorded at the conclusion

of the experiment, approximately 24 hours later during

the next daylight low tide. For all experiments, cages

had been completely inundated by a night high tide.

Crabs for each experiment were collected from Elkhorn

Slough prior to the initiation of the experiment and

were used only once.

To test the af t of species interaction on tidal

height preference, six Pachygrapsus and six

Hernigrapsus oregonensis were simultaneously placed into

large-holed artific 1 substrate cage. Half of the

individuals of each species were placed randomly in the

holes of the upper cage and half in the lower region.

The position of each crab was recorded 24 hours later at

the next low tide.

12

13

Bank Region Preference

An additional cage experiment was used to investigate

which bank region Hemigrapsus oregonensis (carapace width

range 12-22 mm) and Pachygrapsus (carapace

width range 19-38 mm) selects without interference from

the crab species. A similar enclosure cage (1 m X

0.5 m), oval shaped but without styrofoam backing, was

placed directly against a cleared section of the channel

bank at the study area. The upper edge of the cage was

flush with the upper of the bank and the lower edge

flush with the mudflat. The bank within the enclosure

was c ed by excavation of all burrows and removal of

all crabs. Twenty holes (3.5 em in diameter) were bored

into the substratum to a depth of 15 em. Ten holes were

randomly bored in the upper bank region and ten in the

lower. S marked crabs were placed the holes, three

in the upper bank and three in the lower. The position

of each crab was recorded 24 hours later, at the next

daylight low tide, after a night high t had completely

inundated the cage. All crabs were col ted from

Elkhorn Slough before start of the experiment and

used only once.

Substratum Transferal Experiment

A simple observational experiment was used to

determine which crab ecies would occupy a newly cle

section of upper bank substratum transferred to the

lower bank elevation. A swath of channel bank composed

completely of upper bank substratum was provided by

clearing by excavation a 1 m wide section of the

channel bank of all crabs, digging away the lower bank

until a large upper bank section could be cut and trans­

ferred downward into the space provided. Twenty holes

were haphazardly bored into the bank, ten the upper

bank elevation and ten in the lower. Daily observations

were made for six success daylight low t s .

LABORATORY EXPERIMENTS

Desiccation

abilit of Pachygrapsus crassipes, Hemigrapsus

oregonensis and H. nudus to withstand desiccation, the

relationship between desiccation resistance and crab

size was not noted. Therefore, an investigation the

effects crab size on the tolerances of P.

and H. oregonensis to desiccation was performed.

Pairs of similarly sized individuals of Pachygrapsus

crassipes (carapace width range 13-35 mm) and

oregonensis (carapace width e 9-27 mm) were collected

from Elkhorn Slough and placed 500 ml polythylene

0 jars kept at a room temperature of 22 C. Crabs were

14

checked every 30 minutes and the length of time from the

initiation of the experiment to the death of each crab

was noted. Death was signified by the complete lack

of appendage movement when touched.

Tolerance To Silty-Clay Water

In a previous study, Low (1970) found that

Hemigrapsus oregonensis outlived H. nudus in flasks

filled with muddy, poorly oxygenated water. A variation

of this crab tolerance experiment was used to

investigate the effects of small mud particles on H.

oregonensis (carapace width range 13-24 mm) and

Pachygrapsus crassipes (carapace width range 11-24 mm).

Pairs of crabs of each species, collected from

Elkhorn Slough and matched for size, were placed in

500 ml polyethylene jars filled with aerated seawater.

Fifty milliliters of unconsolidated lower bank mud was

added to half of the jars. All jars were kept at a room

temperature of 22°C and the condition of each crab was

checked hourly. The 1 th of time from the initiation

15

of the experiment to the death of each crab was recorded.

Lack of appendage movement signified the death of the crab.

RESULTS

CRAB DISTRIBUTION

Vertical Bank

Different distributional patterns were found for

Pachygrapsus crassipes and Hemigrapsus oregonensis along

the main channel bank of Elkhorn Slough (Table 1).

Pachygrapsus crassipes numerically dominated the mud

burrows of the upper bank region and were found in

significantly fewer numbers in the lower bank (P < 0.001,

Paired-sample t-test). In contrast, H. oregonensis was

found in higher numbers in the lower bank region and

seldom occupied burrows of the upper bank (P <0.001,

Paired-sample t-test). An average of 24.6 P. crassipes

and 1.8 H. oregonensis was found in the 0.25 m2 upper

bank quadrats. A mean of 3.1 P. crassipes was found in

the 0.25 m2 lower bank quadrats, while an average of 12.2

H. oregonensis occupied similar burrow space in that

bank region. Very rarely were P. crassipes and H.

oregonensis found in the same burrow.

Size frequency diagrams of all Hemigrapsus oregonensis

sampled during the six-month period showed that relatively

small, similarly sized individuals were found in both the

upper (Figure 5) and lower (Figure 6) bank regions. Average

carapace widths of 7.3 mrn (SD = 2.8, n =53) and 7.0 mm

16

17

(SD = 3.4, n = 365) were found for H. oregonensis in the

upper and lower bank regions, respectively. Average

sizes of Pachygrapsus crassipes for the upper and lower

bank regions were 13.5 mm (SD = 9.9, n = 737) and 7.9 mm

(SD = 5.8, n = 92), respectively. Size frequency diagrams

of all P. crassipes collected during the sampling period

showed that many large individuals were found in the upper

bank (Figure 7) while very few individuals larger than

16 mrn occupied burrows in the lower bank region (Figure

8) .

Bank Slumps

Pachygrapsus crassipes numerically dominated all bank

slumps which had fallen onto the main channel mudflat

2 with an average of 15.0 individuals/0.25 m (SD = 6.0,

n = 6). Hemigrapsus oregonensis, with a density of

0.7 individuals/0.25 m2 (SD = 1.2, n = 6), occurred

in significantly lower numbers on the bank slumps

(P <0.01, Mann-Whitney U-test).

Physical Properties of the Main Channel Bank

Many of the physical characteristics of the main

channel bank were found to be different for the upper

and lower bank regions (Table 2). While in the erosion

apparatus, the lower bank cores lost a significantly

greater amount of sediment than the upper bank cores

(P <0.001, Mann-Whitney U-test). In all cases, the

lower bank cores deteriorated more rapidly than the

upper bank cores and were reduced to small clumps of

18

mud while retained in the erosion apparatus. The upper

bank cores remained tually unaffected by the flow of

water and maintained the original shape and consistency.

The density of root mat in the upper channel bank

was significantly greater than the lower bank (P <0.01,

Mann-Whitney U-test) with the lower bank almost devoid

of organic material. Moisture content analysis of the

bank cores revealed that there was litt difference

in soil moisture between the upper and lower bank regions

(P >0.05, Mann-Whitney U-test). Soil analysis, which

·ignored weight contributed by root mat material, indicated

that the soils of both ions of the bank were composed

exclus ly of fine silty and clay particles. However,

there were some differences in the gra composition

between the upper and lower bank substrata. The lower

bank substratum had a smaller mean part le size (P <0.05,

Mann-Whitney U-test) and a greater percentage of clay

partie s (P < 0.05, Mann-Whitney U-test) than the sub­

stratum found in the higher bank region.

De the slight difference in vertical tidal

height between the upper and lower bank ions, time

series analysis indicated that the upper half of the

bank is fully exposed 34 percent more during the year

than the lower half. While the upper bank is higher and

exposed more often, high water content is apparently

maintained by the retention of water in a dense root mat

filled with small sediment particles.

FIELD EXPERIMENTS

Tidal Height Preference

Pachygrapsus crassipes and Hemigrapsus oregonensis

utilized the art ial substratum cages placed along the

19

main channel bank of Elkhorn Slough differently (Table 3).

Both small and large P. crassipes showed a preference for

the upper intertidal region of the channel bank (P <0.001,

chi-square test). In contrast, H. oregonensis did not

show a preference for any tidal elevation within the

vertical range of the channel bank (P >0.10, chi-square

test). When both H. oregonensis and P. crass s were

placed into the same cage for 24 hours, P. crassipes

maintained its preference for the higher tidal elevation

while H. oregonens , although sustaining high mortalities,

again showed no tidal height preference (Figure 9).

Mortalit s were assumed to result from predation by

P. crassipes because on several occasions P. crass es

were observed eating H. oregonensis along the banks of

20

Elkhorn Slough. In addition, when both species of crabs

were placed in aquaria, H. oregonensis consistently

underwent heavy mortalities. Willason (1981) also observed

this aggressive interaction between H. oregonensis and

P. crassipes.

Bank Region Preference

Pachygrapsus crassipes and Hemigrapsus oregonensis

had divergent bank region preferences when placed in the

enclosure cage along the main channel bank of Elkhorn

Slough (Table 4) . Pachygrapsus crassipes preferred the

upper bank region (P <0.001, chi-square test), while

H. oregonensis had a distinct preference for the lower

bank region (P <0.001, chi-square test).

Substratum Transferal Experiment

Daily observations showed that only Pachygrapsus crassipes

utilized the burrows in the cleared experimental bank area.

Eight crab individuals were observed at low tide during

the six-day period, with three large P. crassipes (20-30 mm)

found in upper bank substratum placed in the lower tidal

level.

LABORATORY EXPERIMENTS

Desiccation

Hemigrapsus oregonensis and Pachygrapsus crassipes

had approximately the same ability to survive desiccation

(Figure 10). Mean survival time for P. crassipes was

25.7 hours (SD = 12.6, n = 16) with all individuals dead

within 47 hours. Individuals of H. oregonensis survived

on the average of 24.6 hours (SD = 11.1, n = 15) with all

individuals dead within 57 hours. The difference in

survival time between crab species was not significant

21

(P > 0.50, Student's t-test). Larger individuals of both

H. oregonensis (Figure 11) and P. crassipes (Figure 12)

were more tolerant to desiccation than smaller individuals.

Tolerance to Silty-Clay Water

Hemigrapsus oregonensis had a significantly greater

survival time than Pachygrapsus crassipes in experimental

jars containing silty-clay mud (P <0.001, Student's

t-test). Hemigrapsus oregonensis had a mean survival time

of 40.6 hours (SD 16.6, n = 10), while . crassipes

lived on the average of 7.2 hours (SD = 5.1, n 10)

in the muddy water. All P. crassipes were dead within

19 hours, while one H. oregonensis individual survived

more than six days in the muddy water (Figure 13). Size

did not influence survival ability of either crab species.

DISCUSSION

While some overlap does occur along the main channel

bank of Elkhorn Slough, Hemigrapsus oregonensis is

concentrated in burrows located in the lower region of the

bank and Pachygrapsus crassipes is generally found in

burrows located in the upper bank or bank slumps. Field

and laboratory experiments have demonstrated that the

observed distributional pattern can be best explained by

differences in microhabitat preference. Although the

relative importance of various habitat characteristics

is difficult to determine, substratum and tidal elevation

are major factors in the determination of suitable bank

habitats for H. oregonensis and P. crassipes in Elkhorn

Slough.

Degree of tidal exposure appeared to be a major factor

in microhabitat selection of Pachygrapsus crassipes.

Previous work by Hiatt (1948) and Gross (1957), respectively,

demonstrated that P. crassipes was negatively hydrotactic

and preferred to be out of water 50 percent of the time.

The calculated yearly exposure period of 54 percent for the

upper bank zone of Elkhorn Slough would therefore fit their

preference. Results of the tidal height preference

experiment confirmed Willason's (1981) findings that

P. crassipes preferred the more exposed intertial elevations.

22

Although exposure time may be important in the

selection of a suitable burrow habitat for Pachygrapsus

crassipes, substratum characteristics also play an

important role. In Elkhorn Slough, P. crass s was

found in high densities in the lower intertidal on

23

bank slumps and in man-made burrows bored into upper bank

substrate previously transferred to the lower bank region.

Therefore, the tidal height preference of P. crass s can

be modified by the existence of stable, erosion-resistant

upper bank substratum found in less exposed elevations.

The observed vertical distribution of Hemigrapsus

oregonensis along the main channel bank of Elkhorn Slough

appears to be the result of substratum preference. Willason

(1981) has suggested that H. oregonensis is inhibited from

occupying mud burrows in the upper regions of tidal creeks

due to predation pressures from Pachygrapsus crass s.

While antagonistic behavior by P. crass es may be a factor

in limiting the distribution of H. oregonensis, results of

the bank region preference experiment showed that H.

oregonensis preferred the lower bank substratum, selecting

this bank area within the enclosure cage without the

presence of P. crassipes. In an earlier study, Low (1970)

found that H. oregonensis preferred silty mud substratum

to substratum composed of larger particles. Thus,

24

characteristics of the substratum are important parameters

for H. oregonensis in its selection of a burrow habitat

along the banks of Elkhorn Slough.

The importance of substratum composition as a factor

affecting the pattern of distribution of faunal in­

vertebrates has been investigated by a number of authors

(fora review, see Grey, 1974). Earlier studies on the

distribution and ecological requirements of certain

brachyuran crabs, especially of the family Ocypodidae,

found that the distribution of many deposit feeding fiddler

crabs was correlated with substratum characteristics

(Teal, 1958; Ono, 1962, 1965; Whiting and Moshiri, 1974;

Barnes, 1974; Icely and Jones, 1978; Frith and Brunen­

meister, 1980). One substratum property, density of root

mat material, was shown to have a major affect on the

distribution of several species of fiddler crabs (Genus

Uca) in a North Carolina estuary (Ringold, 1979).

The relative susceptibility of the main channel bank

of Elkhorn Slough to erosion by water movement may determine

the stability and longevity of crab burrows found in this

particular habitat. Observations of medium-sized holes

(35 mm in diameter) bored into the lower bank indicated

that they deteriorated rapidly, and even if they occurred

in low densities (ten holes/0.25 m2 ), caused the collapse

25

of the lower bank surface within one or two days. In

contrast, equal-sized holes in equal densities bored into

the upper bank substratum maintained their integrity from

several weeks to several months. A major factor in the

reduced susceptibility of the upper bank substratum to

erosion is its compaction by dense root mat which binds

the soil particles together. The lower bank, devoid of

such material, is highly susceptible to erosional processes.

The difference in the erodibil and longevity of

burrows of the upper and lower bank substrata may be an

important factor in the habitat choice of Pachygrapsus

crassipes. Substratum instability has been found to be

an important influence of the distribution of many marine

organisms (Nickols, 1970; Stephenson et al., 1970;

Biernbaum, 1979). In a recent study, McKillup and Butler

(1979) found that substratum stability directly affected

the burrowing behavior of the grapsid crab, Helo sus

haswellianus, by causing the crab population to limit the

number of burrows dug in a mud bank. Burrows serve both

P. crass es and Hemigrapsus oregonensis as places of

refuge from potential bird (Stenzel et al., 1976), fish

(Low, 1970; Talent, 1973; Nybakken et al., 1977) and

terrestrial (Lindberg, 1980) predators while also providing

insulation from various physical factors. Willason (1981)

found that H. oregonensis readily burrows into mud banks,

but P. crassipes does not. He suggested that individuals

of P. crassipes utilize burrows initially constructed by

26

H. oregonensis and enlarge them by body movements as

needed. The unstable, rapidly deteriorating substratum of

the lower bank may limit the life expectancy and size of

burrows that can be constructed in that habitat. Large

burrows were seldom observed in the lower bank and only

small individuals of P. crassipes were found there.

Therefore, most adult P. crassipes appear to be restricted

to stable, long lasting burrows previously constructed by

H. oregonensis in the organically denser and erosion

resistant upper bank substratum.

In contrast, Hemigrapsus oregonensis may be less

affected by the burrow instability of the lower bank

substratum. Hemigrapsus oregonensis is an efficient

burrower that can rapidly burrow into the mud banks of

Elkhorn Slough (pers. obs.). In addition, H. oregonensis

found along the main channel bank of Elkhorn Slough

were relatively smaller than Pachygrapsus crass es and

subsequently occupied smaller burrows which the lower

bank substratum can support.

The degree of soil unconsolidation and the level of

suspended sediment are additional substratum characteristics

f I

27

which may affect the distribution and habitat selection

of Pachygrapsus crass es and Hemigrapsus oregonensis.

The upper bank region and bank slumps, which originate

from the upper bank, are composed of dense root mat

material which binds the sediment particles together and

reduces the amount of free silt in the burrows or near the

substratum-water/or air interface. The lower bank

substratum, devoid of root mat material, is less resistant

to erosion and may have increased levels of suspended or

loose silt and clay soil particles in and surrounding crab

burrows. Results of the tolerance to silty water

experiment indicated that P. crass es was less tolerant

of muddy water than H. oregonensis and suggested that P.

crassipes lacks some mechanism which prevents gill clogging.

Surrounding the incurrent channels of the gill chambers

of H. oregonensis and P. crassipes are small setae on

the branchiostegites. Generally, setae located on these

structures are thought to function in attracting water

to the gill chamber by capillary action (Warner, 1977).

Hiatt (1948) suggested that these setae also function in

filtering fine particles from the water before it enters

the gill chambers of P. crass s and H. oregonensis. Pachy-

sus crassipes has noticeably fewer and coarser setae ~~----

than H. oregonensis (Hiatt, 1948; pers. obs.), which may

be the morphological basis of its reduced particle f tering

28

efficiency. In waters containing fine suspended particles,

Hiatt (1948) suggested that the gills of P. crassipes

would tend to become clogged much more rapidly than those

of H. oregonensis.

Upon dissection of crabs exposed to fine sediments

during the tolerance to silty water test, Hemigrapsus

oregonensis individuals had heavy deposits of mud on the

setae of their branchiostegites, indicating they efficiently

filtered out sediment particles from the water. The setae

of Pachygrapsus crassipes were clean, suggesting they did

not effectively filter out the silt particles. The gills

and branchial chambers of P. crass s were also observed

to contain a heavier concentration of mud particles than

those of H. oregonensis. Therefore, while the highly

erodible substratum of the lower bank is actively selected

by the silty mud tolerant H. oregonensis, it may be

avoided by the more sensitive P. crass es.

The selection of the lower bank microhabitat by

Hemigrapsus oregonensis may not only be determined by

substratum preference but also by desiccation tolerance.

Small intertidal crabs, because of their high surface to

volume ratio, lose water by evaporation at a higher rate

and are less tolerant than larger crabs to desiccation

(Herreid, 1969; Grant and McDonald, 1979). Studies

29

have shown that smaller crabs are restricted to lower

tidal areas or to substratum with higher moisture contents

(Warner, 1969; Frith and Brunemeister, 1980; Kikuchi

et al., 1981). Although H. oregonensis possesses the same

ability to tolerate desiccation as Pachygrapsus crassipes,

its relatively smaller size may make it less likely to

choose the more exposed tidal elevation of the channel bank.

Although many small P. crassipes were found in the upper

bank, their occurrence here may provide a clue to estimating

the relative importance of desiccation and substratum to the

crab species. The silty, unstable nature of the lower

bank may impose a greater physiological burden on small

individuals of P. crassipes than the increased exposure

to air of burrows found in the upper bank.

SUMMARY

The distribution of Hemigrapsus oregonensis and

Pachygrapsus crassipes along the tidally influenced

vertical banks of Elkhorn Slough is the result of the

interaction of a variety of physical and biological

factors. While the relative importance of the factors

would be difficult to determine, substratum and tidal

elevation seem to be the most important factors in

microhabitat choice for both species of crabs. Although

P. crassipes generally prefers the more exposed levels of

the channel bank, this tidal height preference was

modified by the existence of stable substratum in the

lower intertidal. Therefore, while tidal height does play

a role in microhabitat selection, burrow stability and

level of unconsolidated silty-clay sediment are also very

important in limiting the distribution of P. crassipes.

In contrast, by selecting the less exposed, unstable lower

bank substratum, H. oregonensis avoids desiccation stress

and possibly predation by larger P. crassipes. Efficient

burrowing ability and tolerance to fine particulate mud

may be adaptations related to the substratum preference

by~- oregonensis.

30

LITERATURE CITED

Abele, L.G. 1973. Taxonomy, distribution and ecology of the genus Sesarma (Crustacea: Decapoda: Grapsidae) in eastern North America, with special reference to Florida. Amer. Midl. Nat. 90 (2): 375-386.

Bacon, M.R. 1971. Distribution and ecology of the crabs Cyclograpsus lavauxi and C. insularum in northern New Zealand. . .. of Marine and Fresh. Res. 5: 415-426.

Barnes, R.S.K. 1974. Estuarine biology. Edward Arnold, London. 76 pp.

Batie, R.E. 1974. Population structure of the intertidal shore crab Hemigrapsus oregonensis (Brachyura, Grapsidae) in Yaquina Bay, a Central Oregon coast estuary. Ph.D. dissertation, Oregon State University. 128 pp.

Biernbaum, C.K. 1979. Influence of sedimentary factors on the distribution of benthic amphipods of Fishers Island Sound, Connecticut. J. Exp. Mar. Biol. Ecol. 38: 201-223.

Crane, J. 1975. Fiddler crabs of the world (Ocypodidae: genus Uca). Princeton University Press, Princeton, N.J. 736 pp.

Folk, P.W. and W.C. Ward. 1957. Brazos River Bar: in the significance of grain-size parameters. Pet. 27: 3-26.

a study J. Sed.

Frith, D.W. and S. Brunenmeister. 1980. Ecological and population studies of fiddler crabs (Ocypodidae, genus Uca) on a mangrove shore at Phuket Island, western peninsular Thailand. Crustaceana 39 (2): 157-183.

Grant, J. and J. HcDonald. 1979. Desiccation tolerance of Eurypanopeus depressus (Smith) (Decapoda: Xanthidae) and the exploitation of microhabitat. Estuaries 2 (3): 172-177.

Grey, J.S. 1974. Animal sediment relationships. Oceanogr. Mar. Biol. Ann. Rev. 12: 223-261.

31

Gross, W.J. 1957. A behavioral mechanism for osmotic regulation in a semi-terrestrial crab. Biol. Bull. 113: 268-274.

Herreid, C.F. habitats.

1969. Water loss of crabs from different Comp. Biochem. Physiol. 28: 829-839.

Hiatt, R.W. 1948. Biology of the lined shored crab, Pachygrapsus crassipes (Randall). Pacif. Sci. 2: 134-213.

32

Icely, J.D. and D.A. Jones. 1978. Factors affecting the distribution of the genus Uca (Crustacean: Ocypodidae) on the East African shore.~stuarine and Coastal Mar. Sci. 6: 315-325.

Jones, M.E. 1976. Limiting factors in the distribution of intertidal crabs (Crustacea: Decapoda) in the Avon­Heathcote Estuary Christchurch. N.Z.J. of Marine and Fresh. Res. 10 (4): 577-587.

Kikuchi, T., Tanaka, M., Nojima, S., and T. Takahashi. 1981. Ecological studies on the pebble crab, Gaetice depressus (de Haan). I. Ecological distribution of the crab and environmental conditions. Publ. Amakusa Mar. Biol. Lab. 6 (1): 23-34.

Knudsen, J.W. 1964. Observations of the reproductive cycles and ecology of the common Brachyura and crablike Anomura of Puget Sound, Washington. Pacif. Sci. 18: 3-33.

Krumbein, W.C. and F.J. Pettijohn. 1938. Manual of sedimentary petrology. Appleton-Century-Crofts, New York. 549 pp.

Kuris, A.M. 1971. Population interactions between a shore crab and two symbionts. Ph.D. dissertation, University of California, Berkeley. 477 pp.

Lindberg, W.J. 1980. Behavior of the Oregon mud crab, Hemigrapsus oregonensis (Dana) (Brachyura, Grapsidae). Crustaceana 39 (3): 263-281.

Low, C.J. 1970. Factors affecting the distribution and abundance of two species of beach crabs, Hemigrapsus nudus and H. oregonensis. M.S. thesis, University o tish-Columbia. 70 pp.

33

MacGinitie, G.E. 1935. Ecological aspects of a California marine estuary. Am. Midl. Nat. 35: 629-765.

McKillup, S.C. and A.J. Butler. 1979. Cessation of hole­digging by the crab Helograpsus haswellianus: a resource-conserving adaptation. Mar. Biol. 50: 157-161.

Nickols, F.H. 1970. Benthic polychaete assemblages and their relationship to the sediment in Port Madison, Washington. Mar. Biol. 6: 48-57.

Nybakken, J. Cailliet, G. and W. Broenkow. 1977. Ecologic and hydrographic studies of Elkhorn Slough, Moss Landing harbor nearshore coastal waters. July 1974 to June 1976. Moss Landing: Moss Landing Marine Laboratories. 465 pp.

Ono, Y. 1962. On the habitat preference of ocypoid crabs I. Mem. Fac. Sci. Kyushu Univ. Series E. (Biol.) 3 (2) 143-163.

Ono, Y. 1965. On the ecological distribution of ocypoid crabs in the estuary. Mem. Rae. Sci. Kyushu Univ. Series E. (Biol.) 4 (1): 1-60.

Pestrong, R. 1965. The development of drainage patterns on tidal marshes. Stanford University publications. Geological Sciences. 10 (2): 87 pp.

Ricketts, E.F. and J. Calvin. 1968. Between Pacific Tides. 4th ed. Stanford Univ. Press, Stanford, Calif. 614 pp.

Ringold, P. 1979. Burrowing, root mat density and the distribution of fiddler crabs in the eastern United States. J. Exp. Mar. Biol. Ecol. 36: ll-21.

Seiple, W. 1979. Distribution, habitat preferences and breeding periods in the crustaceans Sesarma cinereum and S. reticulatum (Brachyura: Decapoda: Grapsidae) Mar. Biol. 52: 77-86.

Siegel, S. 1957. Nonparametric statistics for the behavioral sciences. McGraw-Hill, New York. 312 pp.

Smith, R.E. 1974. The hydrology of Elkhorn Slough, a shallow California coastal embayment. M.S. thesis, San Jose State University. 88 pp.

r

Snelling, B. 1959. The distribution of intertidal crabs in the Brisbane River. Aust. J. Mar. Fresh. Res. 10: 67-83.

Stenzel, L.E., Huber, H.R. and G.W. Page. 1976. Feeding behavior and diet of the long-billed curlew and willet. The Wilson Bulletin 88 (2): 314-332.

34

Stephenson, W., Williams, W.T. and G.N. Lance. 1970. The macrobenthos of Moreton Bay. Ecol. Monogr. 40: 459-494.

Talent, L. 1973. The seasonal abundance and food of elasmobranchs occurring in Elkhorn Slough, Monterey Bay, California. M.A. thesis, California State University, Fresno. 58 pp.

Teal, J.M. 1958. salt marshes.

Distribution of fiddler crabs in Georgia Ecology 39: 185-193.

Warner, G.F. 1969. The occurrence and distribution of crabs in a Jamaican mangrove swamp. J. Anim. Ecol. 38: 379-389.

Warner, G.F. London.

1977. The biology of crabs. Paul Elek, Ltd., 202 pp.

Whiting, N.H. and G.A. Moshiri. 1974. Certain organism­substrate relationships affecting the distribution of Uca minax (Crustacea: Decapoda). Hydrobiologia (4): 481-493.

Willason, S.W. 1981. Factors influencing the distribution and coexistence of Pachygrapsus crassipes and

· Hemigrapsus oregonensis (Decapoda: Grapsidae) in a California salt marsh. Mar. Biol. 64: 125-133.

Zar, J.H. 1974. Biostatistical analysis. Prentice-Hall, Inc., New Jersey. 620 pp.

35

Table 1. Mean densities (+ 1 SD) and mean differences in densities (+ 1 SD) of Pachygrapsus crassipes and Hemigrapsus oregonensis collected in 0.25 m2 quadrats in Elkhorn Slough from December 1979 to May 1980. N = 30.

Density Density Mean Upper Bank Lower Bank Difference

P. crassipes 24.6 (+11.0) 3.1 (+3.7) 21.6 ( +9 . 6) *~~·k

H. oregonensis 1.8 (+2.9) 12.2 +9.1) 10.1 +9 3) 'bb~

* 1'"*Significantly different, P < 0. 001, Paired-sample t-test.

36

Table 2. Summary of statistical analysis for the physical characteristics of the upper and lower bank regions of the main channel of Elkhorn Slough. Values are mean + 1 SD with numbers in parentheses indicating the sample size. See text for methods.

Erodibility (% wt. loss)

Root Mat Density (Dry root wt./ wet wt.)

Moisture Content (%water wt.)

Sediment Analysis

Diameter (microns)

% clay

Exposure (% time exposed/ yr)

Upper Bank

3.1 + 3.2 (7)

9.3 + 0.38 (5)

51.8 + 5.4 (5)

3.06 + 0.74 (4)

58.4 + 6.3 (4)

54

Lower Bank

53.2 + 35.7 (7)

0.75 + 0.06 (5)

48.2 + 2.5 (5)

1.38 + 0.88 (4)

76.6 + 10.7 (4)

20

Probability level a

<0.001

< 0. 01

>0.05

<0. 05

< 0. 05

a - Probability that observed differences between mean values are due to random effects as determined by Mann-Whitney U-test (Siegel, 1957).

37

Table 3. Summary of chi-square tests for the tidal height preferences of Pachygrapsus crassipes and Hemigrapsus oregonensis. Ten crabs were introduced into the cage for each experiment.a

b No. of crabs No. of crabs No. of E. upper cage lower cage experiments

P. crassipes 76 14 9 < 0. 001

H. oregonensis c 43 35 8 > 0. 05

a - Nonsignificant, P >0.05, heterogeneity chi-square test indicates pooled data justified (see Zar, 1974).

b - Probability that observed tidal height preference is due to random effects as determined by chi-square with homogeneous sets of data.

c - Two crabs died during the experiment.

Table 4. Summary of chi-square tests for the bank region preferences of Pachygrapsus crassipes and Hemigrapsus oregonensis.a Six crabs were initially introauced into the cage in each experiment. Number of crabs found on cage matgrial and not actual bank substratum are in parentheses.

No. of crabs No. of crabs No. of upper bank region lower bank region experiments p_c

P. crassipes 30 (10) 5 (3) 6 <0.001

H. oregonensis d 4 (0) 39 (3) 9

a -Nonsignificant, P > 0.05, heterogeneity chi-square test indicates pooled data justified (see Zar, 1974).

b - Only crabs found on the bank substratum tested.

c - Probability that observed bank region preference is due to random effects as determined by chi-square procedure with homogeneous sets of data.

d - Nine crabs escaped from the cage during the series of preference experiments.

<0.001

39

Figure 1. Map of Elkhorn Slough showing position of study site.

·1 CALIFORNIA I i I "·'-. ...._

Kilometers

" ·"· "· ~.

1 2

% Miles

. BENNETT ·:· .. · SLOUGH . . .

40

. . ~ .. " ~ .. ·. . . ....

~ . . . . . "'

....

··.~·.::. KIRBY . ·>:·:·:.> .. PARK .

..

. . . ... OYSTER DOCK.·<· ..

41

Figure 2. Photograph of bank slumps at the study site in Elkhorn Slough during low tide.

42

43

Figure 3. Diagram of the device used in the substratum erodibility experiment. Incoming water pressure was approximately 65 lbs/in2. Arrows indicate direction of water flow. Cylinders (10 em X 24 em) were constructed of plexiglass (3 mm in thickness).. A wire cage (6 em X 15 em) with an enclosed substratum core (3.5 em X 15 em) was centered within each cylinder during the experiment.

I It I It I SUBSTRATE I t CORE

I 1t I t :ttj. }:

:t -- ~ -= -==:: -::::::: j f11 /_.... .:::::--1• / ~ 1' .,. . \1' \.\ t . ~J'

'\ ?

RUBBER TUBING diameter= 5mm

ttt

65psi

tl

I 1 I

I

44

45

Figure 4. Artificial substratum cage against the main channel bank of the study site during low tide.

46

47

Figure 5. Size frequency diagram of all Hemigrapsus oregonensis collected from burrows of the upper bank region. Five quadrats (0.25 m2) were sampled monthly (December 1979 to May 1980) in the study area, n = 53

I I I I

1-

I I ! I

Aouenbe,..~,:t

-

-

--

-

48

co N

~ C\..1

(0 .........

N ...-.

00

..., E E

1..-J

..r:. ~ -o •r-1

~

(J) 0 o· a_ 0 L 0 0

49

Figure 6. Size frequency diagram of all Hemigrapsus oregonensis collected from mud burrows of the lower bank region. Five quadrats (0.25 m2) were sampled monthly (December 1979 to May 1980) in the study area, n = 365.

'

• • • '

!-

"'

.

I I I I

Aouenbe...Jj

-....

.

.

-

50

co N

es:a N

U':) .........

N ,......

CD

r-"1 E E

L.....l

..!::. -+> -o ..... == Q) 0 0 a... 0 t. 0 0

51

Figure 7. Size frequency diagram of all Pachygrapsus crassipes collected from mud burrows of the upper bank region. Five quadrats (0.25 m2) were sampled monthly (December 1979 to May 1980) in the study area, n = 737.

52

~ • • • I • • ..q-

~

[ ..q-

co ('0

I ... - N

('0

I ......., - co E

N E

I 1...-J

...c. . -..:t' -+> N -o

I .,.., ill'.:

-I

~ .(1) N 0

0 a_

-I

co 0 !... ........ 0 0

. N ........

I - .

l 1- -

I I I I I • I

Aouenbe..J;}

53

Figure 8. Size frequency diagram of all Pachygrapsus crassipes collected from mud burrows of the lower bank region. Five quadrats were sampled monthly (December 1979 to May 1980) in the study area, n = 92.

54

""':t ~

s ...:t'

UJ en

N en

f""""''

m E N E

I......!

...c ~ +> N -o .....

3:

s G) N 0

0 0...

UJ 0 l. ....... 0 0

Aouenbe-..tj

55

Figure 9. Results of five Vertical Height Preference/ Species Interaction experiments. Six Pachygrapsus crassipes and six Hemigrapsus oregonensis were initially placed into the cage. The position of each crab was recorded 24 hours later at the next low tide.

56

• ' • • ' •

I I I I

57

Figure 10. Tolerance to desiccation of Hemigrapsus oregonensis (n = 15) and Pachygrapsus crassipes (n 16). Difference in survival time was not significant, P >0.50, Student's t-test (Zar, 1974).

(\J ~

::0 oJ 1 (.0 (T) .....,

w L

0 ~ L

+ CSl L-..1

(T) . Q)

I E

I ......

~ 4J ('\j

J co ........

(\J ..--1

CSl (.O~~mN ........ CSlrnro~~~~mN..--~~~ ......... ......... ........ ......... ........ ........ ........

59

Figure 11. Regression of desiccation survival time of Hemigrapsus oregonensis against crab size as determined by carapace width. R '1' 2 = coefficient of determination, Syx = standard error of estimate and n =number of crabs. Broken lines are the 95 percent confidence belts for the regression line.

60

54

48 ,....,

(J) 42 L

L 1.-J

([) 36

E ...... 30 +>

..--1

0 24 > ·rl

+I I

> L 18 :J (J)

12

/ I'

il y I

/ +I /

I I I I

/ I / I

6 / I

/ I / I

I .I ;

0 0 6 12

I

+ I I

I + I I I

.f. I / II

I ,, I I

I I /

/ I

I I

I I

I I

I

I '+* Intercept I

I Slope I

Rt2 Syx

N

18 24 30 36

/ /

I

= :::::

= = =

-L f2157e 2.005e 0.860 4. 125 15

42 48 carapace width [mm]

01 00

54 60

61

Figure 12. Regression of desiccation survival time of Pachygrapsus crassipes against crab size as determined by carapace width. R '1' 2 = coefficient of determination, Syx = standard error of estimate and n =number of crabs. Broken lines are the 95 percent confidence belts for the regression line.

63

Figure 13. Tolerance to silty-clay water. Pairs of Hemigrapsus oregonensis and Pachygrapsus crassipes matched for size and placed in 500 ml jars. Ten jars contained aerated seawater (Control) and 10 contained aerated seawater with 50 ml unconsolidated lower bank mud (Silty-clay water). Dotted lines indicate no change in status until experiment was terminated at 156 hours when the last crab died in the experimental jar.

l

Cl)

> -...... tll

• ~ c..

0

<+-0

&

0 ::z

. 0

::z

64

Silty-clay water

1

. . . . . . . . . . . . . . . . . . . . . . ..

+ - li. gregonens i a

o-~ ocassipes

1 Control

a~~~~--_. __ _. __ ~--~------~--~~~~--~ ra s 12 1a 24 ~ 36 42 48 54 59 66 12

time Chrs]