Migration, Dispersal, and Larval Ecology:

• Many abundant commercially exploited fish spp. conform to the spawning and migration cycle - where the juveniles drift from a spawning area to a nursery area and are recruited to an adult stock area that feeds a feeding ground; adults then migrate to the spawning area

• The gradual movement of fishes from the nursery grounds to different adult feeding grounds (not rich enough) may reflect competition between juveniles and adults for limited food resources

• Spawning area - region where eggs are laid and inseminated. Eggs may be laid in midwater (pelagic) or on the bottom (benthic)

• In salmonids, spawning grounds are located on the bottom of freshwater streams and young smelts then migrate down stream to the sea

• In Atlantic salmon - migration occurs towards the sea after 2-3 years; in Pacific salmon, it happens in a few weeks

• In the Atlantic herring, Clupea harengus, eggs are attached to plant stems and gravel by the female while the males swim and deposit sperm

• In contrast, the cod, Gadus morhua, spawns in the water column - male and female fishes swim in contact as eggs and sperm are released

• Anadromous - (salmon, shad, sea lamprey) spend most of their time in the sea and return to freshwater to breed

• Catadromous (eels) spend their adult lives mainly in freshwater and migrate to the sea to reproduce

• Oceanodromous - (herring, cod, plaice) live and migrate in the ocean

Invertebrate Larval Development

Planktotrophic Development:

• In many invertebrate spp., the time between release of larvae in the plankton and final metamorphosis is genetically programmed and mediated through a series of developmental stages

Risks of mortality and benefits of dispersal

• Metamorphosis may not occur if no suitable substratum exists for adult life – can lead to death

• M. edulis - 4-5 weeks (veligers); Balanus sp.. 3-4 weeks (nauplii) -

Lecithotrophic Development:

• Larva has a short planktonic stage and uses a large egg yolk as a source of energy during development in like plankton

• Ectoprocts - may brood eggs in socially modified chambers called ovicells – Bugula spp.

• Even though the dispersal phase is short, lecithotrophy must have evolved for the purpose of dispersal because even the existence for a short period like an hour will permit a dispersing larva to travel hundreds of meters and escape crowded conditions

Direct Release From Egg Cases:

• Planktonic phase completely absent - development in an egg case is often supplemented by some form of parental protection

• Young released into immediate environment

Gene Flow:

• Allozyme differences of the mud snail Ilyanassa obsoleta - a species with long-dispersing planktotrophic larvae - was compared with the intertidal snail Littorina saxatilis which has direct release

• Littorina has more local geographical differences than does the long disperser I. obsoleta

Within Parent Development (Viviparous):

• Complete development occurs within the parental animal up to the point that the embryo is capable of escaping to the bottom as a fully developed juvenile - few young produced

• Littorina saxatilis - eggs arrive in the brood chamber already fertilized; brooding occurs in an altered part of the oviduct “uterus” containing young in various stages - young snails then emerge as fully shelled juveniles

Distribution

• Latitudinal Trends - Murray (1895-1898); Thorsen (1950) - about 70% of contemporary species of marine-bottom invertebrates have planktonic development

• However, in the Arctic 90% of marine species develop without phase and with large yolky eggs

• Arctic planktotrophic larvae confront 2 problems: a short phytoplankton season and cold temperatures

• Antarctic species - many are brooding species

• 30-45% of sea stars

• Although rich planktonic production occurs in Antarctic seas, production takes place at the very surface of the open ocean while bottom organisms inhabit partly shallow-bottom shelves where food reserves can be low

•A second trend suggested by Thorsen (1950) is a variation in reproduction with depth of water. Claimed that dominance of non-planktonic development in Arctic and Antarctic oceans held for deep sea as well (low food resources)

• Scheltema (1971)- discovery of the large-scale occurrence of long-lived planktonic larval stages in the tropics promoted large-scale survey with larvae of benthic stages occurring in the Gulf Stream, N. Atlantic drift, and equatorial currents

• Scheltema termed such ocean-going larvae of shelf invertebrates teleplanic larvae

• estimates range of 42-320 days of possible life in the plankton for planktotrophic gastropod larvae

• one result of long-distance dispersal is the possibility of large-scale geographic ranges of tropical marine invertebrates

Settlement of Larvae:

• 3 major stages larvae must pass before successful settlement:

• successful development

• retention nearshore

• substratum selection

• Reasons for not succeeding - food shortage, wastage of larvae, predation, crowding

Metamorphosis:

1) physical characteristics of the substratum such as the presence of pits and grooves or the presence of sand grains of suitable diameter

2) presence of adults of the same species (gregarious settling)

3) contact with a substance produced by a species that predictably co-occurs with adults of the larva in question

4) contact with some generalized biological substratum feature - bacterial films

Physical Substratum:

• grain size - polychaete Ophelia bicornis - well- rounded sand grains

- Ilyanassa - fine-grained

• gregarious settling

Theoretical Considerations:

• Vance (1977) attempted to predict optimal reproduction and dispersal strategies

2 questions:

1) Why do we observe only exclusively lecithotrophic or exclusively planktotrophic development for most species? Few species have an extended period of dependence on yolk reserves with subsequent dependence on feeding planktonic larva

2) Under what conditions should a larva feed in the plankton or the benthos, develop living off a yolky egg, or simply develop within an egg case or parent and hatch as a free-living juvenile?

Strathman (1974) Model:

a) egg content energy

b) length of pre-feeding period

c) length of feeding period

d) total larval developmental period

e) mortality rates

Model Predicts- 1) Only extremes of the possible ranges of egg size and method of nutrition are stable in an evolutionary sense

2) Over range of environmental parameters, 2 developmental types are both evolutionary stable states

3) Planktotrophic development is more efficient than lecithotrophic development when more planktonic food is available than the reverse situation

4) Benthic pre-feeding development results in greater efficiency when lecithotrophic developmental time is long and/or plankton predation is more intense than benthic predation

Reproductive Strategies

Allocation of Resources:

• Growth rate of Balanus balanoides decreases during the reproductive or spawning period - egg production

• Semelparous - once (Gadgil & Bossert, 1970)

• Iteroparous - repeatedly

• Current reproductive activity and future reproductive success are inversely correlated (Williams, 1966)

• Must marine animals have a life cycle that can be divided between larval and adult stages.

• In such life cycles the risk of being a larva may be compared with adult mortality - selection of reproductive changes

• Under high adult mortality the species with the highest rmax will win. So, when there is

significant danger to adults, we expect early reproduction and one-time spawning (semelparity)

• Iteroparity is favored if a random component to juvenile mortality is introduced. Under these circumstances, reproducing more than once increases the probability that a year-class of adults will successfully recruit another year- class of juveniles

Asexual vs. Sexual:

• Advantages of asexual - proliferation of genotype that successfully colonizes a given microhabitat

• Secondly, the proliferation of this genotype is not hampered by the suite of adaptations necessary for gamete union

• All are identical - poorly suited for sudden changes

Binary fission and fragmentation – fission (i.e., diatoms – typical doubling time 0.6-6 per day), frag. – multicellular (i.e., Enteromorpha – fragmentation leads to new indiv. (buds)

Parthenogenesis – unfertilized eggs develop into normal indiv. – (Rotifers) – sperm required but does not contribute to genes (rare)

Vegetative Reproduction – division of an animal into many indiv. which may or may not be connected (encrusting sponges and ectoprocts)

Anthopleura elegantissima:

• Contiguous aggregations composed of individuals from a single clone

• planula larva settles, metamorphoses, and divides asexually producing hundreds of clones

• Anthopleura also has separate sexes and normal sexual reproduction. Thus, the need for genetic variation is maintained

• Colonies may have structural function - elkhorn coral - Acropora palmata - minimize effects of currents and waves

Hermaphroditism:

• Single individual contains gametes for both sexes

• Can reproduce with any other individual in the population; self-fertilization is RARE

• Barnacles – Balanus spp.

• Sequential hermaphrodites - can change from male to female (protandry) or female to male (protogony)

• Protandry is the general rule for many marine invertebrates

• Crassostrea virginica - smaller young make transformation into female after a few years - Self-Fertilization is Possible

• Also suspension-feeding gastropod Crepidula fornicata - larger older females below

• Some sequential hermaphroditism in fishes - coral reefs (i.e., wrasses)

• Labroides dimidiatus (wrasse) - small school, about 15 individuals - largest member is male, all others female

• If male dies - largest member of the group of females will change into male

• Male to female change is dependent on predation

• This form of sex change is common among reef fishes

Separate Sexes:

a) Both eggs and sperm may be shed in the water

b) Sperm may be shed in the water and fertilize eggs held by the female

c) Sperm may be mechanically transformed from male to female

• Epidemic spawning - a few individuals induce mass spawning by others

• May be correlated with lunar phases - within a few hours, all have spawned

• Reef sponges - spawn at same time – large clouds in the water

• Adult nereid, syllid, eunicid polychaetes - change morphology into individuals filled with gametes (epitokes) that swim to the surface and spawn - nuptial dance

Sexual Reproduction in Benthic Algae

• May be very complex with alternations of diploid and haploid phases

• gametophyte - gametes in gametangia

• motile – brown algae

• non-motile – red algae