Records of the Western Australian Museum 18: 129-134 (1996).

Description of the eggs and yolk-sac larvae of WhitebaitHyperlophus vittatus (Teleostei: Clupeidae)

R.J. Tregonning, D.J. Gaughan and W.j. Fletcher

Bernard Bowen Fisheries Research Institute,Western Australian Marine Research Laboratories,

p.a. Box 20, North Beach, Western Australia 6020, Australia

Abstract - The eggs and early larval (yolk-sac) stages of Hyperlophus vittatusare described. The eggs are spherical, range from 0.83 - 0.95 mm in diameter(mode = 0.93 mm), have a perivitelline space which occupies 8.6 - 30.9%(mode = 14.5%) of the diameter, a coarsely and completely segmented yolkand one oil globule (0.025 - 0.075 mm in diameter, mode = 0.048 mm). Theeggs hatch in 2 - 3 days at a mean temperature of 17°C. The yolk-sac larvaeare approximately 2.6 mm BL at hatching and lack pigmented eyes or afunctional mouth. The larvae are elongate and have a long, straight gut (pre­anal length = 70 - 79% of BL) with the single oil globule located posteriorlyin the yolk-sac. The presence of melanophores immediately dorsal of theposterior tip of the notochord distinguish the yolk-sac larvae of H. vittatusfrom those of other dupeids in southern Western Australia.

INTRODUCTION

Whitebait (Hyperlophus vittatus Castelnau), orsandy sprat, their common name elsewhere inAustralia, is commercially fished in the southwestof Western Australia (Goh 1992). The confinementof whitebait fishing in this region to beach seiningoperations has restricted the amount ofinformation available on the distribution of thisspecies. Collecting eggs and larvae using planktonnets provides a much more economical means ofsampling relatively large areas than samplingadults. Ichthyoplankton surveys can thus overcomethe problem of limited data on the distribution of ateleost species but requires accurate identificationof its early life stages. While H. vittatus larvae 5.3 ­29.1 mm SL have been described (Miskiewicz1987), eggs and larvae <5.3 mm SL areundescribed.

The aim of this paper is to provide a descriptionof the developmental stages of the fertilized eggsand yolk-sac larvae of H. vittatus to facilitate theuse of ichthyoplankton surveys for determining thespatial distribution of spawning whitebaitpopulations. The embryogeny of H. vittatus eggs isonly described sufficiently to identify the eggs ofthis species in plankton collections.

MATERIALS AND METHODS

Commercial catches of whitebait in WesternAustralia were monitored through 1993. In earlyAugust 1993, 12% of a catch of 200-300 kg ofwhitebait from Warnbro Sound (32°20.8'S,

115°44.5'E) had ripe gonads, suggesting thatspawning was probably occurring at this region.On 17 August 1993, plankton tows were completedat several sites within Warnbro Sound, using 60 cmdiameter bongo nets with 500 pm mesh, towed justbelow the surface at 1 m S-l. Fish eggs wereparticularly abundant at one station close to shore,sampled at 1200 hours. The surface watertemperature at this site was 18°C, with a salinity of35.1. The plankton sample from this tow was takenback to the laboratory, so that fish eggs could beremoved and reared.

Culturing of the eggs past hatching wasnecessary since their identification requiredidentification of the resultant larvae. Although H.vittatus larvae <5.3 mm SL have not beendescribed, individuals smaller than this havepreviously been identified (e.g., Gaughan et al.1990) using Miskiewicz (1987) as a baseline fromwhich to examine a series of sequentially smallerwhitebait larvae, which were identified by theirelongate shape, the relative length of the gut andthe pattern of pigmentation.

At the laboratory, fish eggs were siphoned offfrom the plankton sample and sorted using astereomicroscope. Two types of eggs wereabundant in the sample. Approximately 100 eggsof one of these types, which exhibited thecharacteristics of clupeid eggs, were selected forculturing. These characteristics were the sphericalshape, the presence of a segmented yolk and asingle oil globule (Ahlstrom and Moser 1980;McGowan and Berry 1984).

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The selected eggs were placed in four glasscontainers of seawater, two lightly aerated and twonon-aerated, and maintained at ambienttemperature (15.2 - 24.0°C, mean = 17°C). The eggssurvived well under these conditions and nearlyall them hatched. Forty larvae obtained from thesehatched eggs were transferred to a constanttemperature tank (18°C). Algae and enrichedrotifers were added to the water as a potential foodsource for the developing larvae.

Samples of eggs and larvae were taken daily andpreserved in 5% formalin. The first few rearedlarvae were identified as being those of H. vittatus,thereby confirming that the eggs chosen forculturing were of this species. Hyperlophus vittatuseggs were thus also able to be identified. Whilstthe sample of eggs collected for rearing did notcontain any early embryonic stages, these weredescribed using material collected from previousplankton tows in Warnbro Sound. These earlierstages of whitebait eggs were identified by theirsize, the presence of a segmented yolk and therelative size of the yolk, as determined from thereared eggs of known identity.

Measurements of eggs and larvae were madeusing an eyepiece micrometer fitted in a compoundmicroscope. The measured lengths of the larvaerefers to body length (BL, tip of snout to tip of thenotochord; Leis and Trnski 1989). Descriptions ofpigment refers to black pigment (melanin) unlessstated otherwise. Because future identification ofeggs arid early larvae will most likely involveformalin-preserved samples, the illustrations andmeasurements were done with formalin-preservedmaterial. The illustrations were made with the aidof camera lucida.

RESULTS

Development time of eggsMost (> 95%) of the reared H. vittatus eggs

hatched within 48 hours of capture. The seven,least developed eggs had embryonic keels (seeFigure 1d) at the time the live sample was sorted(1400 hours, 2 hours after capture). Five of theseeggs were allowed to develop through to hatching,which occurred 50 - 51 hours post-capture.

General description of eggsHyperlophus vittatus eggs are planktonic and

spherical. The eggs have a thin chorion thatappears smooth under low magnification. Underhigh magnification, however, evenly spacedcorrugations can be seen. The diameter offormalin-preserved eggs ranges from 0.83 to 0.95mm, with a mode of 0.93 mm.

Each egg contains one oil globule ranging indiameter from 0.025 to 0.075 mm, with a mode of

R.J. Tregonning, D.J. Gaughan, W.J. Fletcher

0.048 mm. The oil globule is not pigmented.The yolk is spherical to ovoid, depending on the

angle of view (Figure 1a,aa), with a diameterranging from 0.50 to 0.87 mm (mode = 0.83 mm).In live eggs, the yolk appeared nearly transparent,but became opaque when preserved in formalinand is semi-translucent both under brighttransmitted and reflected light. The yolk is coarselyand completely segmented with a 'frothy' or'bubbly' appearance which is readily apparentduring all stages of development.

The perivitelline space occupies 8.6 - 30.9% ofthe eggs diameter (mode = 14.5%).

Description of various stages duringdevelopment of the eggs

Initially, the segmented yolk is the mostprominent feature of the egg (Figure 1a,aa). Cellgrowth becomes obvious at the pole opposite tothat containing the oil globule, i.e. the vegetativepole (Figure 1 b). This cap of cells appears to bulgeslightly over the yolk. The cell-cap flattens out ascells multiply and spread in a thinner layer overthe yolk towards the opposite pole (Figure 1c). Incontrast to the opacity of the relatively thick cell­cap, the yolk is visible beneath this thin layer ofcells.

The anterior end of the embryo becomes visibleat the vegetative pole as a slightly raised strip (i.e.,embryonic keel), with a flatter layer of cells lying ateither side (Figure 1d). The embryo is less distincttowards the posterior end and the thin layer ofcells, although difficult to observe, is much widerthan at the anterior end.

The embryo then becomes more distinct, with theeye and some somites visible (Figure le). The tip ofthe tail lies beyond the oil globule. Divisions of thebrain are visible from dorsal view of the head,which lies flat against the yolk. In the majority ofthe eggs, at this and later stages, the yolk-sac lyingbeneath the head is concave, but with a centralbulge (Figure 1£). However, the yolk remainedrounded in a few of the eggs examined (cf. Figures1£,H). The concavity may be due to absorption ofthe yolk by the developing embryo.

With further development, the tail lifts awayfrom the yolk, develops finfolds at the tip andbegins to curve to the right. The tail also becomesmore pointed and increases in length, while thefinfold extends anteriorly almost to the head(Figure 1f). The anterior end of the embryo beginsto lift away from the yolk. The gut which is presentalong the ventral surface of the embryo is difficultto distinguish and has therefore not been shown inFigure 1£.

As development continues, the tail lengthens, thefinfold widens and the hindgut becomes moreobvious (Figure 19). At the time of hatching, thedorsoventrally flattened head of the embryo still

Eggs and yolk-sac larvae of whitebait

ao

b

131

c

f

d

9

yp

Figure 1 Early life history stages of Hyperlophlls vittatlls. a-g, eggs. h, newly hatched yolk-sac larva, 2.7 mm BL, yppatch of yellow pigment; i, yolk-sac larvae, 4.2 mm BL Scale bar equals 1.0 mm.

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lies close to the yolk (or yolk-sac membrane), thetail has grown completely around the yolk andoverlaps the head, but the mouth has notdeveloped and the eyes are not yet pigmented. The2 - 3 small melanophores on the dorsal surface ofthe tail near the tip of the notochord arecharacteristic of H. vittatu5 larvae, and whilstillustrated here (Figure 19), they were often notvisible in preserved eggs.

Description of yolk-sac larvaeNewly-hatched larvae were 2.6 mm BL and had

typical clupeid characteristics, including a longslender body, a long straight gut and lightpigmentation (Figure lh). The pre-anal length is70-79% BL.

The oil globule is located near the posterior endof the yolk-sac. A patch of yellow pigment, which

R.]. Tregonning, D.]. Gaughan, W.]. Fletcher

is often only obvious in fresh or recently preservedspecimens, was observed immediately posterior tothe oil globule (Figure lh,i). Newly hatched H.vittatu5 larvae have very fine melanophoresscattered over the body, particularly over the head.However, these melanophores are difficult toobserve after preservation, so only the denserconcentrations on the head have been illustrated.Moreover, they have 2 - 3 distinct melanophoreson the dorsal side of the trunk near the notochordtip which are homologous to those observed inwell developed embryos within late-stage eggs (cf.Figure 19).

At the time of hatching, larvae had neitherpigmented eyes nor a mouth (Figure lh). Larvaeplaced in the constant temperature tank (18°C) hadutilised their yolk, developed pigmented eyes andappeared to have a functional mouth after 5 - 7

Table 1 The major distinguishing features of the pelagic eggs of dupeids which are common in marine waters ofsouthern Western Australia. The eggs of each species have segmented yolks. The measures of egg and yolkdiameter for S. sagax from New Zealand (Baker 1972), and egg diameter for E. teres from South Africa(O'Toole and King 1974) are presented below those recorded in the present study.

Species

Hyperlophusvittatusn = 73

Sardinellalemurun =40

Sardinopssagaxn= 30

Etrumeusteresn =52

Egg diameter(mm)

range (mode)

0.83-0.95 (0.93)

1.15-1.32 (1.30)

1.34-1.58 (1.44)1.32-1.70 (1.53)

(Baker 1972)

1.24-1.48 (1.35)1.32-1.47 (1.37)(O'Toole and

King 1974)

Oilglobule

yes

yes

yes

no

Yolk diameter(mm)

range (mode)

0.50-0.87 (0.83)

0.69--0.82 (0.75)

0.62--0.85 (0.69)0.71--0.83

0.99-1.28 (1.12)

Perivitelline space(% of diameter)range (mode)

8.6 - 30.9 (14.5)

34.2 - 44.8 (39.7)

42.2 - 57.9 (49.4)

7.1 - 22.6 (18.7)

Diagrammaticrepresentations

•

Eggs and yolk-sac larvae of whitebait

days. They ranged in size from 4.2 to 4.7 mm BL.Rearing of H. vittatus larvae beyond the yolk-sacstage was unsuccessful with the larvae failing tofeed on the supplied rotifers and algae.Consequently no larvae survived past 8 days.

DISCUSSION

EggsHyperlophus vittatus eggs which were collected at

1200 hours and already possessed an embryonickeel, required a further 50 - 51 hours to hatch.Since many dupeoid species spawn at night (e.g.,Blaxter and Hunter 1982; Sommerton et al. 1993),we assumed that the least developed H. vittatuseggs collected for culturing had been fertilizedbetween 2000 and 0400 hours the night beforecapture. Thus, the development time to hatchingfor H. vittatus eggs, at a mean temperature of 17°C,was 58 - 67 hours (2.5 3 d).

Hyperlophus vittatus have eggs which are typicalof many marine teleosts (Ahlstrom and Moser1980; Matarese and Sandknop 1984), beingspherical with a diameter of about 0.9 mm andpossessing a single oil globule. Nonetheless, H.vittatus eggs can be distinguished from those ofother species found in plankton collections insouthwestern Australia by their size, thesegmented yolk, the relative sizes of the yolk andthe perivitelline space, and the small oil globule.However, in other regions, the initial identificationof H. vittatus eggs should ideally be carried out onthe late-stage eggs in which the embryo is welldeveloped. In particular, the 2 - 3 melanophoresdorsal of the notochord tip provide a means ofidentifying late-stage H. vittatus eggs.

The other dupeid species common in southernWestern Australian waters are Spratelloidesrobustus, Sardinella lemuru, Sardinops sagax andEtrumeus teres (Hutchins and Swainston 1986). Theeggs of S. robllstlls have a gelatinous covering andare layed demersally (McGowan and Berry 1984)so would not be expected to occur in planktonsamples. The major features which distinguish H.vittatlls eggs from those of these other dupeids arethe size of the egg and the relative width of theperivitelline space (Table 1). As is the case with alldupeids (McGowan and Berry 1984), the eggs ofeach of these species have segmented yolks. Thedimensions of the eggs of these other species werebased on specimens found in plankton samplestaken in marine waters off southwestern Australiaand preserved in the same way as the H. vittatuseggs used in this study. Baker (1972) provides afull description of S. sagax eggs, while figures ofthe eggs of several Sardinella species can be foundin Bensam (1990). Development of E. teres eggs hasbeen described by O'Toole and King (1974).

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Hyperlop/ws vittatus eggs (diameter of 0.93 mm)are smaller than those of S. lemuru 1 (1.30 mm), S.sagax (1.44 mm) and E. teres (1.35 mm) (Table 1).The perivitelline space occupies 14.5% of the eggdiameter in H. vittatus, but accounts for 39.7 and49.4% of the diameter in S. lemllru and S. sagaxrespectively. Although the relative size of theperivitelline space in the eggs of E. teres (18.7°/,,) ismore similar than these other species to that for H.vittatlls, the eggs of this former species lack an oilglobule (Table 1).

In addition to these features, late-stage H. vittatuseggs can be identified by the dorsal melanophoreson the tail of the embryo and, in fresh specimens,the yellow pigment posterior to the oil globule.

LarvaeThe most easily recognisable feature of the yolk­

sac larvae of H. vittatlls, which can be used todistinguish this species from the other dupeidlarvae which occur in southern Western Australia,is the presence of dorsal melanophores near thenotochord tip. Yolk-sac larvae of S. robustus, S.lemllru, S. sagax and E. teres lack thesemelanophores. Likewise, the yolk-sac larvae ofEngraulis australis, which are similar in body formto those of H. vittatus, do not possessmelanophores dorsal to the notochord tip.

ACKNOWLEDGEMENTS

Thanks to Ken White for assisting in thecollection of the plankton material. This manuscriptbenefited from comments made by staff of the WAMarine Research Laboratory and by twoanonymous referees.

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Manuscript received 13 September 1995; accepted 25 July1996.