Light responses of the larva of the serpulid polychaeteGaleolaria caespitosa

AdultGaleolaria caespitosa were collected at Cape Banks and Balmoral Bay, both in the vicinity of Sydney, Australia, from October 1985 to February 1986. Planktotrophic larvae of this serpulid polychaete are negatively phototactic at irradiance levels between 10 and 2 000x10¹⁴ q cm^-2 s^-1. The effective spectral range lies below 530 nm. Early metatrochophore larvae are less strongly photonegative; later they become photopositive. The photopositive phase persists into the actively swimming 3-setiger stage. Older setigerous larvae are photoneutral, demersal and positively thigmotactic. The strength of the photonegative response is not affected by the level of irradiance. The photopositive response is weaker at low levels of irradiance. The photonegative behaviour of young larvae may keep them at an optimal level for feeding. The subsequent photopositive phase may increase chances of being washed ashore and the terminal, photoneutral, thigmotactic phase may be important in the selection of a suitable site for settlement. Comparisons are made with light responses of two other serpulid species. Larval responses to light may be important infitness, especially in relation to successful niche occupation by adults.     

Behavioral response to hydrostatic pressure in larvae of two species of xanthid crabs

Responses of pelagic larvae of two species of xanthid crabs to manipulations of hydrostatic pressure were examined and compared. One species, Rhithropanopeus harrisii (Gould), is a temperate estuarine species, while the other, Leptodius floridanus (Gibbes), inhabits shallow water and reefs in tropical and subtropical regions. All zoeal stages of R. harrisii detected pressure stimuli presented in increments as small as 0.025 atm. Both Stage I and II zoeae of L. floridanus responded to pressure stimuli presented in increments of 0.1 atm. Stage I responded when pressure was changed by the smallest increment tested, 0.025 atm, but Stage II did not. L. floridanus zoeal Stages III and IV, however, did not change vertical position relative to control larvae when subjected to pressure stimuli. The capacity of R. harrisii zoeae to respond to changes in hydrostatic pressure may be related to their retention in the estuarine environment. In L. floridanus, a non-estuarine species, the pressure response is important only in the first zoeal stage and may function promarily in dispersal.     

How to bite the dust: morphology, motion pattern and function of the feeding appendages of the deposit-feeding thalassinid shrimp Callianassa subterranea

The morphology of the mouthparts of Callianassasubterranea (Montagu, 1808) was studied using light microscopy and scanning electron microscopy. All the mouthparts except the mandible, but including the mandibular palp, appeared to be supplied with a wide variety of setae. The setae of the medial rims of these appendages (the “ventral screens”) show a trend of decreasing passive motility towards the oral side; this applies to the setae themselves as well as the micro-structures on the setae. The motion of the mouthparts was filmed from aboral and rostral views with macro-video and endoscopy equipment, after marking the joints and tips of maxillipeds and the mandibles. Motion analysis showed that all mouthparts except the first maxilla (Mx1) actively moved during deposit feeding. The first and second maxillipeds (MP1, MP2), Mx2 and the mandibles moved at the same frequency, contra-lateral in phase, but with a 50% phase shift between ipsi-lateral mouthparts. The MP3 moved at a lower frequency with a contra-lateral phase shift of about 50% without an obvious phase relation with the other mouthparts. According to the following scenario, the feeding mechanism of C. subterranea seems to be based on the morphology and motion of the appendages in combination with specific setal functions. The MP3 make large excursions over the substrate, suspending the sediment in front of the MP2. The MP2 move through the upper layer of the suspension, trapping particles of 30 μm and smaller. These particles are combed out by the MP1 and transferred to the mouth opening by the cascade of finely structured setae of the inner mouthparts, and finally ingested. Received: 26 September 1997 / Accepted: 27 March 1998     

Feeding and mouthpart morphology in the semaphore crabHeloecius cordiformis (Decapoda: Brachyura: Ocypodidae)

The mouthparts of the Australian semaphore crabHeloecius cordiformis (H. Milne-Edwards, 1837) are described. Male and female semaphore crabs were observed and collected from mangrove swamps around the south side of Botany Bay, Australia between 1983 and 1987. Semaphore crabs inhabit mangrove mud/sand flats between the midlittoral intertidal zone and the supralittoral fringe of bordering saltflats. Scanning electron micrographs provide detailed pictures of mouthpart morphology and the setae present on the third, second, and first maxillipeds; the second and first maxillae; and the mandibles. Amongst other setal types, spoon-tipped, corn, and bottle-brush setae are described. While semaphore crabs are primarily deposit feeders (microphagous), they have well developed mandibles and are, therefore, also able to eat larger plant and animal material (macrophagous). A brief comparison is made between the mouthparts of semaphore crabs and fiddler crabs (Uca spp.), since the two genera are closely related, occupy similar niches, and resemble each other in appearance. This comparison reveals that while the second maxilliped setal morphology differs between the two genera (Heloecius possesses corn setae,Uca possesses spoontipped setae) these setae may have similar functions. The feeding behaviour ofH. cordiformis is related to substrate composition, burrow construction and various mouthpart specialisations. It is conlcuded that, within the context of the present study, certain structural e.g. mouthpart modifications allowing both scavenging and deposit feeding, and behavioural modifications e.g. mound building in wet muddy habitats, enable semaphore crabs to occupy a wide range of habitat types within the mangrove.Please address all correspondence and requests for reprints to Dr Maitland at his present address at the University of the Witwatersrand     

Patterns of growth and triacylglycerol content in snow crab Chionoecetes opilio (Brachyura: Majidae) zoeal stages reared in the laboratory

Snow carb Chionoecetes opilio zoea I and zoea II larvae, hatched from females in a controlled mating experiment, were reared in the laboratory at 10.1 °C and 28.0 salinity, to resolve the patterns of growth (dry weight [DW]) and change in energy reserves (triacylglycerols [TAG]) within a given moult cycle. The patterns of growth and change in TAG reserves were similar in each zoeal stage. Following hatching or a moult, the zoeae entered a phase of rapid size increase, i.e. high daily growth rates (5.5 to 12.8% DWd^-1), for 1/3 to 2/5 of the duration of the moult cycle. During the same period, the zoeae accumulated TAG reserves until a maximum (TAG DW^-1) was reached at the end of the phase of rapid growth. The period of high growth rates and of TAG accumulation is interpreted as the required time for the zoeae to reach a point in development [i.e. point of reserves saturation (PRS); Anger and Dawirs (1981)] where sufficient growth and energy reserves allow moulting to the next stage. Following the phase of rapid growth and TAG accumulation, the zoeae entered a phase of low daily growth rates (0 to 1% DWd^-1) during which the TAG reserves decreased to a minimum at the end of the phase. Prior to, and during the moult to zoea II, a phase of negative growth was observed in the zoea I larvae. We conclude that measurement of zoeal size and TAG content, along with morphometric criteria (e.g. epidermal retraction), can be used to assess growth and nutritional condition of C. opilio zoeal stages from the sea.     

Larval development of the euphausiid Euphausia pacifica in the Yellow Sea

The complete larval development of Euphausia pacifica in the Yellow Sea is described and the stages are compared with larvae of E. nana. Euphausiid larvae examined in the present study were collected at 30 stations in the Yellow Sea in 1989. During the stages of Calyptopis III to Furcilia II, E. pacifica larvae are smaller than E. nana larvae but they are larger from Furcilia III onward. There are geographical variations in body size of Calyptopis III among the southern California waters, the eastern Japan waters, the Japan Sea, and the Yellow Sea; body size is smallest in the Yellow Sea and Japan Sea, while it is largest in southern California waters and intermediate in eastern Japan waters. Segmentation of antennal endopods was observed in the 56 to 51 furcilia forms in the Yellow Sea population, suggesting that this characteristic is not as reliable for identification of furcilia stages as the pleopods and terminal telson spines. Thus, we propose here that there are six furcilia stages of E. pacifica, instead of the seven previously reported by Boden (1950).     

Larval development of Euphausia nana (Crustacea: Euphausiacea)

The larvae of Euphausia nana Brinton in Sagami Bay and Suruga Bay, Central Japan, are described and illustrated. The furcilia stage is separated into six stages on the basis of the form of the pleopods and the number of terminal telson spines. Furcilia I, no pleopod or non-setose pleopods and seven terminal telson spines. Furcilia II, nonsetose and setose pleopods and seven terminal telson spines. Furcilia III, five pairs of setose pleopods and seven terminal telson spines. Furcilia IV, five pairs of setose pleopods and five terminal telson spines. Furcilia V, five pairs of setose pleopods and three terminal telson spines. Furcilia VI, five pairs of setose pleopods and one terminal telson spine. Segmentation of the antennal endopod and mandible palp occurred at the furcilia V stage. The dominant forms of early furcilia larvae of E. nana differed during different seasons of the year. The total lengths of calyptopis III and furcilia I to VI larvae also showed seasonal changes; they were largest in May and smallest from September to December.     

Phototaxis and cannibalism in gammaridean amphipods

Development changes in phototaxis were investigated in three species of estuarine amphipods: Gammarus lawrencianus. G. tigrinus, and G. mucronatus. In water of salinity 16 S, juveniles of G. tigrinus and G. mucronatus were initially photoneutral and became photonegative within two days after hatching. Juveniles of G. lawrencianus were photopositive on hatching and became photoneutral approximately 11 d later. In all three species, the change in phototactic behaviour coincided with the age at which juveniles became less vulnerable to cannibalism by adults. Evidence is presented showing genetic variation for phototactic behaviour in G. lawrencianus. Our data are consistent with the hypothesis that juvenile phototaxis is an evolutionary adaptation to minimise cannibalism by adults.     

Circatidal swimming behavior of brachyuran crab zoea larvae: implications for ebb-tide transport

Larvae of the blue crab Callinectes sapidus and fiddler crab Uca pugilator are exported from estuaries and develop on the continental shelf. Previous studies have shown that the zoea-1 larvae of some crab species use selective tidal-stream transport (STST) to migrate from estuaries to coastal areas. The STST behavior of newly hatched larvae is characterized by upward vertical migration during ebb tide followed by a descent toward the bottom during flood. The objectives of the study were (1) to determine if newly hatched zoeae of U. pugilator and C. sapidus possess endogenous tidal rhythms in vertical migration that could underlie STST, (2) to determine if the rhythms persist in the absence of estuarine chemical cues, and (3) to characterize the photoresponses of zoeae to assess the impact of light on swimming behavior and vertical distribution. Ovigerous crabs with late-stage embryos were collected from June to August 2002 and maintained under constant laboratory conditions. Following hatching, swimming activity of zoeae was monitored in darkness for 72 h. U. pugilator zoeae displayed a circatidal rhythm in swimming with peaks in activity occurring near the expected times of ebb currents in the field. Conversely, C. sapidus zoeae exhibited no clear rhythmic migration patterns. When placed in a light field that simulated the underwater angular light distribution, C. sapidus larvae displayed a weak positive phototaxis at the highest light levels tested, while U. pugilator zoeae were unresponsive. Swimming behaviors and photoresponses of both species were not significantly influenced by the presence of chemical cues associated with offshore or estuarine water. These results are consistent with predictions based on species-specific differences in spawning and the proximity of hatching areas to the mouths of estuaries. U. pugilator larvae are released within estuaries near the adult habitat. Thus, ebb-phased STST behavior by zoeae is adaptive since it enhances export. Selective pressures for a tidal migration in C. sapidus larvae are likely weaker than for U. pugilator since ovigerous females migrate seaward prior to spawning and hatching occurs near inlets and in coastal waters.     

Salinity and temperature tolerance of zoeae of the portunid crab Scylla serrata

The tolerances of the first zoeal stage of the crab Scylla serrata (Forskal) have been investigated in 64 different temperature-salinity combinations. Exposure to temperatures above 25°C or to salinities below 17.5 caused considerable mortality; therefore, zoeae are unsuited to estuarine conditions. The larvae can tolerate temperatures down to 5°C is they are inactive below 10°C. It is suggested that 10°C is probably a lower limit and that female crabs which migrate to sea to release their eggs do not enter water with a temperature below 12°C. Hydrological conditions along the south-east coast of Africa indicate that females would, therefore, migrate less than 10 km offshore.     

Diel,lunar and seasonal rhythms in the reproduction of two tropical damselfishes: Pomacentrus flavicauda and P. wardi

It has been hypothesized that site-attached coral reef fishes breed during periods when their offspring will be rapidly flushed from the reef environment, but within the season when these propagules are least likely to be forever lost from the local populations. Daily, monthly and annual rhythms were observed in the reproduction between November 1978 and February 1980 of two tropical damselfishes, Pomacentrus flavicauda and P. wardi, from the southern Great Barrier Reef, Australia. These data support the first part of the hypothesis, but not the explanation for seasonality. When breeding, females of both species spawned demersal eggs into the care of males during a brief period at first light. After several days, larvae hatched from these eggs during another brief period after sunset. An experiment confirmed that the change from light to dark triggered the hatchings and that the emerging larvae were photopositive. Eggs of P. flavicauda usually hatched on the evening of their fourth day in the nest; eggs of P. wardi hatched on their fifth day. A small proportion of eggs required longer incubation. Hatching success was 90%. When breeding, both species spawned in bi-weekly cycles synchronised with the phases of the moon. The largest clutches, as well as the most clutches, were laid at the peaks of these cycles. In the case of P. flavicauda, maximum hatchings occurred on days during which spring high tides fell near sunset (the cue for hatching to begin). It is likely that these hatchlings were rapidly flushed off the reef by the ebbing tides. The hatching cycles of P. wardi consistently peaked 4 d earlier in the lunar month. Consequently, hatchlings of this species remained in shallow water at least one tidal cycle longer than those of P. flavicauda. This difference may be a consequence of the different habitat specialisations of the two species. Both species bred seasonally during the warmest, calmest months of the year. This is not the period of least current flow on the Great Barrier Reef. The observed seasonality may reflect the best times for larval survival in the open sea rather than an adaptation that limits the loss of offspring from the parent populations.     

A response-surface approach to the combined effects of temperature and salinity on the larval development of Adula californiensis (Pelecypoda: Mytilidae). I. Survival and growth of three and fifteen-day old larvae

Laboratory experiments of a factorial design were used to examine the combined effects of temperature and salinity on the survival and growth of early and late-stage larvae of Adula californiensis (Phillippi, 1847). Response-surface curves were generated to predict optimal conditions for survival and growth in order to better understand the successful recruitment of this species within the Yaquina Bay estuary (Oregon, USA). Three-day old cultured larvae were more sensitive to reduced salinity than were 15-day old larvae. However, the 15-day old larvae showed a narrower temperature tolerance than the 3-day old larvae. A. californiensis larvae survived over a wider range of temperatures near optimum salinities than at salinities near their lower tolerance limit, and conversely. Temperature and salinity ranges for maximum survival (10° to 15°C, 31 to 33) were narrower than the ranges which occur within the estuary where the adult populations exist. Larval size did not increase markedly during the 15-day rearing period, and was not greatly affected by temperature or salinity. No statistically significant temperature-salinity interaction was found for either survival or growth.     

The zooplankton of the inshore waters of Dar es Salaam (Tanzania,S. E. Africa) with observations on reactions to artificial light

From 18th December, 1968 to 5th January, 1970, zooplankton samples were taken in darkness and using artificial light in selected areas of the Dar es Salaam coast (Tanzania, S. E. Africa). Surface sea temperature was measured on most occasions, salinity for the first 7 months only. The neritic waters of Dar es Salaam experience a warm period during January to March and a cool period during July to September. The annual salinity cycle is not known. Thirtysix dark and 17 light zooplankton samples were analysed; where possible, organisms were identified to species, others to generic or higher taxonomic level. The principal taxa and their mean percentage proportions (figures in parentheses) in the dark samples were: Calanoida (49. 1), Larvacea (11. 9), Corycaeus spp. (6.4), Cypridina sinuosa (5.6), Oithona spp. (4.8), caridean larvae (4.0), Sagitta spp. (3.8), Euterpina (2.1), Lucifer (1.2), Oncea (1.2), calyptopis larvae (1.0), Hydromedusae (1.0), Euconchoecia chierchiae (1.0), Creseis acicula (1.0), brachyuran zoeae (0.8), Ctenophora (0.5), Mysidacea (0.5), fish eggs (0.5), postlarval bivalves (0.5), postlarval gastropods (0.5), Cumacea (0.1), Gammaridea (0.1) and Hyperiidea (0.1). Evadne tergestina and Thalia democratica abound in waters at certain times of the year only and then virtually disappear. The remaining groups were numerically unimportant most of the time. Almost every major group showed an annual cycle of abundance; greater numbers were recorded either for the entire or part of the period February to August, compared to the period September to November. From the end of January to early August, 1969, the average numbers per haul of total zooplankton were about three times greater than for the period mid-August to mid-November. During February-March and July-August, several oceanic indicators were observed together in the neritic waters. Artificial light induced the following changes in night zooplankton: Mysidacea, Leptochela sp., Hyperiidea, Cypridina sinuosa, brachyuran megalopae and fish larvae were attracted towards artificial light and aggregated densely under the lamp; Calanoida, Corycaeus spp., Macrosetella, Microsetella and Euterpina avoided regions of strong illumination and aggregated in dim-lit areas; Lucifer, Creseis acicula and postlarval gastropods were more abundant in dim-lit samples compared to dark hauls; caridean larvae, brachyuran zoeae and larvaceans were less abundant in high-intensity light samples compared to dark hauls on the same nights.This work was performed at the Department of Zoology, University of Dar es Salaam, Tanzania, S.E. Africa.     

Larval development in the Antarctic nemertean Parborlasia corrugatus (Heteronemertea: Lineidae)

Embryonic and larval development were followed from fertilisation to settlement in the Antarctic heteronemertean Parborlasia corrugatus (McIntosh, 1876). The first cleavage occurred 10 to 15 h after fertilisation, and the second at 17 h. Larvae hatched at the gastrula stage, between 170 and 200 h post-fertilisation, and were 150 m in diameter. Early larval stages aggregated in dense groups near the surface of incubation vessels and were positively phototactic. Early pilidium larvae were recognisable 435 h post-fertilisation. They were 155×152 m in size, and possessed a complete apical tuft of cilia and a full marginal band of locomotory cilia. At this stage, the gust was visible through the body wall, and the mouth was open and was 40 m in diameter. Late pilidia, 222×193 m in size, were helmet-shaped. They had an apical tuft over 100 m long, and possessed a lobed marginal band of locomotory cilia. Pilidia were observed aggregating close to the bottom of incubation vessels 1200 to 1350 h (50 to 56 d) after fertilisation, and this was interpreted as settlement behaviour. At this stage, the apical tuft had been lost and they were highly contractile, being capable of compressing their bodies. However, neither developing juveniles within the larval envelope nor hatched juveniles were observed. Pilidia consumed the microalgae Tetraselmis suecica, Thalassiosira pseudonana and Isochrysis galbana. They also fed on particulate organic material < 1 m in size, as shown by the presence of material in the guts of larvae offered filtered extracts of algal cultures. There was some indication that larvae could use dissolved organic material, since pilidia held in seawater with organic material removed did not survive as long as those in filtered seawater or in filtered water with added amino acids. However, the only larvae to exhibit settlement behaviour in the feeding experiments were those offered Tetraselmis succica and Thalassiosira pseudonana, and these required a longer development time to reach this stage than pilidia in the standard cultures, where a mixed algal diet was offered.     

A response-surface approach to the combined effects of temperature and salinity on the larval development of Adula californiensis (Pelecypoda: Mytilidae). II. Long-term Larval survival and growth in relation to respiration

Larvae of the bivalve molluso Adula californiensis (Phillippi, 1847) were reared for 3 days, from fertilization to veliger stage, at optimum conditions (15°C, 32.2 S), and then transferred to experimental temperatures and salinities for 22 more days to determine the effects of these factors on survival and growth. For larvae surviving to 25 days, maximum survival was estimated, by response-surface techniques, to occur at temperatures below 10°C and at salinities above 25. A comparison of 60% survival response contours for 3, 15 and 25-day old larvae indicated a progressive shift in temperature and salinity tolerance with age of larvae. The older larvae became more tolerant to reduced salinity, but less tolerant to high temperatures. Growth of the larvae over 25 days of culture was slight, and relatively independent of temperature and salinity conditions found in the environment. Oxygen consumption of 3-day old veliger larvae measured at various combinations of temperature and salinity generally increased from 7° to 18°C, and then sharply decreased from 18° to 21°C. A plateau of oxygen consumption from 9° to 15°C at 32.9 S indicated that the larvae are adapted to oceanic rather than estuarine conditions. A comparison of 25-day larval survival, mean length, and growth, with oxygen consumption of 3-day old veliger larvae indicated that high temperatures (15°C, and above) coupled with reduced salinities (26.1, and below) were unfavorable for prolonged larval life. Because of the lack of larval adaptations to estuarine conditions, larva survival and, hence, successful recruitment of this species within Yaquina Bay (Oregon, USA) depends upon the essentially oceanic conditions found only during the summer in the lower part of the Bay.     

Laboratory rearing of the clupeid fish Harengula pensacolae from fertilized eggs

Pelagic eggs of the scaled sardine Harengula pensacolae (Goode and Bean), have been hatched and reared in the laboratory for the first time. Larvae were reared in two 75 l aquaria under constant illumination, at an average temperature of 26.2°C. Zooplankton collected in a 35 mesh net was fed to the newly hatched larvae, and the diet was supplemented later with Artemia salina nauplii and a pelleted food. Larvae hatched at 4 mm TL (total length), and metamorphosed about 25 days later at 25 to 30 mm TL. Survivors averaged 76 mm TL 100 days after hatching. Of the 500 incubated eggs, 2.8% survived until 20 days, after which no significant natural mortality occurred. Sources of natural mortality included starvation, a copepod parasite (Caligus sp.), and injuries from contact with the sides of the tank. Larvae began feeding at 4.5 mm TL on copepod nauplii averaging 62 in body width. Scaled sardines were photopositive throughout the larval stage.Contribution No. 149, Bureau of Commercial Fisheries Tropical Atlantic Biological Laboratory, Miami, Florida 33149, USA.     

A dispersal model for larvae of the deep sea red crab Geryon quinquedens based upon behavioral regulation of vertical migration in the hatching stage

Behavioral responses to gravity, hydrostatic pressure, and thermoclines are described for Stage I zoeae of the deep sea red crab Geryon quinquedens Smith. Survival and rate of development as a function of temperature is presented for all larval stages. Although temperatures between 10° and 25°C have no direct effect upon survival, development time is five times longer at 10°C than at 25°C. Stage I larvae show strong negative response to gravity. Swimming rate increases with an increase in pressure up to 20 atm above ambient at 11°C, but not at 15°C. Swimming rates at 15°C are higher than those measured at 11°C at each pressure tested. Stage I larvae readily penetrate sharp thermoclines. Potential dispersal ranges of G. quinquedens larvae in the Mid-Atlantic Bight are suggested based on larval behavior, development time, and coastal hydrography. A testable recruitment model is proposed for G. quinquedens.Contribution no. 1365 of the Center for Environmental and Estuarine Studies     

Larval mortality during export to the sea in the fiddler crab Uca minax

Dense populations of the fiddler crab Uca minax (Le Conte 1855) are common along tidally influenced freshwater rivers and streams >50 km from the sea. Adults do not migrate from inland sites to release larvae, but instead release them directly into an environment where the zoeae cannot survive. Laboratory salinity tolerance experiments were used to determine how long larvae from the inland-most population of U. minax along the Pee Dee River, South Carolina, USA can survive zero salinity compared to larvae from a brackish water population (salinity 5) near the mouth of Winyah Bay in the same estuary. Larvae from the brackish water population were also exposed to a salinity of 5 and their survival tracked. These experiments were conducted from May to August 2004 and 2005. To determine if inland larvae suffered significant mortality in transit due to salinity stress, current profiles were measured in the field and used to model the time taken by a larva using ebb-tide transport to travel to permissive salinities. The laboratory tolerance experiments showed that larvae from the inland freshwater population had LT50’s of 4–5 days at 0 salinity, which were significantly longer than those of the brackish water zoeae (2–3 days). Zoeae from the brackish water population survived for at least one larval molt at a salinity of 5 with LT 50’s of ∼12 days. Estimated travel times to reach permissive salinities from the inland-most population based on current profiles were 3–5 days for larvae using night-time only ebb-tide transport and 1.5–2.5 days for those using ebb-tide transport both day and night. Previously published field data indicate that U. minax larvae do use both day- and night-time ebb-tide transport, and are found in high densities in the water column during the day. These results lead to the conclusion that U. minax stage I zoeae do not undergo significant salinity-induced mortality during their 50+ km trip to the sea.     

Synergistic effects of cadmium and salinity combined with constant and cycling temperatures on the larval development of two estuarine crab species

The developmental stages from megalopa to third crab of the blue crab Callinectes sapidus Rathbun were tested in 12 combinations of cadmium (0, 50, and 150 ppb) and salinity (10, 20, 30, and 40) at 25°C. A reduction in survival and a significant delay in development from megalopa to third crab occurred within each salinity regime in 50 ppb compared with the control. Comparison of the delay in development within each salinity regime revealed that the sublethal effect of cadmium was most pronounced in the salinities normally preferred by C. sapidus. A similar comparison within each cadmium concentration, however, showed that the developmental time from megalopa to third crab was approximately the same irrespective of salinity. The developmental stages from hatch to first crab of the mud-crab Rhithropanopeus harrisii (Gould) were examined in 63 combinations of cadmium (0, 50, and 150 ppb), salinity (10, 20, and 30), constant temperature (20°, 25°, 30°, and 35°C) and cycling temperature (20° to 25°C, 25° to 30°C, and 30° to 35°C). The results indicated that cycling temperatures may have a stimulating effect on survival of the larvae compared to constant temperatures, both in the presence and in the absence of cadmium. Effects of cadmium and salinity and their interaction on the survival of the larvae from zoeae to megalopa were documented at most of the temperatures by analyses of variance. The zoeal larvae were more susceptible to cadmium than the megalopa. Effects of different combinations of cadmium and salinity on the duration of larval development were assessed by a t-test.     

Prey selection by larval weakfish (Cynoscion regalis): the effects of prey size,speed, and abundance

We examined feeding by larval weakfish, Cynoscion regalis (Bloch and Schneider), in laboratory experiments conducted during the 1991 spawning season. under natural conditions weakfish larval development is ca. 3 wk, and we ran separate experiments with larvae of five different ages (5, 8, 11, 14, and 17 d post-hatching). We used two different size classes of rotifers (Brachionus plicatilis) and brine shrimp nauplii (Artemia sp.) as prey organisms. Contrary to results of previous research, weakfish larvae did not select prey based on size alone. When prey abundance was above 100 itemsl^-1 weakfish, larvae always chose large rotifers (length = 216 m) over small rotifers (length = 160 m). At 11 d post-hatching, larvae switched their diet from large rotifers to small brine shrimp nauplii (length = 449 m); however, when fed small rotifers and small brine shrimp nauplii the change in diet occurred at 14 d post-hatching. This pattern of selectivity was maintained in each larval age class. Early-stage larvae (5 and 8 d post-hatching) did not feed selectively when prey abundance was less than 100 itemsl^-1. Late-stage larvae (17 d post-hatching) fed selectively at abundances ranging from 10 to 10000 items^-1. Lwimming speeds of prey items, which ranged from 1 to 6 mms^-1, had no consistent effect on prey selection. These results suggest that weakfish larvae are able to feed selectively, that selectivity changes as larvae age, and that selectivity is also influenced by prey abundance.