Ultrastructural study of the exoskeleton of the estuarine crab Rhithropanopeus harrisii: Effect of the insect growth regulator Dimilin® (diflubenzuron) on the formation of the larval cuticle

Ultrastructure of larval cuticle during the molt cycle of the estuarine crab Rhitropanopeus harrisii (Gould) (Crustacea: Brachyura) was studied in control larvae as well as in larvae exposed to 10 ppb of the insect growth regulator Dimilin^® (diflubenzuron). First zoeal larvae were used as test organisms. It has earlier been shown that 10 ppb Dimilin is lethal to zoeal larvae of R. harrisii, and nearly all exposed larvae died during molting to the next stage (Christiansen et al., 1978). Distinct differences in structure of the cuticle were found between the two groups of larvae. Both endocuticle and exocuticle appear to be deformed in Dimilin-treated larvae, whereas formation of epicuticle did not seem to be affected. The results indicate that Dimilin probably inhibits chitin synthesis in crab larvae as shown earlier by several authors for insect larvae.     

Effects of the insect growth regulator Dimilin® (TH 6040) on larval development of two estuarine crabs

Effects of Dimilin^® (TH 6040), an insect growth regulator which interferes with the formation of the insect cuticle, were studied on the larval development of Rhithropanopeus harrisii (Gould) and Sesarma reticulatum (Say) (Crustacea: Brachyura). When larvae were exposed to 0.5 (R. harrisii only), 1, 3, 5, 7, and 10 ppb Dimilin from hatching to the first crab stage, survival in both species decreased in relation to increased concentrations of Dimilin. Survival of R. harrisii larvae wa significantly lower at 1 ppb and higher levels compared with control experiments, and in S. reticulatum a significant decrease in survival began at the 3 ppb level. At 10 ppb Dimilin, no larvae survived to the megalopa stage in either of the two species. The results indicate that early stage larvae of R. harrisii are more sensitive to Dimilin than those of S. reticulatum. When R. harrisii larvae were treated with 10 ppb Dimilin during the intermolt period of each of the 4 zoeal stages, nearly all larvae died during molting to the succeeding stage. First zoeal larvae of R. harrisii exposed to 10 ppb Dimilin at various days during the intermolt period were more sensitive to the compound late than early in the period. It is suggested that Dimilin also may interfere with the formation of the cuticle in crab larvae.     

Dieldrin uptake by larvae of the crab Leptodius floridanus

The rate of uptake of ¹⁴C-dieldrin by crab larvae (Leptodius floridanus) from 0.5 ppb in seawater and from 213 ppb (dry weight) in their food was measured. It was found that, if equal concentrations of dieldrin were available to the larvae in their food and in seawater, the animals would accumulate the pesticide about 8000 times as fast from the water as from the food.     

Comparative vulnerability to predators,and induced defense responses,of eastern oysters Crassostrea virginica and non-native Crassostrea ariakensis oysters in Chesapeake Bay

Management agencies are considering introducing the Suminoe oyster Crassostrea ariakensis into Chesapeake Bay, USA. It is unknown if the growth of feral populations of this non-native oyster would be regulated by the same predators that once controlled the abundance of the native eastern oyster C. virginica. In laboratory studies, we compared the relative susceptibility of juvenile diploids (shell height < 25 mm) of both oyster species to invertebrate predators of eastern oyster juveniles. Predators included four species of mud crabs [Rhithropanopeus harrisii (carapace width 7–11 mm), Eurypanopeus depressus (6–21 mm), Dyspanopeus sayi (8–20 mm), and Panopeus herbstii (9–29 mm)], the blue crab Callinectes sapidus (35–65 mm), and two sizes of polyclad flatworms (Stylochus ellipticus and possibly Euplana gracilis; planar area ≲5 mm² and ∼14 to 88 mm²). All four species of mud crab and the blue crab preyed significantly (ANOVA, P ≤ 0.05) more on C. ariakensis than on C. virginica, but predation by flatworms of both sizes did not differ significantly between oyster species. The greater susceptibility of C. ariakensis to crab predation was likely due to its shell compression strength being 64% lower than that of C. virginica (P = 0.005). To test for predator-induced enhancement of shell strength, we held oysters of both species for 54 days in the presence of, but protected from, C. sapidus and R. harrisii. Crabs were fed congeneric oysters twice weekly within each aquarium. Compared to controls, shell strength of C. virginica exposed to R. harrisii increased significantly (P < 0.043), as did shell strength of both oyster species exposed to C. sapidus (P < 0.01). Despite the changes in shell strength by both oyster species in the presence of C. sapidus, the shell of C. ariakensis remained 57% weaker than C. virginica. We conclude that, because C. ariakensis exposed to predators continued to have a weaker shell relative to C. virginica, the natural suite of crab and flatworm predators in Chesapeake Bay will likely serve to control the abundance of feral C. ariakensis. We caution that the situation in the natural environment may be sufficiently different in some locations that C. ariakensis may be able to compensate for its greater vulnerability to crab predation and hence become a nuisance species.     

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.     

Biochemical changes during larval development of the xanthid crab Rhithropanopeus harrisii. III. Isozyme changes during ontogeny

Isozyme changes in the xanthid crab Rhithropanopeus harrisii (Gould) are described for 6 enzyme systems through all four zoeal stages, megalopa stage, and juveniles. There is considerable variation in the number of isozymes and their expression during the ontogeny of R. harrisii.Contribution No. 806 of the Chesapeake Biological Laboratory, Center for Environmental and Estuarine Studies, University of Maryland, USA.     

Responses of estuarine crab megalopae to pressure,salinity and light: Implications for flood-tide transport

The megalopal larval stage of many estuarine brachyuran crabs appears to return to adult habitats by undergoing rhythmic vertical migrations which result in saltatory up-estuary transport on flood tides. Larval ascent into the water column during rising tides may be cued by changing hydrologic variables. To test this hypothesis, we investigated the responses of field-caught megalopae of the blue crab Callinectes sapidus and the fiddler crab Uca spp. to constant rates of pressure and salinity change under laboratory conditions. For both genera, pressure changes resulted in increased movement (barokinesis) and upward migration in the test chamber, with C. sapidus megalopae having a lower response threshold (2.8×10^-2 mbar s^-1) than Uca spp. larvae (5×10^-2 mbar s^-1). Similarly, larvae ascended in response to increasing salinity, with C. sapidus larvae being more sensitive. Larvae were negatively phototactic and failed to respond to pressure increases at light levels above 1.0×10¹⁵ and 1.0×10¹³ photons m^-2 s^-1 for C. sapidus and Uca spp. megalopae, respectively. Such responses are thought to explain the low abundances of larvae in the water column during daytime flood tides. Nevertheless, threshold sensitivities to increasing pressure for both genera were above levels experienced during floodtide conditions in the field. Similarly, it is unlikely that increasing salinity is sufficient to induce ascent in Uca spp. postlarvae. However, rates of salinity increase during midflood tide typically reach levels necessary to induce an ascent in C. sapidus megalopae. These results are consistent with the hypothesis that fiddler crab megalopae utilize an endogenous activity rhythm for flood-tide transport, while blue crab megalopae rely upon external cues, especially salinity changes, to time their sojourns in the water column.     

A long-term study of the effects of water-soluble fractions of No. 2 fuel oil on the survival,development rate,and growth of the mud crab Rhithropanopeus harrisii

Mud crabs, Rhithropanopeus harrisii (Gould), were exposed continuously for 6 months after hatching to water-soluble fractions (WSF) of No. 2 fuel oil. Survival, growth and development rate were monitored during this time. The zoeal stages were the most sensitive to fuel oil. A 20% WSF (0.36 ppm total naphthalenes, 1.26 ppm total hydrocarbons) was acutely toxic to these stages. Of the zoeal stages, the first stage appeared to be the most sensitive. The combined duration of the 4 zoeal stages was significantly increased by increasing WSF exposure concentrations. The megalopa and crab stages were not particularly sensitive to continued petroleum hydrocarbon exposure, particularly when compared to zoeal stages. However, mean duration of the megalopa and first crab stages was significantly affected by oil exposure. Individuals which survived the highest exposure concentrations as larvae appeared to grow larger during the crab stages, so that at the end of 6 months comparably staged crabs were equal to or larger than both control crabs and those exposed to low WSF concentrations. Stage distributions at the end of 6 months showed no differences due to WSF exposure. Sex ratios, which could be determined at the end of 6 months, were approximately 1, indicating no sex-related differential sensitivity to WSF exposure, at least as larvae or juveniles. The data indicate that these crabs possess considerable ability to recover from the effects of chronic sublethal exposure to petroleum hydrocarbons. The most deleterious effects of oil pollution on this species may be due to its impact on larval recruitment into the adult population.     

Effects of the juvenile hormone mimic ZR-512 (Altozar®) on larval development of the mud-crab Rhithropanopeus harrisii at various cyclic temperatures

Effects of the juvenile hormone (JH) mimic hydroprene (Altozar^®: ZR-512), which exhibits high activity against Lepidoptera, were studied on the larval development of the mud-crab Rhithropanopeus harrisii (Gould) (Brachyura: Xanthidae). Larvae reared in 20 S at 3 cycles of temperature of 20° to 25°C, 25° to 30°C and 30° to 35°C, were exposed to 0.01, 0.1 and 0.5 ppm hydroprene from hatching to the first crab stage. Larvae were also exposed to 0.1 and 0.5 ppm hydroprene only from the megalopa stage to the first crab stage. When larvae were treated with hydroprene throughout larval life, survival was significantly reduced with increasing concentrations of the compound at all temperature cycles. Synergistic effect between hydroprene and temperature on survival of zoeal larvae was not observed. On the average there was 11% less survival in the zoeal stages at the 0.01 ppm concentration. of hydroprene than in the control, an additional reduction of 13% occurred at 0.1 ppm, and finally there was a further decrease of 46% at 0.5 ppm hydroprene. Significant decrease in survival in the megalopa stage occurred only in the 0.5 ppm concentration of hydroprene at the lowest temperature cycle when larvae were exposed to the compound from hatching. When larvae were treated with hydroprene only within the megalopa stage, a significant reduction in survival was not observed. First-stage zoeae were the most sensitive of the larval stages to hydroprene. Duration of zoeal development was significantly delayed at 0.5 ppm hydroprene at the two lower temperature cycles, whereas in the megalopa stage the delay began at the 0.1 ppm level at all 3 temperature cycles when larvae were exposed to hydroprene from hatching. A significant delay was also observed at 0.1 ppm hydroprene at the two lower cycles when larvae were exposed to hydroprene only in the megalopa stage; at 30° to 35°C a significant delay was observed only at the 0.5 ppm level. The results show that metamorphosis to the first crab stage was not inhibited at the 0.5 ppm level of hydroprene or lower. Reduction in survival and increase in duration of larval development were presumably related to stress conditions caused by hydroprene. The results also suggest an interaction between temperature and hydroprene on survival of megalopa larvae and duration of larval development.     

Population-related toxicity responses to two butyltin compounds by zoeae of the mud crab Rhithropanopeus harrisii

We exposed zoeae of the mud crab Rhithropanopeus harrisii to either bis(tri-n-butyltin) oxide (TBT) or di-n-butyltin dichloride (DBT). Experiments were repeated with zoeae from females collected from the Petaluma River, California in June–August 1983 and 1984 or from Sykes Creek, Florida (USA) in February 1985. Using probit analysis, we calculated LC₅₀ values for exposure lasting the duration of zoeal development. Tributyltin was 54 to 65 times more toxic than dibutyltin, the lower value characterizing the response of Florida zoeae. Increases in duration of zoeal development and reduction of dry weights of megalops, both sublethal responses, were dose-dependent for the two populations. However, zoeae from Florida consistently had shorter duration of zoeal development and higher megalopal weights at metamorphosis, indicating less sensitivity to an identical exposure to either organotin compound. The results of these experiments show that dibutyltin, a putative degradation product of tributyltin, is less toxic than the parent compound. In addition, early life-history stages of two populations may have significantly different responses to xenobiotic stress which, in the case of brachyuran larvae, is evident in a differential reduction of survival and growth and an increase in duration of zoeal development.     

Early growth and mortality of the Caribbean king crab Mithrax spinosissimus reared in the laboratory

The berried females of the Caribbean king crab Mithrax spinosissimus (Lamarck) used in this study were collected from canals on Big Pine Key, Sugarloaf Key and Lower Matecumbe Key (south Florida, USA) on 9 August, 8 October and 15 November 1986. Viable spawns hatched as first zoeae and molted to second zoeae within ca. 10 to 12 h. Most of the larvae reached the megalopa stage 1 d later, and molted to first crab 4 to 8 d after hatching (water temperature: 27.2° to 28.8°C). Low water temperature and/or early lack of food had a negative effect not only on stage duration, but also on the size of the early crab stages. Successful molt to first crabs occurred, however, in the absence of food. The growth rate (carapace length) between molts in early crab stages varied between ca. 20 and 30%. When provided with good water exchange, stocking density could be very high (>22 500 individuals m^-2), with no increase in mortality. The highest mortality rate was recorded when the larvae molted to first crab, and the highest rates of survival were always recorded when feeding was not initiated until after 5 to 8 d after hatching. No cannibalism was observed among larvae, and cannibalism was low in early crab stages. The study indicates that to achieve viable hatches and high larval survival in rearing M. spinosissimus, a continuous and adequate supply of high-quality seawater is a prerequisite both in larviculture and in maintaining brooding females.Contribution No. 93, Department of Oceanography and Ocean Engineering, Florida Institute of Technology     

Biology and ecology of the crab Rhithropanopeus harrisii ssp. tridentatus

Data are presented on the geographic and local distributions as well as on the life cycle of the crab Rhithropanopeus harrisii (Gould) ssp. tridentatus (Maitland). Extensive studies on this subspecies were carried out in its natural habitat (an abandoned arm of the Vistula, Poland), and under laboratory conditions. The effects of some environmental factors on distribution, life cycles, and abundance of the crab are discussed. The results obtained are compared with those reported by other investigators.     

Behavioral responses of larvae of the crab Rhithropanopeus harrisii (Brachyura: Xanthidae) during diel vertical migration

Larval Rhithropanopeus harrisii (Gould) show nocturnal vertical migration. Larval behavioral responses to different rates of increase and decrease in light intensity were measured in an apparatus with a natural angular light distribution. A central objective was to establish whether phototaxis actually participates in vertical migration. At sunset the level of light adaptation controlled the readiness of the larvae to migrate, while an ascent was initiated by a preductable relative decrease in intensity (e.g. 4.0x10^-3s^-1). Rates of relative decrease around sunset would evoke continuous upward swimming. Gravity was the orienting cue and there was no change in swimming speed during the ascent. At sunrise, the larval descent was initiated by exposure to an absolute light intensity of about 0.23 log unit above the lower visual threshold. Light served as an orienting cue, as larvae descended by a negative phototaxis. Thus, phototaxis is not a laboratory artifact and does participate in vertical migration. A consideration of behavioral responses of other crustacean zooplankton indicates there is considerable variation in the initiating and directing cues for vertical movements. The variety of behavioral responses of R. harrisii suggests that a synthesis of hypotheses about migration may provide the proper basis for explaining the mechanisms underlying diel vertical migration.     

Managing the offspring environment: brood care in the bromeliad crab,Metopaulias depressus

Summary Maternal modification of the larval environment was studied in the bromeliad crab, Metopaulias depressus (Decapoda, Grapsidae). The bromeliad crab breeds in rain water stored in the leaf axils of large Jamaican bromeliads. The physico-chemical water conditions are highly unfavorable for the development of the larvae. Water in nursery axils contained no leaf litter and up to 14 empty snail shells, whereas other axils were partly or completely covered with leaf litter and rarely held snail shells. Mother crabs removed leaf litter from nursery axils, and occasionally from neighboring axils. Nursery axils contained twice as much dissolved oxygen (DO₂) at night when DO₂ is at a minimum, and less dissolved carbon dioxide (DCO₂) than axils unattended by a crab. The higher DO₂ content in nurseries meets the respiratory needs of an average sized brood, but the lower concentration in axils with litter may not. It is suggested that the higher DO₂ and lower DCO₂ concentrations in the nursery axils result from a water circulation produced by the mother and her leaf litter cleaning behavior. The low pH and the Ca²⁺ content in the water of unattended bromeliad axils are unfavorable for development of larvae and young. Mother crabs significantly increased pH and Ca²⁺ by adding empty snail shells to the nursery-axil water. In experiments where the brood was transferred to other axils, mothers switched from the nursery axil to the new axil containing the brood, and there continued caring for their offspring.     

Metabolic disposition of lactate in the horseshoe crab Limulus polyphemus and the stone crab Menippe mercenaria

The metabolism of ¹⁴C-labelled lactate was investigated in the horseshoe crab Limulus polyphemus and the stone crab Menippe mercenaria. When a bolus of (¹⁴C-U)-D-lactate was injected into L. polyphemus, there was substantial release pf ¹⁴CO₂ into the medium. In the case of M. mercenaria, ¹⁴CO₂ release was also observed after injection of (¹⁴C-U)-L-lactate into experimental individuals. Analysis of the distribution of radioactivity in whole body extracts of both species revealed easily detectable amounts of radioactivity in the glycogen fraction, although the bulk of the radioactivity was in the cation, anion/neutral and CO₂ fractions. To investigate the metabolism of lactate further, ¹⁴C-labelled lactate was injected into large individuals of L. polyphemus and M. mercenaria, and the distribution of radioactivity was determined in the hemolymph, muscle and hepatopancreas. Utilization of (¹⁴C-U)-D-lactate by L. polyphemus resulted in the accumulation of significant amounts of labelled glucose in all three body compartments as well as the production of labelled glycogen in the telson levator muscle and hepatopancreas. Utilization of (¹⁴C-U)-L-lactate resulted in a similar pattern of glucose and glycogen labelling in the hemolymph, cheliped muscle and hepatopancreas of M. mercenaria. These studies demonstrate that both L. polyphemus and M. mercenaria have the capacity for glyco- and gluconeogenesis using lactate as the substrate.     

Effect of the insect growth regulator diflubenzuron (Dimilin®) on the uptake of glucose and N-acetylglucosamine into the cuticle of crab larvae

Incorporation of ³H-labelled glucose and ³H-labelled N-acetylglucosamine (NAGA) — both precursors to chitin —into the cuticle of Rhithropanopeus harrisii (Gould) larvae (Crustacea: Brachyura) has been examined at different stages of the moult cycle in control larvae as well as in larvae treated with the insect growth regulator diflubenzuron (Dimilin^®). As far as the control larvae were concerned, the incorporation of both precursors was high at the postmoult stage when endocuticle was secreted. NAGA appeared to be a more specific precursor of cuticular material than glucose during the premoult stage when exocuticle was produced. Incorporation of both precursors was low immediately before ecdysis and during the intermoult stage when secretion of the cuticle is complete. The results show that incorporation of glucose into chitin was greatly inhibited by the pollutant during the postmoult stage when endocuticle is produced, while incorporation of NAGA was reduced to a lesser extent at this stage. Diflubenzuron treatment markedly affected the incorporation of both NAGA and glucose in the premoult stage during secretion of exocuticle.     

Toxicity of the fungicide captan to the Dungeness crab Cancer magister

Captan did not affect the survival of Dungeness crab (Cancer magister Dana) zoea exposed to 30 g l^-1 during a chronic toxicity test lasting 69 days, but larvae were quickly killed (mean survival time = 9 days) in the same test by exposure to 450 g l^-1 of the fungicide. Delay of molting occurred, however, for later stages at 30 g l^-1. Survival of juvenile crabs was not reduced by exposure to captan for 36 days at 510 g l^-1 or, in a second test, for 80 days at 290 g l^-1. No deaths of adults exposed for 75 days to 340 g l^-1 of captan were observed. Captan appeared to accelerate hatching of eggs at all concentrations tested from 100 to 10,000 g l^-1. The development from prezoeae during a 24-h period was not inhibited by the fungicide, but at 3,300 and 10,00 g l^-1, the two highest concentrations tested, developing zoeae exhibited a morphological deformity and were largely inactive. Under the prevailing conditions in the toxicity tests, the half-life of captan was estimated to be from 23 to 54 h. Because of the relatively low toxicity of captan to crab stages and its high rate of degradation in sewater, it is suggested that the agricultural application of captan near marine waters is not likely to affect natural crab populations or crabs in laboratory culture. Further-more, the prophylactic use of captan as a fungicidal treatment for Lagenidium sp. in larval crab cultures is considered safe when used at recommended dosages.Technical Paper No. 4131, Oregon Agricultural Experiment Station.     

Effects of the juvenile hormone mimic ZR-515 (Altosid®) on larval development of the mud-crab Rhithropanopeus harrisii in various salinities and cyclic temperatures

Effects of 0.01, 0.1 and 1.0 ppm methoprene (Altosid^®: ZR-515), a juvenile hormone (JH) mimic which shows high activity against some economically important insect pests, especially Diptera, were tested on larvae of the mud-crab Rhithropanopeus harrisii (Gould) (Brachyura: Xanthidae) from hatching to the first crab stage under optimum and stress conditions of a number of salinities and cyclic temperatures. There was a significant reduction in survival of zoeal larvae with increasing concentrations of methoprene in nearly all combinations of salinity and temperature. On the average there was 9% less survival in the 0.01 ppm concentration of methoprene than in the control, and in the 0.1 ppm concentration the survival was further reduced by another 16%. At 1.0 ppm methoprene no larvae survived beyond the first zoeal stage under optimum conditions or under stressful combinations of salinity and temperature. Except at 0.2 ppm in 27.5% S, survival of the megalopa was not significantly reduced in 0.01 or 0.1 ppm methoprene in any salinity or temperature, although the percentage of abnormal megalopa increased under stress conditions. The first zoeal stage was the most sensitive of the larval stages to methoprene as well as to salinity and temperature stress. The duration of zoeal development was significantly lengthened with an increase in concentration of methoprene under nearly all conditions of salinity and temperature. The JH mimic had, however, no significant effect on the duration of megalopa development. A significant synergism between methoprene, salinity and temperature was not observed. It can be concluded from the results that methoprene does not inhibit metamophosis of R. harrisii larvae at the 0.1 ppm level or lower. Reduction in survival of zoeal stages and increased duration of zoeal development with increasing concentrations of methoprene are presumably related to stress.     

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.     

The ontogeny of phototaxis by larvae of the crab Rhithropanopeus harrisii

Phototaxis by each zoeal stage of the crab Rhithropanopeus harrisii (Gould) was quantitatively measured by means of a microscope closed-circuit television system. The megalopa stage is indifferent to light stimulation and was thus not tested. The action spectrum for positive phototaxis is similar at each zoeal stage, having the most pronounced maximum at about 500 nm and smaller maxima at 400 and 280 nm. Responsiveness to various intensities of 500 nm light is also similar at each stage. After dark-adaptation strongest positive phototaxis occurs at intensities between 1.0 and 3.0x10^-4 W/m², with no negative phototaxis to lower intensities. After light-adaptation, the positive response occurs to higher intensities between 20 and 0.01 W/m², with a pronounced negative response to lower intensities. Generally, mean swimming speeds during positive phototaxis do not change with stimulation intensity, although during negative phototaxis mean speeds do vary with intensity. At the highest intensities which initiate a negative response, mean values are significantly greater than values for positive phototaxis. Based on the pattern of phototaxis at each developmental stage, a prediction of events during diurnal vertical migration is possible. The negative response after light-adaptation might also function as a shadow reflex.