Behavioral responses of a larval crustacean to hydrostatic pressure: Rhithropanopeus harrisii (Brachyura: Xanthidae)

Responses of the four zoeal stages of the crab Rhithropanopeus harrisii (Gould) to step and continuous changes in hydrostatic pressure were analyzed with a video system. Crabs were collected from the Neuse River estuary (North Carolina, USA) from June to August, 1987. The lower thresholds for step increases and decreases in pressure were 3 and 8 to 10 mbar, respectively. There was little change in sensitivity with zoeal development. Tests of larval responses in a light field that simulated the underwater angular light distribution indicated positive phototaxis does not occur upon pressure changes. In darkness, rates of pressure increase at and above 0.175 mbar s^-1 induced high barokinesis and negative geotaxis in all but Stage IV zoeae, which had a threshold of 1.19 mbar s^-1. Since larval sinking and descent swimming speeds exceed these threshold rates, larvae can move rapidly enough to produce suprathreshold changes in pressure which evoke behavioral responses. Slow rates of pressure decrease induced passive sinking while rapid rates caused an active ascent. This ascent response upon a pressure decrease is unreported among crustaceans, and is hypothesized to function for avoidance of feeding and respiratory currents of benthic invertebrates. The descent response occurs in all zoeal stages, except IV, at rates of pressure decrease (0.4 to 0.71 mbar s^-1) that are within the range of ascent swimming speeds. These results support Sulkin's negative feedback depth regulation model. The absolute distances moved before corrective vertical responses to threshold rates of pressure change are initated delimit the depth regulatory window. In darkness, the asymmetry of the window would lead to an ascent. It is hypothesized that light is an additional component in depth regulation, and that the limits and symmetry of the depth regulatory window may be controlled by the level of light adaptation.     

Entrainment of the larval release rhythm of the crab Rhithropanopeus harrisii (Brachyura: Xanthidae) by cycles in salinity change

Influences of salinity, Na, K, Ca and Mg on Na–K-ATPase activity in the posterior gills of Carcinus maenas (L.) have been investigated with respect to the role of the enzyme in hyperosmotic regulation. K and Mg ions were obligatory for enzyme activity. The dependence on the K concentration can be seen in a saturation curve of the Michaelis-Menten type. Low concentrations of Ca (0.2–3 mM) in the incubation medium strongly inhibited Na–K-ATPase activity. Activities inhibited by Ca could be reactivated to non-inhibited values by the addition of higher amounts of Mg (25 mM). Activity increased along with the salinity of the sea water used as incubation medium up to about 10 S. Here, maximum activity was observed. Further salinity increases of the incubating sea water were inhibitory. Salinity dependence is assumed to be based on Na dependence of the Na-pump. Comparative investigations of the Na–K-ATPase activity and its affinity to sodium in five species of decapod crustaceans indicated that levels of Na–K-ATPase differed in the posterior gills of stenohaline and euryhaline species. The results obtained confirm previous assumptions of a central role of the branchial Na–K-ATPase in hyperosmotic regulation. Properties of the Na–K-ATPase, such as affinity for substrates or dependence on ionic sea water constituents, are kept constant with respect to salinity changes. Modifications due to salinity only concern enzyme amounts especially in the posterior gills. The finding that the Na-pump is localized in basolateral parts of ion-transporting epithelial cells confirms the aforementioned results.     

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.     

The effect of salinity and cyclic temperature on larval development of the mud-crab Rhithropanopeus harrisii (Brachyura: Xanthidae) reared in the laboratory

Larvae of Rhithropanopeus harrisii (Gould) were reared from hatching to the first or second crab stages in 11 combinations of salinities and cyclic temperatures (5, 20, and 35 S at 20° to 25°C, 25° to 30°C, and 30° to 35°C; 25 S at 20° to 25°C and 30° to 35°C). The larvae survived to the megalops and first crab stages in all salinities and cycles of temperature other than 5 S at 30° to 35°C. The best survival to the megalops (94%) and first crab (90%) stages occurred in 20 S, 20° to 25°C. In all other combinations of salinities and temperatures there was a reduction in survival to the first crab stage. The duration of the larval stages was affected significantly by temperature, whereas the effect of salinity on the mean days from hatching to the first crab stage was not consistent at the different temperature cycles. Development to the first crab stage required the shortest time in 20 S, 30° to 35°C (mean 12.3 days), and the longest time in 5 and 35 S, 20° to 25°C (mean 22.6 days and 21.6 days, respectively). Megalops larvae reared in 35 S at all cycles of temperature, as well as larvae in 20 and 25 S, 30° to 35°C, showed a high percentage of abnormality, with the highest percentage occurring in 35 S, 30° to 35°C. It appears that larval development of R. harrisii is strongly influenced by environmental factors and not solely related to genetic differences.This research was supported by grants from the Nordic Council for Marine Biology and the U.S. Atomic Energy Commission [Grant No. At-(40-1)-4377].Contribution No. 116, Zoological Museum, University of Oslo, Norway.     

Absence of sterol synthesis in larvae of the mud crab Rhithropanopeus harrisii and of the spider crab Libinia emarginata

A dietary requirement for sterols in crustacean larvae was established by the use of isotopic tracer techniques. Larvae of the mud crab Rhithropanopeus harrisii and larvae of the spider crab Libinia emarginata were exposed to acetate-¹⁴C or mevalonate-¹⁴C. Radioanalysis of the fatty acids of each species indicated sufficient incorporation of acetate for lipid synthesis. No radioactivity was detected in the sterols of animals exposed to either acetate-¹⁴C or mevalonate-¹⁴C. It was concluded that R. harrisii and L. emarginata larvae are unable to synthesize sterols from acetate or mevalonate and, therefore, require them in their diet.     

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.     

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.     

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.     

Interactions between temperature and salinity during brooding on subsequent zoeal development of the mud crab Rhithropanopeus harrisii

Female mud crabs, Rhithropanopeus harrisii, carrying newly extruded eggs, were collected from the Petaluma River (San Francisco Bay Estuarine System, California, USA) in summer 1985, and exposed to factorial combinations of temperature (20°, 25° or 30°C) and salinity (2, 5, 15, 25, or 32%.). Upon hatching, dry weights of 12 to 15 h-old zoeae were determined. Subgroups of the remaining zoeae were transferred from hatching salinities to the salinities listed above and raised until metamorphosis to megalopa. Low salinities reduced zoeal dry weights by as much as 25%. Temperature played a secondary role in reduction of hatching weight of zoeae. Survival of larvae through zoeal development was best when hatching and rearing salinities were the same; in this case, overall survival increased with temperature. Both duration of zoeal development and megalopal dry weights were strongly influenced by temperature and rearing salinity, with only a small contribution from hatching salinity. The influence of hatching salinity was most obvious at extremes of the range tested. These studies indicate that physical conditions during embryogenesis profoundly influence subsequent larval development. Interpretation of experimental approaches to study ecophysiological adaptations of larval stages should not neglect the role of physical conditions during embryogenesis.     

Larval development of Pilumnoides perlatus (Brachyura: Xanthidae) under laboratory conditions

Larvae of the xanthid crab Pilumnoides perlatus (Poeppig, 1836) have been reared in the laboratory at 3 different temperatures (10.2°, 15° and 20°C) from hatching to megalopa stage. The 5 zoea stages and the megalopa, as well as the setation of the functional appendages are described and illustrated. The main characteristics useful to differentiate the larval stages of P. perlatus from those of Homalaspis plana, the other Chilean species of the same family so far reared, are discussed. Data on duration of zoea development, length of moulting intervals, and mortality at the 3 test temperatures are also given.This study was partially supported by the Chilean National Commission for Scientific and Technological Research (CONICYT).     

Measuring the effects of thermoclines on the vertical migration of larvae of Callinectes sapidus (Brachyura: Portunidae) in the laboratory

An experimental system is described that is capable of maintaining precise and stable thermoclines of specified magnitude for laboratory study of behavior of planktonic organisms. Behavioral responses to encounters with thermoclines of various magnitudes were measured in stage I larvae of the portunid crab Callinectes sapidus Rathbun. Larvae adapted to an initial temperature of 15°C were able to swim up through a temperature gradient of 10°C, but showed increasing inhibition of upward migration when t=12.5°C. Larvae adapted to 17.5°C were unable to penetrate t=10°C. Thermoclines of 5° or 10°C slowed passive sinking rates. Results suggest that larvae avoid an upper temperature of 27.5°C by a cessation of swimming, accompanied by passive sinking. Thermal gradients of the magnitude present in the natural habitat are unlikely to alter larval vertical migration that is regulated by other exogenous stimuli. Naturally occurring thermoclines should seldom influence dispersal characteristics of C. sapidus larvae.Contribution No. 1535 of the Center for Environmental and Estuarine Studies, University of Maryland     

Differences in responses to factorial combinations of temperature and salinity by zoeae from two geographically isolated populations of the mud crab Rhithropanopeus harrisii

Ovigerous mud crabs, Rhithropanopeus harrisii, were collected from the Petaluma River (San Francisco Bay Estuarine System, California, USA) and from Sykes Creek (Indian River Lagoon System, Florida, USA) during the summer of 1984 and during February 1985, respectively. Their zoeae were reared in factorial combinations of temperature (20°, 25° or 30°C) and salinity (2, 5, 10, 15, 20, 25, or 30%.). Survival and megalopal dry weight were maximal over a far larger range of temperature-salinity combinations for the Florida population. Absolute values of the two parameters were also greater for this group. Temperature dominated effects on duration of zoeal development in both populations. California zoeae developed more slowly at any of the temperatures tested compared with those from Florida. The pattern of all three indices was markedly different under non-optimal conditions. Putatively adaptive modification of survival, development rate and growth of zoeae is evident in response to prevailing environmental conditions which are, in part, a function of latitudinal position. Even though populations in the Petaluma River, California, are less capable of reaching maximal performance under the prevailing physical regimes than the Florida population, they still can live in habitats where physical conditions exclude competitors and predators.     

Action of eyestalks on the ovary in Rhithropanopeus harrisii and Crangon crangon (Crustacea: Decapoda)

The annual cycle of ovarian activity and eyestalk effects on the ovary in different phases of its activity were examined in the crab Rhithropanopeus harrisii Gould ssp. tridentatus Maitland, and the sand shrimp Crangon crangon L. In both species, the annual ovarian cycle consists of two phases of vitellogenesis and the resting phase. In R. harrisii, the eyestalks exert an inhibitory effect on vitellogenesis during the phase of vitellogenesis outside the breeding season, and are necessary for the process of oviposition during the breeding season. In C. crangon, ovary-inhibiting hormone is present in the eyestalks outside the breeding season, and has not been detected during this season. The resting phase of the ovary after the breeding season is caused by the ovary-inhibitiing hormone. Females of C. crangon are a suitable material for testing ovary-inhibiting hormone activity, while female R. harrisii are not.     

Diel vertical migration and timing of metamorphosis of larvae of the Dungeness crab Cancer magister

Joint USA/USSR ichthyoplankton surveys off the coasts of Washington, Oregon and northern California during the years 1981 to 1985 sampled more than 120 stations each year, from 5 to 360 km offshore and between Latitude 40° and 48° N, providing information on ontogeny and diel migration of larvae of the Dungeness crab Cancer magister on a scale not studied previously. We developed a maximum likelihood method for estimating abundance and fraction in the neuston at each station from a neuston tow and an oblique bongo tow. Latestage megalopae migrate vertically on a diel basis, with the fraction in the neuston being (on average) 62% at night (19.00 to 08.00 hrs Pacific Standard Time, PST) and 8% during the day (08.00 to 19.00 hrs PST). The hourly pattern of this migration includes a peak in the early evening, possibly another in the early morning, and an intermediate level in the late afternoon. We detected no dependence of vertical migration on cloud cover or sea state. Early-stage megalopae were present in much lower fractions in the neuston, but weakly displayed the same diel pattern of migration. Zoeae appeared to be below the neuston at all times, except for 2 or 3 h in the evening. From an abrupt change in larval stage in samples from a north-south cruise, we concluded that the majority of the larvae metamorphose from zoeae to megalopae over a fairly short time span (2 to 4 wk) at a given latitude. In later cruises, 95% of the larvae were megalopae, indicating that metamorphosis over the study area either occurs at the same time or proceeds from south to north over a time span of less than a month in early spring.     

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.     

Population genetics of eight species of Trapezia (Brachyura: Xanthidae), symbionts of corals

Starch-gel electrophoresis was used to study gene-enzyme variation in thirteen populations of eight species of the genus Trapezia from Hawaii, Panamá, and Enewetak Atoll (Marshall Islands). Between 20 and 30 (mean = 27.8) gene-enzyme systems were resolved in each population, with 20 systems in common among all populations. The distribution of electrophenotypes was in agreement with Hardy-Weinberg-Castle expectations, except for T. digitalis, which consistently showed heterozygote deficiencies. Diagnostic loci among color forms support the hypothesis that color forms are distinct species. Low values of genetic distance among species suggest a recent radiation, perhaps during the Pleistocene. Genetic distance between the Hawaiian and Panamanian populations of T. ferruginea did not significantly differ from zero, indicating that the Eastern Pacific population of T. ferruginea has recently immigrated from the central Pacific, and/or that there is gene flow between the two areas. There were diagnostic loci between T. ferruginea and T. formosa from Enewetak and the populations of these species from Hawaii (T. ferruginea only) and Panamá (both species). Therefore, these geographic populations may represent separate species. The level and pattern of genetic variability in Trapezia spp. are in agreement with those observed in most other organisms.     

Biochemical changes during larval development of the xanthid crab Rhithropanopeus harrisii. I. Protein,total lipid,alkaline phosphatase,and glutamic oxaloacetic transaminase

Protein, total lipid, and the activities of the enzymes alkaline phosphatase (AP) and glutamic oxaloacetic transaminase (GOT) were measured daily during larval development of the xanthid crab Rhithropanopeus harrisii (Gould). Percent increase in fresh weight was greatest on the first day after each molt. Protein and total lipid per larva increased during the second, third, and fourth zoeal stages, with the most marked increase during the third. When calculated as percent fresh weight, marked accumulation of both protein and total lipid occurred during the third zoeal stage. GOT activity remained stable during the third zoeal stage, but increased significantly during the fourth zoeal stage. AP showed a 4-day cycle of activity apparently related to the molt cycle. It is suggested that increased protein and total lipid content during the third zoeal stage indicate nutritional requirements prior to metamorphosis. Biochemical change may be a more relevant indicator of growth during larval development than either size increase or molt frequency when assessing the value of various diets in the culture of crabs.     

Developmental changes in body weight and elemental composition of laboratory-reared larvae of the red frog crab,Ranina ranina (Decapoda: Brachyura)

Ranina ranina larvae were reared at 29°C from hatching to the megalopa stage to measure daily changes in body weight, water content and elemental composition. Energy, estimated from carbon content, was also examined. The water content was 85 to 92% of body weight immediately after ecdysis but decreased with days after ecdysis. Gains in body weight, carbon, nitrogen, hydrogen and energy during each instar ranged from 52 to 245% and increased with instar after instar II (body weight and carbon), instar III (hydrogen and energy), and instar IV (nitrogen). Cumulative gains of these elements from hatching to 2 d before metamorphosis into megalopa ranged from 11 567% (carbon) to 12 209% (energy). Most cumulative gains (57 to 59%) in elemental composition were contributed by instar VII. Carbon, nitrogen and hydrogen content in body weight decreased to a minimum on the day of ecdysis and increased on the subsequent days. C:N ratios after instar IV were lowest on the day after ecdysis and reached a plateau by the second day. Energy, estimated as J mg^-1 dry weight (DW), decreased with instar and within a molt cycle, and was at a minimum on the day after ecdysis. Gains in elemental composition could be described by an exponential function of days after hatching and by a quadratic function in each instar.     

Rhythms in larval release of the sublittoral crab Neopanope sayi and the supralittoral crab Sesarma cinereum (Decapoda: Brachyura)

Ovigerous females of the subtidal xanthid crab Neopanope sayi (Smith) and the high intertidal grapsid crab Sesarma cinereum (Bosc) were collected during the summers of 1986 and 1987 in the Beaufort, North Carolina (USA), area and brought into the laboratory, where rhythms in larval release were monitored. When crabs with late-stage embryos were put under a 14 h light:10 h dark cycle in an otherwise constant-environment room, an apparent tidal rhythm in release of larvae was observed for both species, with N. sayi releasing near the time of day and night high tides, and S. cinereum releasing around the time of night high tides. The time of sunset relative to high tide was a complicating factor, since larval release for both species was often concentrated around sunset when evening high tides fell several hours before sunset. When a group of N. sayi and S. cinereum were brought into the laboratory and placed under constant lowlevel light for 5 d, the release rhythm of the population persisted, thus implying that the rhythm is endogenous. Larval release near the time of high tide and often at night is common among brachyurans living in tidal areas, regardless of specific adult habitat, suggesting a common functional advantage. Possibilities include transport of larvae from areas where predation and the likelihood of stranding and exposure to low-salinity waters are high, as well as a reduced probability of predation on adult females. Results of the present study suggest that the importance of release after darkness may increase with increasing tidal height of the adult.     

Ontogenetic diel vertical migration of the planktonic copepodCalanus sinicus in the Inland Sea of Japan

The ontogenetic diel vertical migration of the planktonic copepodCalanus sinicus was investigated in the Inland Sea of Japan in summer 1988, when the water was thermally stratified with a thermocline of ca 5 °C between 35 and 45 m. Stage-specific differences in the diel vertical migration behavior ofC. sinicus were found. Eggs were spawned primarily within the surface-waters between midnight and dawn by ascending females, and sank gradually to deeper waters until they hatched into nauplii. Non-feeding nauplius stages (NI and II) were distributed throughout the water column, but the first feeding stage (NIII) performed an ontogenetic upward migration. NIV to VI and copepodite (C) stages I to III continuously aggregated in the phytoplankton-rich euphotic layer. However, the depth of the median CI to III populations descended as stage progressed. The onset of prominent diel vertical migration took place in CIV, and the amplitude of vertical migration increased with age, being maximal in adult females (CVI). Adult males (CVI), however, remained in the layer below 20 m, and did not migrate dielly. The ecological significance of ontogenetic diel vertical migration is discussed.