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.     

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     

Temperature preference and tolerance,and oxygen consumption of the marbled rockfish,Sebastiscus marmoratus

Behavioral and metabolic responses of the marbled rockfish, Sebastiscus marmoratus (Cuvier), to temperature were measured to define optimal thermal habitat. Preferred temperature was determined by means of a newly developed horizontal temperature-gradient tank. Acclimation temperature had a direct positive effect on critical thermal maxima and minima and upper lethal temperatures but no effect on final preferred temperature. It was indicated that upper temperature tolerance and final preferred temperature of the marbled rockfish were closely connected. Oxygen consumption rate increased with temperature to 23°C but Q ₁₀ (the increase in rate caused by 10°C increase in temperature) declined above 20°C. The maximum Q ₁₀ (4.69) occurred between 15 and 20°C. The final preferred temperature of 20.7 ± 1.5 °C corresponded well to the temperature at which increase in oxygen consumption rate with temperature gradually lessened, approximately 20°C.     

Effects of constant and cyclic temperatures on the salinity tolerance of the estuarine sandhopper Orchestia chiliensis

The survival of Orchestia chiliensis (Milne Edwards, 1840) was investigated at salinities between 0.3 and 68 and constant or 10 C° cyclic temperatures between 5° and 25° C. Mortality increased with age, temperature and at salinity extremes. Small individuals show little seasonal acclimatisation apart from increased thermal tolerance at the highest exposure temperature. Larger individuals show a lateral shift in the mortality curve to the right in summer, giving increased survival at most salinities. Salinity had less effect on amphipods in cyclic regimes and survival was similar in 5° to 15° C and 10° to 20° C cycles. Mortality of larger individuals was higher in the 15° to 25° C cycle, but seasonal acclimatisation gave increased resistance at all fluctuating temperatures during the summer. Mortality in cyclic temperatures was higher than at similar constant temperatures. O. chiliensis does not actively evade immersion and diel temperature changes of 10 C° represent an important stress factor. This would affect all life stages and influence field populations both in the winter and the summer.     

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.     

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     

Metabolic responses of the sympagic amphipodsGammarus wilkitzkii andOnisimus glacialis to acute temperature variations

Acutely elevated seawater temperatures had pronounced metabolic effect on the Arctic under-ice amphipodsGammarus wilkitzkii andOnisimus glacialis, collected in May 1986 and 1988 in the Barents Sea. An increased rate of oxygen consumption vs temperature was observed for both species. In the range from 0° to 10 °CG. wilkitzkii andO. glacialis exhibit Q₁₀ values of 3.4 and 3.6, respectively. The results also indicate increased ammonia excretion rates forG. wilkitzkii andO. glacialis by an elevation of temperature from 0° to 10°C, with an overall Q₁₀ of 1.9 and 2.3, respectively. The present study demonstrates an increased O:N ratio with ambient temperature elevation from 0° to 10°C forG. wilkitzkii andO. glacialis, with overall Q₁₀ values of 2.0 and 1.6, respectively. This indicates a temperature-induced change in the metabolic substrate towards lipids.     

Filter-feeding ethology of benthic invertebrates (ascidians). V. Influence of temperature on pumping,filtration and digestion rates and rhythms in Phallusia mamillata

The influence of temperature has been studied simultaneously on the pumping, filtration, and digestion rates of Phallusia mammillata (Cuvier, 1815). Eighteen experiments were made between 7° and 25°C on 5 individuals. The average velocities of the water current varied between 3.37 and 9.65 cm sec^-1 (maximum 34.90 cm sec^-1). No recognizable rhythm emerged; pumping was continuous except at 7°C, where it soon ceased. Above 20°C, the curves were irregular and reflected the high sensitivity of the ascidian. The pumping rate was highest at 15°C (mean=5,788 ml h^-1 g^-1 dry weight of organs). At 10°C, the mean was 3,560; at 20°C, 2,629 ml h^-1 g^-1 dry weight of organs. At 20°C, the coefficients of variation displayed higher values, indicating a more irregular pumping at this temperature. Although there was no filtration rhythm, the variability of the results was higher at 20°C and above. As for pumping, maximum values were observed at 15°C (mean=4,286 ml h^-1 g^-1 dry weight of organs) decreasing with lower and higher temperatures, such decreases being more marked at the higher temperatures. Means were 352 ml h^-1 g^-1 dry weight of organs at 7°C; 2,935 at 10°C; 1,995 at 20°C; 973 at 25°C. The mean temperature coefficients for the filtration rates were: Q₁₀ for 7° to 15°C=11.86, Q₁₀ for 10° to 20°C=0,66, Q₁₀ for 15° to 25°C=0.22. The filtering efficiency was fairly constant throughout an experiment; the pumping and filtration curves were in fact almost parallel. The filtering efficiency of the branchial sac was high (75 to 85%), with constant values at 10° and 15°C; it became smaller (59%) at 20°C, with a higher coefficient of variation. The digestion rate also displayed maximum values at 15°C (mean=5.47 mg of albumin equivalent 24 h^-1 g^-1 dry weight of organs). It was lower at 10°C (mean=3.60 mg) and reached its minimum at 20°C (mean=1.71 mg). The higher temperature affected the percentage of food utilization, which showed smaller values at 20°C (59%) than at 10°C (89%) and 15°C (87%).     

Effects of acute changes in temperature and salinity on pulsation rates in ephyrae of the scyphozoan Aurelia aurita

The response of Aurelia aurita ephyrae to abrupt temperature and salinity differentials is expressed as changes in bell pulsation rates. Acute temperature rate-responses of Texas (USA) ephyrae reflect a reduced temperature sensitivity over a broad range (10° to 35°C), with a Q₁₀ value of 0.97 between 20° and 25°C. The initial relationship between salinity change and pulsation rate is linear and direct. This pattern is disrupted after 24 h, with those ephyrae experiencing a salinity decrease pulsing significantly faster than those experiencing no change or an increase in salinity. This response to low salinities dissipates after 2 days. Holding osmotic pressure constant and disrupting ionic ratios has more of an immediate and persistent effect than solely decreasing salinity.     

Latitudinal variation in the Dungeness crab,Cancer magister: zoeal morphology explained by incubation temperature

The difference in morphology between zoeae of Cancer magister Dana from Alaskan and Californian waters was documented to determine if the morphological variation is attributable to environmental influences. First-stage zoeae from Alaska have significantly longer carapace spines than zoeae from central California. The dorsal, rostral and lateral carapace spines were 14, 14 and 29% longer, respectively, in the Alaskan zoeae. The effect of temperature was tested on zoeal morphology as it is an obvious environmental difference between Alaskan and Californian waters. Ovigerous female crabs collected in southeastern Alaska in 1984 were held at 1°, 5°, 10° and 15° C until hatching occurred. Eggs were sampled seven times during the incubation period, and relative mortality, egg diameter and development stage were measured. All of the crabs and eggs at 1° C died before hatching occurred. Egg mortality averaged less than 2% in the other temperature treatments. Egg diameter increased significantly over the incubation period for all temperatures. Developmental rate of the embryos was inversely related to temperature. Hatching first occurred in 42 d at 15° C, 60 at 10° C and 160 d at 5° C. Newly hatched zoeae were collected and body length, dorsal, rostral and lateral carapace spines were measured. Significant differences existed between all temperatures for all spine lengths, with longer spines occurring at lower temperatures. Zoeal body lengths were also significantly different between the three temperatures. The results of this study question the use of spine lengths to distinguish similar larval species.     

Laboratory study of survival and duration of individual zoeal stages as a function of temperature in the brachyuran crab Cancer magister

Larvae were hatched from ovigerous Dungeness crabs, Cancer magister, collected from Puget Sound Basin, Washington, USA, in April, 1986, and the effects of temperature on rates of survival and development were studied for each of the five zoeal stages both in small batch-culture and in individual culture. Culture method had little effect on the results at 10°, 15°, and 20°C. Increased mortality was measured at all stages at 20°C, with 100% mortality occurring during the terminal fifth stage. Fifth stage larvae may also show higher mortality at 15°C than at 10°C. Stage duration varied inversely with temperature at all stages, although differences between 10° and 15°C were greater than between 15° and 20°C. The results indicate that survival and stage duration are independent of the values for the previous and subsequent stages, that variability among larvae in instar duration increases with temperature, and that the terminal fifth zoeal stage is the most sensitive to temperature stress. Duration of a late zoeal instar is not related to its earlier development rate nor can early development rates be used to predict whether individual zoeae will successfully develop to the megalopa. Measurements of megalopa dry weights indicate no differences due either to previous culture temperatures or to total time to the megalopa. Predictive models of larval transport that require estimates of larval duration should account for both changes in temperature response that can affect individual stage duration, and variability among individuals in stage duration that can influence the degree of larval dispersion.     

Über den Einfluß der Nahrungskonzentration und anderer Faktoren auf Filtrierleistung und Nahrungsausnutzung der Muscheln Arctica islandica und Modiolus modiolus

Filtration rates and the extent of phagocytosed food particles were determined in the offshore lamellibranchs Artica islandica and Modiolus modiolus in relation to particle concentration, body size and temperature. Pure cultures of the algae Chlamydomonas sp. and Dunaliella sp. were used as food. A new method for determining filtration rates was developed by modifying the classical indirect method. The concentration of the experimental medium (100%) was kept constant to ±1%. Whenever the bivalves removed algae from the medium, additional algae were added and the filtration rate of the bivalves expressed in terms of percentage amount of algae added per unit time. The concentration of the experimental medium was measured continuously by a flow colorimeter. By keeping the concentration constant, filtration rates could be determined even in relation to different definite concentrations and over long periods of time. The amount of phagocytosed food was measured by employing the biuret-method (algae cells ingested minus algae cells in faeces). Filtration rates vary continuously. As a rule, however, during a period of 24 h, two phases of high food consumption alternate with two phases of low food consumption during which the mussels' activities are almost exclusively occupied by food digestion. Filtration rate and amount of phagocytosed algae increase with increasing body size. Specimens of A. islandica with a body length of 33 to 83 mm filter between 0.7 to 71/h (30–280 mg dry weight of algae/24 h) and phagocytose 21 to 122 mg dry weight of algae during a period of 24 h. The extent of food utilization declines from 75 to 43% with increasing body size. In M. modiolus of 40 to 88 mm body length, the corresponding values of filtration rate and amount of phagocytosed algae range between 0.5 and 2.5 l/h (20–100 mg dry weight of algae) and 17 to 90 mg dry weight of algae, respectively; the percentage of food utilization does not vary much and lies near 87%. Filtration rate and amount of phagocytosed algae follow the allometric equation y=a·x ^b. In this equation, y represents the filtration rate (or the amount of phagocytosed algae), a the specific capacity of a mussel of 1 g soft parts (wet weight), x the wet weight of the bivalves' soft parts, and b the specific form of relationship between body size and filtration rate (or the amount of phagocytosed algae). The values obtained for b lie within a range which indicates that the filtration rate (or the amount of phagocytosed algae) is sometimes more or less proportional to body surface area, sometimes to body weight. Temperature coefficients for the filtration rate are in Arctica islandica Q₁₀ (4°–14°C)=2.05 and Q₁₀ (10°–20°C)=1.23, in Modiolus modiolus Q₁₀ (4°–14°C)=2.33 and Q₁₀ (10°–20°C)=1.63. In A. islandica, temperature coefficients for the amount of phagocytosed algae amount to Q₁₀ (4°–14°C)=2.15 and Q₁₀ (10°–20°C)=1.55, in M. modiolus to Q₁₀ (4°–14°C)=2.54 and Q₁₀ (10°–20°C)=1.92. Upon a temperature decrease from 12° to 4°C, filtration rate and amount of phagocytosed algae are reduced to 50%. At the increasing concentrations of 10×10⁶, 20×10⁶ and 40×10⁶ cells of Chlamydomonas/l offered, filtration rates of both mollusc species decrease at the ratios 3:2:1. At 12°C, pseudofaeces production occurs in both species in a suspension of 40×10⁶, at 20°C in 60×10⁶ cells of Chlamydomonas/l. At 12°C and 10–20×10⁶ cells of Chlamydomonas/l, the maximum amount of algae is phagocytosed. At 40×10⁶ cells/l, the amount of phagocytosed cells is reduced by 26% as a consequence of low filtration rates and intensive production of pseudofaeces. At 20°C and 20–50×10⁶ cells of Chlamydomonas/l, the maximum amount of algae is sieved out and phagocytosed; the concentration of 10×10⁶ cells/l is too low and cannot be compensated for by increased activity of the molluscs. With increasing temperatures, the amount of suspended matter, allowing higher rates of filtration and food utilization, shifts toward higher particle concentrations; but at each temperature a threshold exists, above which increase in particle density is not followed by increase in the amount of particles ingested. Based on theoretical considerations and facts known from literature, 7 different levels of food concentration are distinguishable. Experiments with Chlamydomonas sp. and Dunaliella sp. used as food, reveal the combined influence of particle concentration and particle size on filtration rate. Supplementary experiments with Mytilus edulis resulted in filtration rates similar to those obtained for M. modiolus, whereas, experiments with Cardium edule, Mya arenaria, Mya truncata and Venerupis pullastra revealed low filtration rates. These species, inhabiting waters with high seston contents, seem to be adapted to higher food concentrations, and unable to compensate for low concentrations by higher filtration activities. Adaptation to higher food concentrations makes it possible to ingest large amounts of particles even at low filtration rates. Suspension feeding bivalves are subdivided into four groups on the basis of their different food filtration behaviour.     

A controlled-temperature plankton wheel

A controlled-temperature plankton wheel is described that is suitable for use on board a ship. The IMER plankton wheel system allows the use of various sizes of experimental bottles, up to 2.2 litres, the simulation of ambient light regimes and variable speed control for the rotation of the experimental bottles. The flexibility of the system was demonstrated by investigating the relationship between temperature and ingestion rate of an herbivorous copepod. Using four of the IMER plankton wheels simultaneously at four different temperatures (5°, 10°, 15° and 20° C), the ingestion rate of Calanus helgolandicus, feeding on Thalassiosira weissflogii, was shown to increase with increasing temperature; from a transformation of log_e (ingestion rate), this relationship was calculated as a Q₁₀ (10° to 20°C) for Copepodite Stage V (Q₁₀ 4.5) and adult female (Q₁₀ 2.7) C. helgolandicus. The possibility of damaging cells, by rotation at 2 rpm, was investigated using the spinose form of the diatom T. weissflogii. Such rotation did not cause any damage to the spines of T. weissflogii, but mixing this diatom with a magnetic stirrer bar did damage the spines to varying degrees, depending on the volume being mixed.     

Effects of various temperature cycles on the larval development of the gastropod molluscCrepidula fornicata

Veligers ofCrepidula fornicata (L.) were reared for 12 days at constant temperatures of 15°, 20°, 25°, 30° and 35°C, and at 5 C° daily cycles of equal periodicity (COEP) over the temperature ranges 15° to 20°C, 20° to 25°C, 25° to 30°C and 30° to 35°C. COEP consisted of equal periods (6 h) of maximum temperature, minimum temperature, and uniformly increasing and decreasing temperature each 24 h period. Survival was high and not influenced by cyclic or constant temperature from 15° to 30°C. At 35°C and COEP 30° to 35°C, all larvae died before Day 6. Shell growth rate increased markedly over the range 15° to 25°C, and growth rates at cyclic temperatures in this range were intermediate between growth rates at the corresponding constant temperatures. Larvae reared at COEP 15° to 20°C and COEP 30° to 35°C had discontinuities in their shells due to inhibition of shell secretion during the adverse part of each temperature cycle. Groups ofc. fornicata veligers were exposed for 2 days to daily temperature cycles of equal and unequal periodicity in the critical 30° to 35°C range. [Cycles of unequal periodicity (COUP) consisted of unequal periods (varying between 3 and 15 h) of maximum and minimum temperature and uniformly increasing and decreasing temperature each 24 h period.] These veligers showed shell growth although their body tissue declined, as indicated by decreasing carbon content per larva. Least shell growth and most body tissue loss occurred in those cycles with the longest exposure to higher temperature. Larvae exposed for arious days to the mildest 30° to 35°C COUP (15 h at 30°C, 3 h increasing temperature, 3 h at 35°C and 3 h decreasing temperature) recovered and resumed normal growth when transferred to constant 30°C, but their growth was retarded in proportion to the number of days in the temperature cycle. Rates of shell growth of veligers in temperature cycles show an immediate effect of environmental temperature, while changes in carbon content per larva better reflect the effects of temperature on general metabolism and survival.     

Rearing Pandalus borealis larvae in the laboratory

Larvae of the northern shrimp Pandalus borealis (Krøyer) are pelagic. In the Estuary and Gulf of St. Lawrence, Canada, the early stages are found in the upper 25-m of the water column in spring and early summer and are expected to experience a range of water temperatures from as low as 0°C to as high at 6°C. Little is known of the impact of water temperature on metabolic requirements of northern shrimp larvae. In this study, routine respiration (VO₂), maximum respiration (electron transport system activity, ETSA) and metabolic scope for growth (MS, ETSA–VO₂) of northern shrimp larvae were measured as a function of temperature (3, 5 and 8°C), developmental stage (I–V at 3°C, I–VII at 5°C and 8°C) and growth rate in dry mass. After logarithmic transformation, all three metabolic variables were linearly related to dry mass. The increase in VO₂ with body mass was faster at 5°C than at 3 or 8°C, whereas with ETSA this increase was slower. As a result, MS increased more slowly with dry mass at 5°C than at 3 and 8°C. However, MS did not limit growth in this study, since it explained only 39% of the variability in growth. All three metabolic variables as well as growth varied together as a function of temperature and ontogeny. Q₁₀ of all three metabolic variables ranged from 1.6 and 2.2 for stages I–V larvae, except for VO₂ at stage I (3.9) and stage III (2.9).     

Neue ergebnisse über die aufzucht von Carcinus maenas im laboratorium

The shore crab Carcinus maenas was reared in the laboratory from egg deposition to sexual maturity. Special enclosures were developed for cultivation of the larvae. Food and temperature proved to be the most important exogenous factors for rearing success. Fresh Artemia salina nauplii were the only food suitable for all larval stages. The following rearing temperatures proved most successful during larval development: (1) embryonic development, 10°C; (2) zoea stages, 15°C; (3) megalopa stage, 17.5°C. The larvae hatch preferably in darkness when reared under short-day conditions.     

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.     

Combined effects of temperature and salinity on the complete development ofEurytemora velox (Crustacea: Calanoidea)

The calanoid copepodEurytemora velox was collected from rock pools at Castletown, Isle of Man, UK. Its optimum environmental requirements, particularly temperature and salinity, were determined, with a view to its possible future use as living food in intensive fish and shellfish farming. The species was cultured in 21 different temperature and salinity combinations. Investigations covered a period of two years from December 1983 to December 1985. Complete development from hatching to adult stage was followed in 21 temperature and salinity combinations. Nauplii suffered relatively high mortalities, indicating the sensitivity of this development stage to variations in temperature and salinity. Highest nauplii survival was observed in the combinations 15°C with 25 and 20 S and 20°C with 20 S, the highest copepodite survival at 10°C and 20 S. Lower salinities were tolerated better at higher temperatures and higher salinities at lower temperatures. Development time varied with the temperature and salinity combinations. Lower salinities at the lower temperatures of 10° and 15°C and both lower and higher salinities at 20°C prolonged development, particularly of the naupliar stage. Highest Q₅ values (i.e., rate of change of development with a 5 C° increase in temperature) were recorded for the naupliar stage. Statistical analysis indicated that salinity influences the survival of both nauplii and copepodites; however, this effect is not linear.     

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.     

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.