Effects of high temperature on larval development and metamorphosis of Arachnoides placenta (Echinodermata: Echinoidea)

Adults of the sea urchin Arachnoides placenta (L.) were induced to spawn, and eggs were fertilized at 28°C in September 1989. After 5 min, eggs were transferred to 28, 31, 34, or 37°C and reared to metamorphosis. Embryos were observed at 20-min intervals during the first 2 h; larvae were observed daily. The cleavage was higher at higher temperatures. Embryos reared at 28°C were still at the 4th cleavage (16-cell stage) after 100 min, while those at 34°C had reached the 5th cleavage (32-cell stage). All embryos reared at 37°C died on the second day. Incidence of abnormality was 20 to 30% at 28 and 31°C, 48% at 34°C, and 77% at 37°C. The 8-arm stage was reached after 4 d at 28°C, 3 d at 31°C and 2 d at 34°C. Larvae displayed decreasing body length and arm length with increasing temperature. Larvae at 31°C have relatively long arms, as a result of a decrease in body length, not because of increased arm length. Incidence of metamorphosis was 43.9±1.7% (mean/plusmn;SD) at 28°C, 24.5±1.9% at 31°C, and 5.3% at 34°C. The size of metamorphosed juveniles was significantly larger at 28°C than at 31 and 34°C. Temperatures of 31°C negatively affect larvae and juveniles of the sand dollar.     

Responses of ovaries and testes of Lytechinus variegatus (Echinodermata: Echinoidea) to dietary administration of estradiol, progesterone and testosterone

 In many vertebrates, environmental factors influence gamete differentiation and growth of the mature gonad through alteration of sex steroid production or action; however, it is unclear how gamete differentiation and gonadal growth are regulated in echinoids. The purpose of this study was to examine the influence of dietary administration of estradiol (E2), progesterone (P4), testosterone (T) and finasteride (F, a 5α-reductase inhibitor) on the ovaries and testes of mature Lytechinus variegatus (Lamarck) during gonadal growth. Echinoids were fed a formulated diet supplemented with steroids or steroids in combination with finasteride for 36 d. The effects of dietary administration of steroids on L. variegatus were both steroid- and sex-specific. The mean ovary index was 54% greater in individuals fed E2 than from individuals fed the control (C) diet (10.0 ± 1.1 vs 6.5 ± 0.7, respectively; P < 0.05). Individuals fed E2, P4, E2/P4, and P4 in combination with F had significantly smaller oocytes (P < 0.005) than individuals fed C. The volume fraction occupied by nutritive phagocytes in ovarian tubules from individuals fed E2/P4 or P4 were significantly larger than the volume fraction occupied by nutritive phagocytes from individuals fed C, indicating that E2 and 5α-reduced progestins may promote nutrient allocation to nutritive phagocytes. Although oocytes from the individuals fed T alone were significantly smaller than those fed T in combination with F, oocytes from individuals in both treatments were significantly larger compared to oocytes from individuals fed C. These data suggest that upon removal of 5α-reduced androgens, T is able to promote an increase in oocyte diameters. In contrast, the mean testis index was 56% greater in individuals fed P4 than in individuals fed C (8.9 ± 0.6 vs 5.7 ± 0.9, respectively; P < 0.05); the testis index did not increase in individuals fed P4/F, suggesting that 5α-reduced progestins stimulate testicular growth. Testes growth in the presence of 5α-reduced progestins was accomplished by significant increases in the volume fraction occupied by nutritive phagocytes and by a significant reduction in the volume fraction occupied by spermatogenic columns in testicular tubules. These data further indicate that 5α-reduced progestins (or 5α-reduced androgens) may inhibit spermatogenic column formation. In conclusion, E2 stimulated ovarian growth but inhibited oocyte growth, whereas T had no affect on ovarian growth but promoted oocyte growth in L. variegatus. We hypothesize that the E2 (or E2 metabolites) and/or 5α-reduced androgens in combination with T regulate oocyte growth in the echinoid L. variegatus. In addition, 5α-reduced progestins promoted nutrient accumulation in nutritive phagocytes within the ovaries and the testes. Furthermore, 5α-reduced progestins stimulated growth of the testes and inhibited spermatogenic column formation, suggesting that 5α-reduced progestins regulate nutrient accumulation into nutritive phagocytes and spermatogenic column formation in L. variegatus. The differences in estrogen effects between echinoids and asteroids may be related to differences in gonad morphology and, ultimately, the differences in cellular signalling pathways (paracrine vs endocrine). Received: 22 May 1999 / Accepted: 24 May 2000     

Effects of temperature and salinity on larval development ofNecora puber (Brachyura: Portunidae)

Temperature and salinity affected both length of larval development and mortality inNecora puber collected in the Ría de A Coruña during December 1984 and January 1985. Development time decreased considerably with increased temperature. This decrease was sharper when temperature increased from 15° to 20°C than when it increased from 20° to 25°C. At 35S, average development took 48, 32 and 28 d at 15°, 20° and 25°C, respectively. At the three salinities tested (25, 30 and 35), larval development was completed only at 15°C, at 20°C/30 and 35S, and at 25°C/35S. Development times at 15° and 20°C were highly significantly different at both 35 and 30S (P 0.01). However, there were no significant differences between development times at 20° and 25°C (P > 0.05). Within any one specific temperature series, no significant difference was observed between the salinity values tested (P > 0.05). The duration of each of the five zoeal stages was similar within each and the same temperature/salinity combination, whereas the duration of the megalop was twice as long as any of the zoeal stages. The combination of the lowest temperature (15°C) and the highest salinity (35) tested resulted in the greatest larval survival of 28%. Highest mortality occurred at 25°C, at which temperature development was completed only at 35S. A sharp drop in larval survival was observed in the transition period Zoea V — megalop in all combinations of temperature and salinity tested. Within the limits of tolerance to temperature and salinity, the former effected more pronounced differences in the duration of larval development, while salinity appeared to constitute a limiting factor for survival.     

Biochemical and morphometric study of growth in the stomach and intestine of the echinoid Lytechinus variegatus (Echinodermata)

In June 1990, previously starved Lytechnius variegatus were fed a 5% fish meal-agar diet for 32 d at 20°C and 32. Stomach and intestine indices increased by 139 and 66%, respectively, with the most growth occurring by Day 16. The stomach had a faster growth rate than the intestine, as indicated by an increase in the stomach: intestine ratio. Gonadal growth did not begin until Day 16, after substantial gut growth. DNA and morphometric analysis revealed that cellular growth in the stomach and intestine was first hyperplastic and then hypertrophic. DNA concentration and histological examination indicated that the stomach was composed of large voluminous cells, and the intestine of small cells. The stomach of starved individuals contained numerous depressions that significantly decreased in number during growth. The number of depressions significantly increased in the intestine. Lipid concentration and amount were higher in the stomach, and increased during growth by 43 and 258%, respectively, by Day 32; little change in lipid occurred in the intestine. The stomach lipid:DNA ratio increased by 66% by Day 16, with little change in the intestine. The concentration of soluble protein remained constant in the stomach, but had decreased in the intestine by Day 16. The total energy stored in the stomach and the intestine increased by 152 and 66%, respectively, by Day 32. The stomach mucosa increased in thickness and appeared to decrease its surface area. These data suggest that the stomach constitutes the primary storage organ of the gut. The intestinal mucosa also inreased in thickness but remained thinner than the stomich while apparently increasing in surface area, suggesting an enhanced role in residual nutrient absorption and ranslocation, respiration or water ion-transport.     

Effect of temperature and salinity on larval development of southern king crab (Lithodes antarcticus)

Larvae of Lithodes antarcticus Jacquinot were reared in October, 1981 from hatching to the glaucothoe stage at 16 temperature/salinity combinations (5.5°; 7.5°; 9.5° and 13.5°C; 26, 29, 32 and 35 S) to determine optimal environmental conditions for larval development. The highest survival percentage was obtained in the culture at 7.5°C and diminished according to temperature increase or decrease. High temperature cultures significantly shorten the larval life duration, but produce large mortalities. At 5.5°C mortality occurred almost exclusively during the moult to glaucothoe stage. Higher survival percentages were obtained as salinity was increased. In the lowest salinity culture (26 S) no zoea reached the post-larvae stage at culture temperatures. The best T/S combination was obtained at 7.5°C and 35 S, with a survival percentage of 29%. The shortest zoeal developments were obtained at 32 S in all culture temperatures. Salinity also affects larvae coloration: there is a pigment concentration on erythrophores, which causes a color decrease.     

Ecology of Echinometra mathaei (Echinoidea: Echinodermata) at Diani Beach,Kenya

Studies were carried out on the inner and outer coral reefs at Diani Beach on the Kenya coast to assess the distribution, density and behaviour of Echinometra mathaei (de Blainville). Transects 1 m wide were run on the two reefs in April, June and September, 1970. Test measurements on representative samples from the animal populations on both reefs were also taken. Direct observations on specimens of E. mathaci in selected rock pools on the outer reef were made to determine their movement, gregariousness, homing and feeding behaviour. Population density was higher on the inner reef furthest from the sea at low tide than on the outer reef. On the submerged inner coral reef at low tide, E. mathaei occurred mainly exposed on the seaweeds, but, on the exposed outer reef, its main niches were crevices in rock pools and under coral ledges. Sizefrequency distributions revealed that smaller individuals occurred on the inner reef and larger ones on the outer reef. The growth rate of E. mathaei was estimated from the positions of modal values, calculated from size-frequency distributions. No gregarious or homing behaviour was observed and, once settled in a suitable crevice, E. mathaei showed little movement.     

Influence of food concentration,temperature and salinity on the larval development ofBalanus amphitrite

Influence of food concentration (0.5, 1 and 2 x 10⁵ cell ml^–1 ofSkeletonema costatum), temperature (20 and 30°C) and salinity (15, 25 and 35) on the larval development ofBalanus amphitrite (Cirripedia: Thoracica) was examined. The mortality rate at 20°C was lower than at 30°C in general. Increase in food concentration from 0.5 to 1 x 105 cells ml^–1 improved the survival rate, but this was not evident when food concentration was increased to 2 x 105 cells ml^–1. The results indicate that food availability and temperature jointly determine the energy allocation for metamorphic progress. It was observed that the influence of the tested variables varied with instar. At 20 °C the mean duration of the second instar exceeded 3 d and was much longer than other instar durations. The fourth, fifth and sixth instars and the total naupliar period showed that the effect of different salinities at given food concentrations was negligible at 20°C, while at 30°C there was a marked decrease in duration with increasing salinity.     

Effects of salinity and calcium concentration on arm regeneration byOphiothrix angulata (Echinodermata: Ophiuroidea)

At 33 salinity a tissue stump formed 2 to 3 d after autotomy and developing ossicles were present by the fourth day inOphiothrix angulata (Say). Regeneration proceeded rapidly from the sixth day until the thirteenth day, when the rate decreased greatly. The length of the regenerated arm and the number of ossicles formed did not vary over a salinity range of 28 to 38 S, but were significantly less at 23 S. The number of ossicles regenerated increased linearly (y=1.9 x-7.7;r=0.9089) with the calcium concentrations ranging from 3.8 to 9.5 mM. No ossicle formation occurred at 3.8 mM calcium concentration. Rate of net uptake of calcium-45 into the ossicles of intact individuals in salinities of 28 and 33 was significantly greater than that in 23 and 38 S. However, net uptake rate of calcium into the soft tissues of the arms was significantly higher at 18S than at the lower two salinities.     

Growth and production of Sterechinus neumayeri (Echinoidea: Echinodermata) in McMurdo Sound,Antarctica

Population dynamics of Sterechinus neumayeri were investigated at four sites in McMurdo Sound: Cape Evans, McMurdo Station, East Cape Armitage and New Harbor. The annual formation of natural growth bands in the jaws of the Aristotle's lantern was verified by a tagging-recapture experiment. Growth functions based on natural growth bands indicated differences among stations but showed S. neumayeri to be a slow growing species, reaching its maximum diameter of 70 mm at an age of about 40 yr. Annual production ranged between 2.4 (C. Evans) and 0.65 g ash free dry mass m^-2 (New Harbor) and was related to differences in food conditions. More than 95% of total production was invested in reproduction, and less than 5% was invested in somatic growth. Consumption estimates for S. neumayeri showed this species to play a significant role in the benthic trophic web of McMurdo Sound.     

Mass mortality of Strongylocentrotus droebachiensis (Echinodermata: Echinoidea) off Nova Scotia,Canada

A mass mortality of Strongylocentrotus droebachiensis, attributed to disease, was monitored in an echinoiddominated barren ground at Eagle Head on the south-western coast of Nova Scotia, Canada, in 1982. Mortality was 70% in a shallow (3 m) nearshore area, resulting in a loss of echinoid biomass of 2 042 g fresh weight m^-2, and 6% in deeper (7 m, 10 m) offshore areas. Echinoid density, size and nutritional condition (gonad index) were highest in the nearshore area. Survivorship was higher in juveniles (<15 mm diameter) than in adults resulting in the formation of a bimodal size distribution in the nearshore area. Mortality began around early October, near the peak of the annual cycle of seawater temperature (15°C), and was arrested by early December (seawater temperature 7°C) when morbid echinoids appeared to recover. In laboratory experiments, time to morbidity of S. droebachiensis exposed to morbid conspecifics increased exponentially with decreasing temperature (20° to 8°C). There was no survival at 20° and 16°C, 20% survival at 12°C and 100% survival at 8°C after 60 d; suggesting a lower temperature limit (between 12° and 8°C) for possible transmission of a pathogenic agent. Morbid laboratory echinoids from experiments at 16°C, and recovering echinoids collected in the nearshore area in early December, showed 100 and 85% survival respectively at <=8°C, and 0 and 15% survival respectively at 16°C, after 30 d. Time to morbidity was not affected significantly by nutritional condition and was similar for juvenile and adult echinoids. Time to morbidity was greater in echinoids exposed to one or three morbid individuals continuously, or seven morbid individuals for 1 h, relative to higher levels of exposure (up to seven morbid individuals continuously). Recent mass mortalities in S. droebachiensis have occurred in years of record high sea surface temperatures. The extent of mortality is correlated with the magnitude and duration of temperatures above a lower limit.     

The effects of dietary protein concentration on feeding and growth of small<Emphasis Type="Italic"> Lytechinus variegatus</Emphasis> (Echinodermata: Echinoidea)

Understanding the nutritional requirements of Lytechinus variegatus is essential to define its role in the marine environment and necessary for successful culture of this species. Protein is an essential component for growth, yet the requirements for protein in most marine organisms are not known. Small Lytechinus variegatus (ca. 14 mm diameter, n=160) were collected at St. Joseph Bay, Florida in March 2000. They were held in aquaria and fed ad libitum one of four diets varying in protein concentration (9, 15, 21, or 33% dry weight) for 14 weeks. Diets 21% dry weight protein were sufficient for maximal growth and survivorship. Individuals fed a diet low in protein compensated by consuming higher quantities of food per gram wet weight. However, the protein was insufficient to support maximal growth and survivorship. Total dry matter absorption, protein absorption, and carbohydrate absorption of the prepared feeds were higher than reported for natural diets. Total production and production efficiency was lowest in those fed the low-protein diet. Gonad production occurred in individuals with test diameters smaller than reported previously in field populations and was lowest in those fed the low-protein diet. The acini of both females and males were predominantly composed of nutritive phagocytes, suggesting that growing L. variegatus use the gonad primarily as a nutrient storage organ and not for reproduction. In females the growth rate of oocytes increased with dietary protein. In males, protein concentration 21% resulted in an increase in the volume of germinal epithelium and a decrease in the volume of nutritive phagocytes. The minimal requirement for protein in L. variegatus from approximately 1 g to 15 g total weight is >15% dry weight for the conditions of this experiment. The requirement for a diet >15% protein corroborates with field studies that indicate L. variegatus consumes epibionts and animal tissues preferentially to seagrass, and that the microbial, microfloral, and microfaunal community living on the seagrass may provide the extra protein required for growth and survival of small L. variegatus.Communicated by P.W. Sammarco, Chauvin     

Electron microscope study of the digestive system of Strongylocentrotus droebachiensis (Echinodermata: Echinoidea)

The fine structure of esophagus, stomach and intestinal epithelial cells of the sea urchin Strongylocentrotus droebachiensis have been examined. Three types of mucosal cells secreting neutral, acid and sulfated mucopolysaccharides have been found in the esophagus. The modes of origin of different types of mucous granules are described. Storage and zymogen cells have been identified in the stomach epithelium. Storage cells are adapted for intracellular digestion and synthesis of nutritive substance. Zymogen cells are adapted for enzyme synthesis. The columnar cells occur in the intestinal epithelium; they are adapted for nutrient absorption and lipid storage. Digestion in different parts of the digestive tract is considered in context with the ultrastructural and histochemical data obtained. Some problems concerning the relation between intracellular and extracellular digestion are discussed.     

Reproductive cycle,larval development,juvenile growth and population dynamics of Patiriella pseudoexigua (Echinodermata: Asteroidea) in Taiwan

A field study was conducted at Wanlitung, southern Taiwan, in 1986–1089, to determine the reproductive cycle, development mode, growth rate and population dynamics of the small seastar Patiriella pseudoexigua (Dartnall), which occurs in highly stressful and disturbed intertidal pools in this area. An inverse relationship between gonad index and pyloric-caccum index was only recorded immediately prior to spawning. A short, well-synchronized seasonal spawning occurs in October. When reared at 25 °C, lecithotrophic larvae develop directly, lack a bipinnaria stage, and metamorphose completely on the seventh day after fertilization. The growth curves of field juveniles are linear, those of laboratory-reared juveniles are sigmoid. Juveniles appear in tide pools in spring-early summer of each year. Adults spawn mainly in late fall, enabling spawning to occur in time for the larvae to benefit from the environmentally favorable winter season. Populations in high-tidal pools decrease in later summer, but remain more stable in lowtidal pools and lagoons.     

Population dynamics of Diadema antillarum (Echinodermata: Echinoidea) following mass mortality in Panamá

In 1983, Diadema antillarum suffered mass mortality throughout the Caribbean Sea and the western Atlantic Ocean. I followed the dynamics of populations at the San Blas Islands, Panamá from April 1983 to November 1987. Density measurements indicate that populations of D. antillarum have not recovered from the die-offs that killed nearly 97% of the individuals. There was recruitment to the 1 to 1.5 cm class immediately after the mass mortality, but there has been little additional influx of juveniles since then. The low number of observable juveniles could not be attributed to elevated rates of predation on very small individuals. Rates of recruitment did not differ between reefs with artificially increased densities of D. antillarum and reefs kept free of sea urchins; thus, the lack of recruitment did not arise from absence of adults that could provide settlement cues to the larvae or protection to newly settled juveniles. Other species of sea urchins did not show a clear pattern of increase after the demise of D. antillarum. Therefore, interspecific competition directed towards D. antillarum juveniles did not increase after the mass mortality. Two reefs where Echinometra viridis, Eucidaris tribuloides and Lytechinus williamsi, were removed showed no significant differences in recruitment of D. antillarum relative to two reefs where these species were allowed to remain at their natural densities. Resident D. antillarum after the mass mortality produced gametes with the same per capita intensity and lunar synchrony as before the mass mortality. However, it is possible that the probability of fertilization of their gametes decreased because of low population density. The most likely explanation for lack of recruitment is that the reduced numbers of reproducing adults at Panamá and upstream locations resulted in levels of larval supply that were inadequate to sustain recruitment on Panamanian reefs.     

Growth and spatial distribution of Cassidulus mitis (Echinodermata: Echinoidea) on a sandy beach in Southeastern Brazil

Using a hand dredge, a population of Cassidulus mitis Krau, 1954 was sampled monthly from February 1986 to January 1987 at 15 stations offshore from Vermelha Beach (22°5718 S; 43°948 W), in water depths ranging from 0 to 6 m. The sex ratio indicated a slight numerical dominance of females for adults and a larger numerical female dominance for juveniles. A von Bertalanffy growth curve was estimated as L=71.00 (1-e^(-0.390t)), where L=length (mm) and t=time (yr). The instantaneous natural mortality rate (M) was estimated as 0.812. The population occupies a zone parallel to the beach, and displays a spatially aggregated pattern, with higher densities in areas characterized by coarse and very well sorted sand containing low amounts of organic matter.     

Effects of temperature on morphological landmarks critical to growth and survival in larval Atlantic cod (Gadus morhua)

The morphology and function of structures important to energy acquisition were studied from spawning to the stage of transformation of larva to pelagic juvenile in Atlantic cod, Gadus morhua L., from December 1991 to July 1992. Fertilized eggs produced by adult fish from two genetically discrete populations (Newfoundland and Scotian Shelf) were raised under similar conditions in the laboratory at temperatures of 5 and 10°C. Subsamples of larvae were removed from cultures daily for 10 d, and then less frequently, and fixed for light microscopy and scanning electron microscopy. Nine functional morphological landmarks important to feeding, respiration and locomotion were chosen from observation of 280 ind. These landmarks defined 12 major developmental stages, from hatching to the pelagic juvenile stage. One of the feeding landmarks, intestinal stage, varied as a function of age and size and the variance in development was higher at 10°C than at 5°C; Newfoundland larvae developed more complex intestines than did Scotian Shelf larvae. In addition, Newfoundland larvae had significantly higher growth rates than those of Scotian Shelf larvae. Despite the higher growth rates and greater structural complexity of the intestine in Newfoundland larvae, the rate of yolk utilization was not significantly different between Newfoundland and Scotian Shelf larvae. Staging of respiratory landmarks showed that the gill arches were probably used preferentially in feeding while respiration was cutaneous. The gills, operculum and gill rakers developed late in larval life and accompanied the transition from cutaneous to branchial respiration. In the yolk-sac period, development of feeding and respiratory structures may be largely genetically controlled. During exogenous feeding, extrinsic factors also become important, as shown by the size and age-independent variation in intestinal development of larval cod raised at different temperatures.     

Effects of duration of larval swimming period on early colony development in Bugula stolonifera (Bryozoa: Cheilostomata)

Laboratory experiments with larvae of the cheilostome bryozoan Bugula stolonifera Ryland, 1960 assessed the time to settlement in the presence of a constantly available polystryrene substrate, the development of competence for metamorphosis, and the effects of the duration of swimming period on early colony development. Sexually mature colonies of B. stolonifera were collected on 11 and 18 September 1987; 2 and 18 August, 1988; and 6, 12, 19, and 26 September 1988, from Eel Pond (Woods Hole, Massachusetts, USA) and were maintained at 20°C. In the presence of a constantly available substrate, cumulative percent settlement curves were sigmoid, with 75% of larvae settled in 3.2±0.5 h. Typically, 50% of the larvae settled in less than 3 h and 95% settled in 6.1±1.2 h. The number of settled individuals that developed feeding ancestrulae by 3 d and the number that developed first-feeding autozooids by 6 d was assessed as a function of duration of larval swimming. Individuals which were kept swimming for 8 and 10 h after hatching developed significantly more slowly to the ancestrula and autozooid stages in 13 out of 14 experiments than did larvae that swam 2 or 6 h. This is the first report for any bryozoan that prolongation of the larval free-swimming period affects the rate of colony development.     

Effects of parental acclimation temperature and egg-incubation temperature on egg-hatching time in Acartia tonsa (Copepoda: Calanoida)

Experimental results indicate that both parental acclimation temperature and egg-incubation temperature have significant effects on the egg-hatching time of Acartia tonsa Dana between 15° and 25°C. Effects of parental acclimation temperature and egg-incubation temperature are additive if the long-term parental acclimation temperature is constant. A. tonsa are sensitive to temperature and changes of temperature during development (in vivo) and after they are laid (in vitro), up to the time of hatching. A temperature change of the parent culture for time periods of 86 h to 8 d is sufficient to change egg-hatching times. The time required for a temperature change to affect egg-hatching times depends on the magnitude and direction of the temperature change. Field collected A. tonsa demonstrate that changes in egg-hatching times occur in the field populations and can be predicted by mean weekly water temperature.     

Seasonal effects of light,temperature, nutrient concentration and salinity on the physiology and growth of Caulerpa paspaloides (Chlorophyceae)

Caulerpa paspaloides (Bory) Greville were collected during the winter and summer (1978 to 1979) from the Florida Keys, USA. Thalli collected during the winter photosynthesized more efficiently at low light intensities (I_c<1, I_k=38 Exm^-2xs^-1) than did thalli collected in the summer (I_c=13, I_k=111 Exm^-2xs^-1). Summer thalli exhibited higher P_max values (2.20 mgO₂xg^-1 dry wtxh^-1) than winter thalli (1.70 mg O₂xg^-1 dry wtxh^-1). Rates of rhizome elongation and frond initiation were strongly inhibited by winter temperatures. The maximum lethal temperature for summer thalli was 37° to 38°C as measured by both growth and photosynthesis. No evidence of nitrogen or phosphorus limitation was found. Relatively minor reductions in salinity (3S) resulted in significant increases in rhizome apex motality. Results indicate that low winter temperatures are responsible for reduced winter growth rates previously reported for the Key Largo population. Increased photosynthetic efficiency at low light intensities and preferential maintenance of rhizome elongation over frond initiation appear to allow this tropical macroalga to optimize growth and survival during the winter.     

Effects of algal and larval densities on development and survival of asteroid larvae

Effects of larval and algal culture density and diet composition on development and survival of temperate asteroid larvae were studied in the laboratory at Santa Cruz, California, USA, during summer and fall of 1990. Larvae of Asterina miniata were reared at two densities, 0.5 or 1.0 ml^-1, and fed one or two species of cultured phytoflagellates — Dunaliella tertiolecta alone or mixed with Rhodomonas sp. — at three concentrations of 5x10², 5x10³, and 5x10⁴ total cells ml^-1. Algal concentration strongly influenced larval development; however, larval density also had a marked effect. Development progressed further with increasing algal concentration. Larval growth and differentiation were sometimes uncoupled; i.e., growth measures were directly related to food level, while differentiation indicators were less so. At the lowest food level, growth was negative and differentiation was arrested at early precompetent stages; these larvae never formed juvenile rudiments or brachiolar attachment structures. Development times of larvae given more food ranged from 26 to 50 d and depended directly on food availability. Development time to metamorphosis at the highest food concentration was similar for siblings fed D. tertiolecta alone or mixed with Rhodomonas sp. In contrast, when food level was an order of magnitude lower, larvae fed the algal mixture metamorphosed significantly earlier than larvae fed the unialgal diet. This suggests interactive effects of food quantity and food quality. Survival was little affected by larval or food density, except at the lowest ration. Feeding experiments in well-controlled laboratory conditions are useful to predict and compare the physiological or developmental scope of response of larvae to defined environmental factors; however, results from such studies should not be extrapolated to predict rates and processes of larval development in nature.