Foods and predators of the green sea urchin Strongylocentrotus droebachiensis in Newfoundland waters

Gut analyses of the green sea urchin Strongylocentrotus droebachiensis (O. F. Müller) demonstrated that perennial phaeophytes, mostly fucoids and Alaria esculenta, were predominant in the diet. Ephemeral species, coralline algae and animals, were consumed in smaller amounts when available. Grazing by the urchins is evidently responsible for the dearth of non-coralline sublittoral algae in Newfoundland waters. Lobsters, rock crabs, purple sea stars, other urchins, and a variety of fishes and birds feed on S. droebachiensis, but predation is apparently not effective in limiting the abundance of the urchin.Studies in Biology from the Memorial University of Newfoundland No. 234.Contribution from the Marine Sciences Research Laboratory No. 66.     

Density-related reproductive trade-offs in the green sea urchin, Strongylocentrotus droebachiensis

Green sea urchin, Strongylocentrotus droebachiensis (O.F. Müller), populations are being depleted rapidly in the Gulf of Maine and there is justified concern that the potential of this free-spawner to produce larvae may be severely inhibited. We evaluated the opposing effects of different population densities on gonad development and fertilization success, using population surveys and fertilization experiments. We determined gonad indices (gonad mass/body mass) over a range of population densities (0.1 to 250 ind. m⁻²) at seven sites in coastal Maine, USA, sampled at two depths (5 and 15 m). At shallow sites, we found that gonad indices declined by 50% over the 1500-fold range in adult population density. At 15 m deep locations, gonad mass was consistently low and did not vary significantly with density. Patterns of macroalgal abundance suggest urchins at high density and in deeper water were food limited. Because macroalgal cover co-varies inversely with sea urchin density, we designed field experiments to determine the interaction between sea urchin density and kelp canopy on fertilization success. On square arrays we manipulated the spacing of simulated urchins, but held their numbers constant (five sperm-filled syringes interspersed with four Nitex mesh egg containers permeable to sperm). These experiments, simulating the observed range of natural density, suggested that (1) fertilization rates decreased many times faster than individual gamete production increased over the same range in density, and (2) kelp increased fertilization success at high density when eggs were within 25 cm of a sperm source, but not when spaced 1 m apart. Additional laboratory fertilization experiments at ambient temperatures (3 to 5 °C) indicated that diluted sperm were viable for <1 h, but egg viability was virtually unchanged for >8 h. In short, to the individual the reproductive benefits of aggregating appear to outweigh the costs; and while sperm may be limiting at low population density, eggs may remain viable long enough to be fertilized by sperm from more distant males. Received: 10 February 1998 / Accepted: 22 January 1999     

Experimental evaluation of aggregation behavior in the sea urchin Strongylocentrotus droebachiensis

Defensive aggregation by Strongylocentrotus droebachiensis has been invoked as the fulcrum for the transformation of macroalgal beds into coralline barrens in the northwest Atlantic. We critically examined some of the mechanisms contributing to aggregation behavior by experimentally manipulating sea urchins, purported predators and food during autumn 1983 and spring 1984 both in the laboratory and field. We utilized several approaches to examine a range of sea urchin responses to the presence of food, tethered predators, caged predators, crushed con-specifics in the field and predators in laboratory tanks. Some of the field cages had the property of allowing free passage to sea urchins while retaining lobsters; this allowed distinctions to be made between artifacts caused by cage walls or topographic barriers and unrestricted behavioral responses of sea urchins. The results falsified the hypothesis that aggregations of S. droebachiensis are elicited by predators. Except in the presence of algae, sea urchins always avoided decapod predators (but not sea stars) and fled from the vicinity of injured conspecifics. However, avoidance behavior was subordinate to feeding responses, demonstrated by mass migrations of sea urchins into cages (with lobsters) to feed on algae. We reaffirmed by observation and manipulation previous studies which showed that sea urchins aggregate only in the presence of food. Two types of sea urchin groupings were delineated: (1) surficial two dimensional associations, often caused by topographic or other features which inhibited dispersal and (2) cohesive three-dimensional aggregations induced by food.     

The role of behavioral responses to predators in modifying urchins' (Strongylocentrotus droebachiensis) destructive grazing and seasonal foraging patterns

We documented spatial and temporal patterns of urchins (Strongylocentrotus droebachiensis) and periwinkles (Littorina littorea) in three habitats: a persistent Laminaria longicruris and L. digitata bed; an urchin dominated barrens, and the edge of the kelp bed that formed a boundary between the two. Urchins were rare in the kelp and, when present, always large and well hidden, a pattern we interpret as a response to crab and lobster predation. Urchins were abundant in the barrens, and, in the summer when predaceous fish were active during the day, foraged only at nigh. We observed the formation of a dense urchin feeding front along the kelp bed edge, and these urchins remained exposed and feeding even during the summer. Laboratory experiments demonstrated that aggregations are an effective defense against some predators, and that the presence of crabs increases the tendency of large urchins to aggregate. We hypothesize that healthy Laminaria spp. beds persist because kelp bed associated predators keep urchins at low densities and in hiding. A reduction in predation pressure permits urchin densities to increase to the point where they form aggregations, which provide better defense than hiding. These aggregations then graze destructively on Laminaria spp., forming barrens. These barrens seem to be a new, stable configuration of the system.     

Dietary accumulation of UV-absorbing mycosporine-like amino acids (MAAs) by the green sea urchin (Strongylocentrotus droebachiensis)

The bioaccumulation of ultraviolet radiation-absorbing mycosporine-like amino acids (MAAs) by Strongylocentrotus droebachiensis (Müller) was determined in specimens maintained on MAA-rich and MAA-deficient diets. Individuals were fed either the red alga Mastocarpus stellatus (Stackhouse), which has a high concentration of the MAA shinorine (6.98 nmolmg^–1 dry wt), and trace amounts of the MAAs porphyra-334, asterina-330 and palythine, or the brown alga Laminaria saccharina (Lamouroux), which contains no detectable MAAs. Reproductively spent urchins were fed ad libitum during a 9 mo period in 1992 to 1993 until they were once again gravid. To monitor accumulation, samples of urchin tissues and ingested food were taken throughout the 9 mo period from males and females maintained on each diet, and the concentrations of MAAs determined using high performance liquid chromatography. Urchins maintained on a diet of M. stellatus showed an ovarian shinorine concentration (8.33 nmol mg^–1 dry wt) 25 times higher than those fed L. saccharina. There was no change in MAA content of testes or somatic tissues in either feeding group. More than 99% of the MAAs present in an experimental meal of M. stellatus were removed during passage through the gut. Previous studies have inferred dietary accumulation of MAAs by comparing MAAs present in animal tissues with the consumer's potential diet in the field. This is the first study to have monitored MAA accumulation in animal tissues in conjunction with a controlled diet having a known MAA composition. Concentrating MAAs in the ovaries may provide eggs released into the water column with protection from damaging solar ultraviolet radiation.     

Anaerobic and aerobic energy metabolism in ovaries of the sea urchin Strongylocentrotus droebachiensis

The gonads of sea urchins undergo large changes in mass during their gametogenic cycle. In addition, they have relatively low aerobic capacities and are poorly perfused by the circulatory system and thus are continually hypoxic or anoxic. The present study of Strongylocentrotus droebachiensis investigates seasonal changes in the relationships among mass of the ovaries, pH and P_O2 of the perivisceral coelomic fluid which bathes the ovaries, and partitioning of ovary energy metabolism into its anaerobic and aerobic components. S. droebachiensis were collected at Blue Hill Falls, Maine, USA, from August 1982 to March 1984. We found that from 76 to 92% of the heat dissipated by isolated ovaries of the sea urchin S. droebachiensis derives from anaerobic energy metabolism at partial pressures of oxygen prevailing in vivo. Ovaries from S. droebachiensis have the capacity to produce large amounts of lactate under imposed anoxia, but lactate accounts for only 37% of the total anoxic heat dissipation, which suggests that other end products of anaerobiosis are present. Seasonal changes in pH and P_O2 of the perivisceral coelomic fluid can be explained by a complex relationship among changes in temperature, reproductive condition, and anaerobic and aerobic metabolism in the ovaries, gut and body wall. Seasonal changes in the buffering capacity of the perivisceral coelomic fluid must be determined before the effects of respiratory and metabolic acid production on the acid-base status of the coelomic fluid can be fully understood.     

Alarm responses of the green sea urchin, Strongylocentrotus droebachiensis, induced by chemically labelled durophagous predators and simulated acts of predation

The green sea urchin, Strongylocentrotus droebachiensis, exhibited immediate behavioural responses to waterborne chemosensory cues from two durophagous predators, the Atlantic wolffish, Anarhichas lupus, and the edible crab, Cancer pagurus; as well as from crushed conspecifics and crushed blue mussels, Mytilus edulis. The response patterns were dose dependent and diet dependent. Strong responses were elicited by water conditioned by echinivorous wolffish (97.5%), undiluted urchin extract (73%), undiluted mussel extract (52.5%), and by water conditioned by echinivorous crab (45%). In contrast, urchin extract diluted to 1%, mussel extract diluted to 10%, and water conditioned by predators on a mussel diet elicited weak responses (~20%). Infection by the endoparasitic nematode Echinomermella matsi had no significant effect on the response pattern of S. droebachiensis. There was no conclusive evidence of an alarm response to the predators per se, as the weak response to stimuli from non-echinivorous wolffish and crab, as well as from the seastar Asterias rubens, may have been caused by chemical cues transmitted from their mussel diet. The diet-dependent response to predators suggests that active predators were labelled by chemical cues from their echinoid prey. The chemical cue from echinivorous wolffish acted as both an arrestant and as a repellent, whereas the response to other cues was predominantly or exclusively repellent. The response to echinivorous wolffish was quantitatively stronger than, and qualitatively different from, the response to other stimuli, including undiluted urchin extract. The wolffish is apparently being labelled by a latent chemical cue which derives its potency from activation by, or interaction with, substances in the digestive system of the wolffish. We interpret this novel phenomenon as evidence of alarm signal magnification. The induced behavioural modifications demonstrate the green sea urchins' ability to detect chemical cues associated with active durophagous predators, and would therefore seem to have adaptive potential as a predator defence mechanism.     

Movement and feeding activity of red sea urchins (Strongylocentrotus franciscanus) adjacent to a kelp forest

Movement and feeding were studied in a population of red sea urchins, Strongylocentrotus franciscanus (Agassiz, 1863), found within and immediately seaward of a kelp forest offshore from Santa Cruz, California, USA. Mean sea urchin movements varied from 7.5 cm/day inside the kelp forest to over 50 cm/day at 15 and 100 m outside the kelp forest. The percentage of sea urchins feeding decreased from 66% inside the kelp forest to 16 and 15% at 15 and 100 m outside the kelp forest. These data indicate that movement by these sea urchins is a response to a low food supply.     

Feeding, growth and reproduction of sea urchins (Strongylocentrotus droebachiensis) on single and mixed diets of kelp (Laminaria spp.) and the invasive alga Codium fragile ssp. tomentosoides

Since its introduction to Nova Scotia in the early 1990s, the invasive green alga Codium fragile ssp. tomentosoides has spread rapidly to become a dominant and persistent component of seaweed assemblages in the rocky subtidal zone. To examine the potential of sea urchins (Strongylocentrotus droebachiensis) to regulate Codium, and the potential of the alga to support urchin growth and reproduction, we fed urchins Codium and/or kelp (Laminaria spp., a high quality and preferred native food) in the laboratory for 11 months. Urchins showed a strong and active preference for kelp: they consumed more Laminaria than Codium (as dry weight) in single and mixed diet treatments. Urchins fed only Codium consumed 3.5 times more of the alga than those on a mixed diet, but did not increase their feeding rate in winter when kelp consumption was greatest. Laminaria was consumed at the same rate in single and mixed diets, indicating that the presence of an alternative food had no effect on kelp consumption. Survival and growth (change in test diameter) were lowest on the Codium diet, although the differences among diets were not statistically significant. Urchins on the Codium diet showed no gonadal production over the annual reproductive cycle, compared to a marked rise in gonad index on the Laminaria and mixed diets. Our results indicate that Codium is an unattractive, unpalatable and poor quality food, which is not readily consumed by urchins. Thus we predict that urchins at moderate densities will be much more likely to graze kelp than Codium in mixed stands, creating a mosaic of Codium patches and barren areas. At high densities, however, urchins are likely to destructively graze all seaweeds, although dense stands of the invasive alga may prolong the widespread formation of barrens.     

Allocation of45calcium to body components of starved and fed purple sea urchins (Strongylocentrotus purpuratus)

Several lines of evidence in the literature indicate that environmental stress such as starvation may initiate reallocation of sea urchin endoskeletal tissue. For example, Aristotle's lantern enlarges under conditions of starvation, and sea urchins tagged with tetracycline and then fed develop a distinct growth line, while starved individuals develop a diffuse pattern. We designed anin vivo system to examine stress-related changes in calcification in the purple sea urchinStrongylocentrotus purpuratus. SmallS. purpuratus (ca. 2 cm test diam) were collected from the Mission Bay jetty or Imperial Beach (San Diego, California, USA) in 1987.⁴⁵Ca was incorporated from seawater into all body fractions including the organic tissue/coelomic fluid. In an initial experiment, sea urchins were fed or starved for 4 wk and then post-incubated in isotope. Overall, starved individuals deposited new calcite more slowly than did fed individuals; however, allocation was very different and calcification of teeth of starved sea urchins was nearly as great as in fed individuals. In a second experiment,S. purpuratus were first pre-labeled with isotope and then treated by feeding or starving. More of the labeled calcium was mobilized from the soft tissues and coelomic fluid into calcite in fed than in starved individuals. Growth of the teeth in starved sea urchins was significantly greater than in those fed. We conclude that starvation changes the metabolism of calcium in order to preferentially build teeth. However, we also found no evidence that calcium was resorbed from old skeletal calcite in order to build new skeleton.     

Genetic heterogeneity among adult and recruit red sea urchins, Strongylocentrotus franciscanus

Allozyme electrophoresis was used to characterize genetic variation within and among natural populations of the red sea urchin Strongylocentrotus franciscanus. In 1995 to 1996, adult urchins were sampled from twelve geographically separated populations, seven from northern California and five from southern California (including Santa Rosa Island). Significant population heterogeneity in allelic frequencies was observed at five of six polymorphic loci. No geographic pattern of differentiation was evident; neighboring populations were often more genetically differentiated than distant populations. Northern and southern populations were not consistently distinguishable at any of the six loci. In order to assess within-population genetic variation and patterns of recruitment, large samples were collected from several northern California populations in 1996 and 1997, and were divided into three size classes, roughly representing large adults (>60 mm), medium-sized individuals (31 to 60 mm, “subadults”) and individuals <2 yr of age (≤30 mm test diam, referred to as “recruits”). Comparisons of allelic counts revealed significant spatial and temporal differentiation among size-stratified population samples. Recruit samples differed significantly from adult samples collected at the same locale, and showed extensive between-year variation. Genetic differentiation among recruit samples was much higher in 1997 than in 1996. Between-year differences within populations were always greater for recruits than for adults. Potential explanations for the differentiation of recruit samples include pre- and post-settlement natural selection and high interfamily variance in reproductive success or “sweepstakes” recruitment. Unless recruit differentiation can be attributed to an improbable combination of strong and spatially diverse selection, such differentiation across northern California populations indicates that the larval pool is not well mixed geographically (even on spatial scales <20 km), despite long planktonic larval duration. Received: 6 July 1999 / Accepted: 25 January 2000     

Long-term and trans-life-cycle effects of exposure to ocean acidification in the green sea urchin Strongylocentrotus droebachiensis

Anthropogenic CO₂ emissions are acidifying the world’s oceans. A growing body of evidence demonstrates that ocean acidification can impact survival, growth, development and physiology of marine invertebrates. Here, we tested the impact of long-term (up to 16 months) and trans-life-cycle (adult, embryo/larvae and juvenile) exposure to elevated pCO₂ (1,200 μatm, compared to control 400 μatm) on the green sea urchin Strongylocentrotus droebachiensis. Female fecundity was decreased 4.5-fold when acclimated to elevated pCO₂ for 4 months during reproductive conditioning, while no difference was observed in females acclimated for 16 months. Moreover, adult pre-exposure for 4 months to elevated pCO₂ had a direct negative impact on subsequent larval settlement success. Five to nine times fewer offspring reached the juvenile stage in cultures using gametes collected from adults previously acclimated to high pCO₂ for 4 months. However, no difference in larval survival was observed when adults were pre-exposed for 16 months to elevated pCO₂. pCO₂ had no direct negative impact on juvenile survival except when both larvae and juveniles were raised in elevated pCO₂. These negative effects on settlement success and juvenile survival can be attributed to carry-over effects from adults to larvae and from larvae to juveniles. Our results support the contention that adult sea urchins can acclimate to moderately elevated pCO₂ in a matter of a few months and that carry-over effects can exacerbate the negative impact of ocean acidification on larvae and juveniles.     

Sulfuric acid in the phaeophyte alga Desmarestia munda deters feeding by the sea urchin Strongylocentrotus droebachiensis

The phaeophyte alga Desmarestia munda (Order Desmarestiales) concentrates sulfuric acid (H₂SO₄) in cell vacuoles. The primary function of the accumulated acid in the alga's life history is unknown. To investigate if sulfuric acid works as a chemical defense, grazing by the sea urchin herbivore Strongylocentrotus droebachiensis on D. munda was compared to grazing on four common non-acidic phaeophyte algae (Alaria marginata, Laminaria saccharina, Costaria costata, and Nereocystis luetkeana) in multiple-choice feeding preference experiments. Both fed and starved urchins discriminated among algae; N. luetkeana and L. saccharina were consumed preferentially to D. munda and A. marginata. Fed urchins consumed a smaller quantity of A. marginata and D. munda than the starved urchins (15-28% less). The lack of preference for D. munda was not correlated with the presence of polyphenolic compounds or with nitrogen concentration. A. marginata had a high polyphenolic concentration (0.8% DM, dry mass) compared to that of the other four algae (0.08-0.24% DM). The nutritional content (C:N ratio and percent nitrogen) of the five algae ranged from 10.8 to 16.2 and from 2.08 to 3.55, respectively, but was not correlated with food choice by the urchins. Sulfuric acid made up nearly 16% DM of D. munda tissue and the pH of homogenized tissues averaged 1.99 pH units. Sulfuric acid, added to prepared food to provide a range of pH values from 1.9 to 6.2 pH units, deterred feeding by urchins at pH values Г.5 pH units. This result provides direct evidence that sulfuric acid at concentrations found in algal tissues deters grazing by S. droebachiensis. Therefore, acid accumulation in D. munda provides an important ecological defense against grazing by this herbivore.     

Effects of food type and ration on reproductive maturation and growth of the sea urchin Strongylocentrotus droebachiensis

We investigated the effects of food quality and quantity on reproductive maturation and growth of juveniles of Strongylocentrotus droebachiensis (Müller) in a 22 month laboratory experiment in which we fed sea urchins four diets: (1) kelp (Laminaria spp.) for 6 d wk⁻¹ and mussel (Mytilus spp.) flesh for 1 d wk⁻¹ (KM); (2) kelp for 7 d wk⁻¹ (high ration, KH); (3) kelp for 1 d wk⁻¹ (low ration, KL) and (4) no food other than encrusting coralline algae (NF). At their first and second opportunity for reproduction, all sea urchins in the KM and KH treatments, and most in the KL treatment were reproductively mature, whereas all sea urchins in the NF treatment remained immature. Gonad index differed significantly among all fed treatments at first and second reproduction, and was highest in the KM and lowest in the KL treatment. Gonad index was similar in both sexes at first reproduction, but it was higher in females than in males at second reproduction. Diet had little or no effect on the relative abundance of spermatocytes, spermatozoa, or nutritive phagocytes in testes at first and second reproduction. In ovaries, nutritive phagocytes were significantly more abundant in females in the KM and KH treatments than in the KL treatment at first reproduction, and significantly more abundant in unfed (NF) than fed (KM, KH, KL) females at second reproduction. Mean oocyte size was similar in all fed females at first reproduction, but significantly larger in fed than unfed females at second reproduction. Mean ovum size was similar in all fed females in both reproductive periods. Increase in test diameter was greatest in the KM treatment and smallest in the KL treatment; sea urchins in the NF treatment decreased slightly in size. Survival was 95 to 100% in all fed sea urchins but significantly lower in unfed ones. The feeding rate on kelp was significantly greater in the KL than the KM and KH treatments. In the KM treatment, the feeding rate on kelp increased significantly over a 6 d period after mussel flesh was provided. Our results demonstrate that a diet of high food quality and quantity accelerates reproductive maturation and growth rate, and enhances gonad production and survival in juvenile and young adult S. droebachiensis. These findings contribute to our understanding of the reproductive ecology of S. droebachiensis in habitats with differing food supplies (e.g., kelp beds and barrens). Our results also can be used to improve aquacultural practices for sea urchins. Received: 3 June 1998 / Accepted: 2 February 1999     

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.     

Effects of food ration and feeding regime on growth and reproduction of the sea urchin Strongylocentrotus droebachiensis

To determine the effects food ration and feeding regime on growth and reproduction of Strongylocentrotus droebachiensis (Müller), sea urchins in laboratory aquaria were fed kelp (Laminaria longicruris) supplied at either a high (H, ad libidum daily) or a low (L, ad libidum 1 d wk⁻¹) ration in two successive 12-wk intervals during the reproductive period. After 24 wk, urchins fed the high ration continuously (HH) or for the last 12 wk only (LH) had a significantly greater mean gonad index [(gonad weight/total body weight) × 100] and body weight than urchins fed the low ration continuously (LL) or for the last 12 wk only (HL). Urchins in the HL treatment had a significantly greater gonad index than those in the LL treatment; there was no significant difference in gonad index between the LH and HH treatments. Females had a greater gonad index than males in the low ration (LL and HL) treatments at the end of the experiment; there was no significant difference between sexes in the high ration (LH, HH) treatments. Gametogenesis proceeded to maturation in all treatments and some individuals spawned at the end of the experiment. Females in the high ration (HH and LH) treatments had a greater proportion of nutritive phagocytes in their ovaries than females in the low ration treatments, but there was no effect of feeding treatment on oocyte or ovum size. Feeding treatment had no effect on the relative abundance of nutritive phagocytes in the testes, although the proportion of spermatocytes was higher (and that of spermatozoa lower) in the high ration than in the low ration treatments. Urchins in the high ration treatments had a lower mean jaw height index [(jaw height/test diameter) × 100] and greater mean test diameter than those in the low ration treatments at the end of the experiment, although these differences were not statistically significant. Feeding rate on kelp at the end of the experiment was significantly greater for urchins in the low ration than in the high ration treatments. Our experimental results show that even relatively low rations of kelp support somatic and gonadal growth in S. droebachiensis. Increasing the supply of kelp, particularly during the period of active gametogenesis, results in maximal rates of growth and reproduction. These results suggest that populations of S. droebachiensis in barrens may derive a substantial proportion of their nutrition from drift kelp, which may contribute to their persistence in these habitats. They also explain the large body size, high reproductive effort and fecundity of urchins grazing on kelp beds. These findings have important implications for understanding the dynamics of natural populations of S. droebachiensis and for development of effective aquacultural practices. Received: 17 February 1997 / Accepted: 5 March 1997     

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.     

Effects of temperature and food ration on gonad growth and oogenesis of the green sea urchin, Strongylocentrotus droebachiensis

To assess the effects of both temperature and food ration on gonad growth and oogenesis of the green sea urchin, Strongylocentrotus droebachiensis (O.F. Müller), individuals collected December 1996 (winter experiment) and June 1997 (summer experiment) were maintained for 3 months in one of four experimental treatments: (1) 3 °C and fed ad libitum (high ration), (2) 3 °C and fed one-seventh of the maximum ration (low ration), and (3) 12 °C and fed the high ration; (4) 12 °C and fed the low ration. All individuals were fed an artificial diet and exposed to only 1 h of light every day. At the end of both experiments, mean gonad indices of sea urchins fed the high ration had increased significantly (11–24% and 6–19% in the winter and summer experiments, respectively), while the gonad indices of individuals fed the low ration did not change. At the high ration (both experiments), the increase in gonad index of sea urchins occurred primarily as the result of a significant increase in the mass of nutritive phagocytes, as revealed by histological analyses. Primary oocytes were significantly larger in individuals held at 3 °C than at 12 °C throughout the winter experiment, regardless of food ration; during the summer experiment, primary oocytes were significantly larger in individuals receiving the high ration, regardless of the temperature at which they were held. These results suggest that: (1) food availability is the most important factor regulating energy storage and the relative size of gonads throughout the year, (2) temperature affects the rate of growth and maturation of primary oocytes during the later stages of oogenesis, and (3) once gametogenesis has been initiated, mature ova will be produced, even under conditions of low food availability. Conditions of high food availability in summer and low temperature in winter would thus favor reproductive output in sea urchin populations. Received: 1 March 2000 / Accepted: 4 October 2000     

Growth and mortality of subtidal red sea urchins (Strongylocentrotus franciscanus) at San Nicolas Island,California, USA: problems with models

Red sea urchins, Strongylocentrotus franciscanus, were tagged with tetracycline in 1990 at subtidal sites off San Nicolas Island, California, USA. After one year in the field, the sea urchins were collected and growth increments were measured based on tetracycline marks, which indicated initial slow growth, a maximum rate, and finally a prolonged period of very slow growth. Small red sea urchins (4 cm diam) were estimated to be 3 to 4 yr old, which is much older than has previously been reported. It is estimated that about 12 yr would be required to attain 10 cm diam. Survival has previously been modeled assuming a constant rate. If the population of red sea urchins is assumed to be stable and stationary, annual survival rate was between 71 and 77% yr^-1. Census data for the two years of study have permitted annual survival to be estimated without assuming stable and stationary population structure. Under these conditions, annual survival rate was between 79 and 86% yr^-1. Analysis of transitions in the size distributions from 1990 to 1991 suggested that annual survival may have been sizespecific: 91% yr^-1 for individuals 1.1 to 4.0 cm diam, 82% yr^-1 for individuals 4.1 to 7.0 cm diam, and 63% yr^-1 for those of 7.1 to 10.0 cm diam. An alternative explanation to size-specific survival in our study is sizespecific immigration.     

Manipulation of food and photoperiod promotes out-of-season gametogenesis in the green sea urchin, Strongylocentrotus droebachiensis: implications for aquaculture

Green sea urchins, Strongylocentrotus droebachiensis OF Müller, collected off the coast of New Hampshire, USA, in late February 1995, were brought into the laboratory and fed an artificial diet ad libitum, and subjected to a photoperiod advanced by 4 months. During this study, temperatures and salinities for experimental urchins mirrored those recorded at the collection site. We examined the effects of changes in feeding regime and photoperiod on gametogenesis and compared the experimental urchins with those from the source population. During the 7-month period, experimental urchins showed no detectable changes in mean test height or diameter. Experimental urchins had a significantly higher gonad index (GI) in March, April and May (18 ± 6%) compared with field urchins in March (11 ± 3%). Subsequently, experimental urchins had a mean monthly GI of 25 to 30%, while the mean GI for field urchins was 11 to 13%. Gonial cell mitosis and gametogenesis occurred earlier in experimental male and female urchins compared with field urchins. Stereological and histological observations and stage–frequency data showed that the ovaries of experimental urchins were large because of the accelerated development of nutritive phagocytes, the volume fraction (V _v) of which was 89 to 90% of the gonad, while new vitellogenic primary oocytes occupied <1% V _v. In males, stereological and histological observations and stage–frequency data suggested a mobilization of materials from the nutritive phagocytes beginning between June and August, i.e. earlier than in females, and, by September, new gametes occupied a V _v of 49 ± 3% of the testes. Oocyte size–frequency distributions demonstrated that most primary oocytes were <80 μm in diameter between March and September, suggesting that cold temperatures may be needed for completion of vitellogenesis. We describe changes in the two principal cell types in the germinal epithelium of urchin gonads and indicate how knowledge of their population dynamics may be useful in aquaculture applications. Received: 21 February 1997 / Accepted: 3 June 1998