Small Sea Urchins Research Articles

A cost-effective approach to decreasing the disease transmission of the sea urchin Strongylocentrotus intermedius: New information for seed production and longline culture

Developing a cost-effective approach to effectively removing problematic (dead, diseased and asymptomatic) individuals is important to decrease the disease transmission in seed production and longline culture of the sea urchin Strongylocentrotus intermedius. A simple facility, which was divided into areas M (the upper 70 mm part) and W (the lower 70 mm part), was proposed to separate the problematic and healthy sea urchins based on the differences in their behavioral performances. The V-shaped plastic corridor was inside these two unseparated areas and connected by an upper platform.In experiment 1, fresh kelp was placed in the platform of area M, twenty-five healthy small sea urchins (~10 mm of test diameter) and another twenty-five problematic individuals (previously exposed to the bacteria liquid) were put on the area W of the facility. The present study found that the average proportion of sea urchins distributed in areas M and W was 0.47/0.53. Significantly higher morbidity (~9 times) and poorer performances in righting behavior, foraging behavior, Aristotle's lantern reflex and adhesion force were consistently found in area W than those in area M. The number of sea urchins with diseased Aristotle's lantern was significantly higher (~24 times) than that of those without diseased Aristotle's lantern, among sea urchins without the black-month disease performances in area W. This approach, therefore, is effective in separating problematic sea urchins (including asymptomatic infectors).In experiment 2, five problematic sea urchins (previously exposed to the bacteria liquid but without disease symptoms) and thirty-five healthy sea urchins were maintained in both the new facility group (group S) and the control group (group C) for 7 days. This study found significantly higher morbidity (~2 times) and significantly poorer performances in adhesion force, righting behavior and Aristotle's lantern reflex occurred in group C compared to those in group S. These results indicate that the new facility greatly decreases disease transmission and leads to better performance in fitness-related behaviors of cultured sea urchins.The present study provides valuable information on effectively decreasing the disease transmission in seed production and longline culture of sea urchins.

An effective facility decreases disease transmission and promotes resistance ability of small sea urchins Strongylocentrotus intermedius: A potential application in the longline culture

Mass mortality of small sea urchins Strongylocentrotus intermedius in summer greatly decreases the production efficiency of the longline culture. An effective facility that separates sea urchins from each other and separates sea urchins from macroalgae was proposed to address this problem.Here, a laboratory simulation was conducted for ~6 weeks to investigate survival, food consumption, feeding behavior and resistance ability of S. intermedius cultured in a newly designed facility (the experimental group) and commonly used facility for sea urchins (the control group) at high temperature (23 °C). The present study found that significantly lower morbidity occurred in the experimental group than that in the control group. The disease challenge assays consistently showed that the experimental group had significantly lower morbidity compared to that in the control group. These results suggest that the new facility potentially decreases the disease transmission of sea urchins of longline culture in summer. Further, the critical thermal limit (CTmax) and Aristotle's lantern reflex in the experimental group were significantly greater than those in the control group, suggesting that the new facility leads a better acclimation to the high water temperature for cultured sea urchins.The present study suggests an effective facility specialized for the longline culture of sea urchins, which has great application potential to improve the production efficiency at high water temperatures.

Effects of eliminating interactions in multi-layer culture on survival, food utilization and growth of small sea urchins Strongylocentrotus intermedius at high temperatures

Poor growth and disease transmission of small sea urchins Strongylocentrotus intermedius in summer greatly hamper the production efficiency of the longline culture. Reducing the adverse effects of high stocking density while maintaining high biomass is essential to address these problems. Here, we conducted a laboratory experiment to simulate the multi-layer culture for sea urchins at ambient high temperatures (from 22.2 to 24.5 °C) in summer for ~ 7 weeks. Survival, body size, lantern growth, gut weight, food consumption, Aristotle's lantern reflex, 5-hydroxytryptamine concentration, pepsin activity and gut morphology were subsequently evaluated. The present study found that multi-layer culture led to significantly larger body size than those without multi-layer culture (the control group). This was probably because of the greater feeding capacity (indicated by lantern growth and Aristotle's lantern reflex) and food digestion (indicated by morphology and pepsin activity of gut) in the multi-layer cultured sea urchins. These results indicate that multi-layer is an effective approach to improving the growth efficiency of sea urchins at high temperatures. We assessed whether eliminating interaction further improve these commercially important traits of sea urchins in multi-layer culture. This study found that eliminating interactions displayed greater body size and Aristotle's lantern reflex than those not separated in the multi-layer culture. This approach also significantly reduced the morbidity compared with the control group. These novel findings indicate that eliminating interactions in multi-layer culture greatly contributes to the growth and disease prevention of sea urchins at high temperatures. The present study establishes a new technique for the longline culture of sea urchins in summer and provides valuable information into the longline culture management of other commercially important species (e.g. scallops, abalones and oysters).

Carryover effects of long-term high water temperatures on fitness-related traits of the offspring of the sea urchin Strongylocentrotus intermedius

It is important to study the fitness of marine invertebrates in exposure to high water temperature. We studied whether the long-term high temperatures work on the fitness-related traits (righting behavior, covering behavior, foraging behavior, Aristotle's lantern reflex, body size) of S. intermedius whose parents (males and females) were exposed to ambient or high temperatures (~3 °C higher than the ambient) for a long period of time. The present study found that test diameter, wet body weight and test weight of offspring were not significantly different between temperature treatments, indicating that the parental sea urchins in exposure to high temperatures develop no carryover effects on the body size of the offspring sea urchins. We found no significant difference in foraging behavior, Aristotle's lantern reflex, lantern length and lantern weight of sea urchins after their parents had experienced long-term high temperatures. In addition, no significant change was found in the righting and covering behaviors of sea urchins whose parents were at long-term high temperatures. These results indicate that no significant lasting effects exhibited in the fitness-related behaviors and tissue size after their parents were exposed to high temperatures for a long time. The crushing force of test and test thickness showed no significant difference in the offspring of S. intermedius, no matter whether their parents were exposed to long-term high temperatures or not. The current results enrich our understanding that the parental sea urchin experiencing long-term high temperatures probably develop no carryover effects on the test of their offspring. We found that sea urchins whose parents were exposed to long-term elevated temperatures showed a significantly higher lantern length/test diameter and a significantly lower test height/test diameter in offspring sea urchins due to the thermal experience of their parents, showing the plasticity of lantern and test of offspring sea urchins in response to the thermal experience of their parents. Together with our previous investigation, the present study indicates that small sea urchins are less susceptible to the carryover effects of high temperatures in comparison with the developmental stages of embryos and larvae.

Long-term effects of light spectra on fitness related behaviors and growth of the sea urchin Strongylocentrotus intermedius

The appropriate light spectrum for reseeding small sea urchins is crucial for stock enhancement. Here, we investigated righting, foraging behavior and growth of small sea urchins Strongylocentrotus intermedius exposed to five light spectra (blue light: 440–490 nm, green light: 510–550 nm, red light: 630–670 nm, yellow light: 570–600 nm and white light) for 60 days. In the present study, S. intermedius exposed to red light had a smaller test diameter than those exposed to other light spectra (P < 0.05). No significant difference of daily weight gain was found among light spectra groups (P > 0.05). Consistently, the lantern length and weight of S. intermedius under red light were smallest among five light spectra. These results indicate that red light significantly inhibits the growth of S. intermedius. The successful foraging proportion showed no significant difference among the five light spectra (P > 0.05). Righting response time and foraging time, however, were significantly higher in S. intermedius under blue light than those exposed to other light spectra (both P < 0.01). This highlights the importance and necessity of kelp beds and shelters at the reseeding site to avoid the long-term effects of blue light on foraging and righting behaviors.

Impacts of water temperature on the physiology and behaviours of the sea urchinsHeliocidaris crassispinaandMesocentrotus nudusthat reflect their range extension and disappearance in the Oga Peninsula, northern Honshu, Japan

Ocean warming has facilitated the extension of Heliocidaris crassispina to Oga Peninsula, Japan, where the native species Mesocentrotus nudus has disappeared. To verify the temperature impacts on the physiology and behaviour of the two species, we reared small sea urchins at the increasing–decreasing temperature rate of 2.5 °C·week−1. The righting response, lantern reflex, gonad and gut carbon (C) and nitrogen (N) contents, and feeding rate were investigated. The high and low temperature limits of H. crassispina were 33.3 and 3.9 °C, respectively, which were higher than those of M. nudus. The optimal temperature ranges for behaviour and feeding in H. crassispina were 10.3–31.0 and 10.3–33.4 °C, respectively, which were higher than those in M. nudus. Feeding rates decreased significantly in both species when the temperature approached the high or low temperature limit, but the gut C and N contents of were not greatly affected. At 26–31 °C, the feeding rate significantly decreased in M. nudus but not in H. crassispina, which may explain the replacement of M. nudus by H. crassispina in the Oga Peninsula.

Light spectra regulated foraging and feeding behaviors shed light on stock enhancement of the sea urchin Strongylocentrotus intermedius

Light is an important environmental factor for the fitness of small sea urchins. It is thus important to reveal the appropriate light environments for their stock enhancement. However, the effects of light spectra on fitness related behaviors of sea urchins remain mostly unknown. To reveal appropriate light spectra for stock enhancement of the sea urchin Strongylocentrotus intermedius, we investigated the effects of light spectra on the foraging behavior, phototaxis, Aristotle's lantern reflex of S. intermedius (test diameter: 10.35 ± 0.05 mm). There were five light spectra, including red (630–670 nm), yellow (570–600 nm), green (510–550 nm), blue (440–490 nm) and white light (a mix of blue-based and a little red). We found that successful foraging proportion under blue light (65 %) was obviously higher than those under other light spectra, although no significant difference was detected (P > 0.05). The foraging time of S. intermedius was significantly higher under red light than under blue (P < 0.05) and white light (P < 0.01). Light spectra did not significantly affect the movement distance of S. intermedius (P> 0.05). Light spectra showed no significant effect on phototaxis of S. intermedius (P > 0.05). This indicates that the effect of light spectra on the foraging behavior is not due to phototaxis and movement distance in S. intermedius. Further, Aristotle's lantern reflex of S. intermedius under white light was significantly higher than that under red light (P < 0.01). In summary, S. intermedius showed significantly better foraging and feeding ability, when they were exposed to blue and white light. Since most of the spectrum is short-wavelength light (blue light) in the water below 10 m, we suggest that small S. intermedius are appropriate to be reseeded into clear water areas with the depths more than 10 m.

Light intensity regulates phototaxis, foraging and righting behaviors of the sea urchin Strongylocentrotus intermedius

Small sea urchins Strongylocentrotus intermedius (1–2 cm of test diameter) are exposed to different environments of light intensities after being reseeded to the sea bottom. With little information available about the behavioral responses of S. intermedius to different light intensities in the environment, we carried out an investigation on how S. intermedius is affected by three light intensity environments in terms of phototaxis, foraging and righting behaviors. They were no light (zero lx), low light intensity (24–209 lx) and high light intensity (252–2,280 lx). Light intensity had obvious different effects on phototaxis. In low light intensity, sea urchins moved more and spent significantly more time at the higher intensity (69–209 lx) (P = 0.046). S. intermedius in high light intensity, in contrast, spent significantly more time at lower intensity (252–690 lx) (P = 0.005). Unexpectedly, no significant difference of movement (average velocity and total distance covered) was found among the three light intensities (P > 0.05). Foraging behavior of S. intermedius was significantly different among the light intensities. In the no light environment, only three of ten S. intermedius found food within 7 min. In low light intensity, nine of 10 sea urchins showed successful foraging behavior to the food placed at 209 lx, which was significantly higher than the ratio of the number (two of 10) when food was placed at 24 lx (P = 0.005). In the high light intensity, in contrast, significantly less sea urchins (three of 10) found food placed at the higher light intensity (2,280 lx) compared with the lower light intensity (252 lx) (10/10, P = 0.003). Furthermore, S. intermedius showed significantly longer righting response time in the high light intensity compared with both no light (P = 0.001) and low light intensity (P = 0.031). No significant difference was found in righting behavior between no light and low light intensity (P = 0.892). The present study indicates that light intensity significantly affects phototaxis, foraging and righting behaviors of S. intermedius and that ~200 lx might be the appropriate light intensity for reseeding small S. intermedius.

Growth performance and survival of Holothuria theeli (holothurian) fed with feces of Tripneustes depressus (echinoid): A multi-trophic aquaculture approach

Integrate Multi-Trophic Aquaculture (IMTA) for deposit-feeding sea cucumbers and herbivorous sea urchin is currently a booming industry worldwide, including South America. Hence, it requires novel biotechnological protocols for culturing juvenile sea urchins and sea cucumbers with a high quality of natural food that meet organisms' nutritional needs. This study evaluated the effect on the growth performance (Specific Growth Rate, Feed Conversion Ratio, Feces Production) and survival of the sea urchin Tripneustes depressus (producer of feces) and the sea cucumber Holothuria theeli (consumer sea urchinʼs feces). Two independent experiments were conducted. For T. depressus, experiment #1: diet of seaweeds (D1: Padina durvillaei, D2: Sargassum ecuadoreanum, D3: Kappaphycus alvarezii, D4: mix of three seaweeds) and size categories (small, S: 15.0 ± 1.1 mm test diameter, TD and large, L: 25.0 ± 1.1 mm TD) were considered. For H. theeli, experiment #2: feces of diets of experiment #1 and sediment (with, without sediment) were considered. Experiment #1, SGR: small and large sea urchins fed P. durvillaei grew and gained weight faster, respectively (S: 3.0 ± 0.05 cm TD, 7.9 ± 0.2 g; L: 2.1 ± 0.1 cm TD, 5.3 ± 0.1 g). Sea urchins fed P. durvillaei and those of small size had lower FCR (diet: 7.2 ± 0.6 g; size: 13.1 ± 2.4 g, P < .05) and large and small sea urchins fed P. durvillaei (L: 31.1 ± 2.7 g d−1; S: 11.1 ± 1.2 g d−1) and mixture of seaweeds produced higher amount of feces per day, respectively (L: 28.9 ± 1.9 g d−1; S: 10.9 ± 1.2 g d−1, P < .05). Experiment #2, sea cucumbers fed diets plus sediment grew and gained weight faster (1.69 ± 0.31 cm, 4.1 ± 0.4 g, P < .05) than those diets without sediment (0.96 ± 0.25 cm, 3.3 ± 0.4 g). Sea urchins kept in the starved condition massively died at day 80, but sea cucumbers in the starved condition without sediment reduced their length and weight, by 85%, but did not die. This study highlighted the importance of feces of T. depressus on the growth of the sea cucumber H. theeli. This is a perfectible protocol for production of juveniles of T. depressus and H. theeli at IMTAs in Ecuador.

Feeding of the short-spined sea urchin Strongylocentrotus intermedius on macroalgae and benthic animals

To examine the feeding ecology of the sea urchin Strongylocentrotus intermedius, two types of macroalgal diet (kelp Saccharina longissima and red alga Ptilota filicina) and four types of benthic animal diet [barnacle Balanus glandula, limpet Lottia cassis, free-living Perinereis aibuhitensis and P. aibuhitensis attached to a stainless steel wire (wired polychaete)], were separately or simultaneously provided for urchins of 5- to 60-mm test diameter (TD). Sea urchins of even 3-mm TD could graze the kelp, and began to graze the red alga, the wired polychaete and limpet by 5-mm TD, the barnacle by 8-mm TD and the free-living polychaete by 15-mm TD, respectively. These results suggest that the variety of macroalgae and benthic animals which the urchin can graze increases with the ontogeny of their feeding organs, motility and ability to catch animals. Both juvenile and adult sea urchins preferred to eat the polychaete as well as the kelp fronds. Feed conversion efficiency and daily growth rate of the small sea urchins fed benthic animals like the polychaete were higher than those fed macroalgae. The sea urchins are likely to grow better when feeding on animals than on macroalgae, even in habitats where they can access adequate quantities of kelp.

Small macro‐benthic invertebrates influence the survival, growth and behaviour of juvenile green sea urchins Strongylocentrotus droebachiensis in the laboratory

AbstractRecent settlers of many marine benthic invertebrates are cryptic, which exposes them to a suite of animals that differs from those they may experience as adults, potentially resulting in interactions causing mortality and/or reducing growth. Previous field experiments have indicated that such is the case with small juvenile green sea urchins Strongylocentrotus droebachiensis but which taxa are responsible for the mortality and reduced growth was not determined. A laboratory study was conducted to examine the effects of small macro‐benthic invertebrates, specifically chitons, scaleworms and larger juvenile conspecifics, as well as the full suite of cobble‐dwelling organisms, on the mortality, growth and behaviour of small (&lt;3 mm) juvenile sea urchins. The likelihood of survival of small juvenile sea urchins was lower in the presence of larger juvenile sea urchins or with the full suite of cobble‐dwelling organisms than in the absence of animals. The small juvenile sea urchins survived and grew the best when they were with chitons and scaleworms. The behaviour of small sea urchins with the full suite of cobble‐dwelling organisms was more cryptic than the behaviour of urchins with scaleworms. This study indicates that interactions with the suite of small organisms living amongst cobbles can affect survival, growth and behaviour of small juvenile sea urchins, and that larger juvenile sea urchins can be a source of mortality for smaller conspecifics.

Effects of Dietary Carbohydrate on Weight Gain and Gonad Production in Small Sea Urchins, Lytechinus variegatus.

In experiment 1, juvenile sea urchins (n = 80, 0.088 ± 0.001 g wet weight and 5.72 ± 0.04 mm diameter) were held individually and fed ad libitum one of three semi-purified formulated diets (n = 16 individuals treatment-1). In the diets, protein was held constant (310g kg-1 dry, as fed) and carbohydrate level varied (190, 260, or 380 g kg-1 dry, as fed). Wet weights were measured every 2 weeks. Total wet weight gain was inversely proportional to dietary carbohydrate level and energy content of the respective diet. In experiment 2, sea urchins (5.60 ± 0.48g wet weight, n= 40) fed 190 g kg-1 carbohydrate consumed significantly more dry feed than those fed 260 g kg-1, but not more than those fed 380 g kg-1 carbohydrate. Based on differential feed intake rates, sea urchins that consumed more feed also consumed higher levels of protein and had the highest weight gain. Consequently, protein content and/or protein: energy ratio may be important in determining feed utilization and growth among sea urchins in this study. The average digestible energy intake was approximately 70 kcal kg-1 body weight day-1, suggesting daily caloric intake of juvenile Lytechinus variegatus is lower than in shrimp and fish.

The effect of temperature on feeding and growth characteristics of the sea urchin Lytechinus variegatus fed a formulated feed

We investigated the effect of culture temperature on feed intake, absorption, organismal growth, and tissue production of Lytechinus variegatus by culturing individuals at three different temperatures representing the normal range of temperature exposure in wild populations in the northern Gulf of Mexico. Large L. variegatus (ca. 42 + 0.6 mm diameter, 36 + 1.3 g wet weight, n = 97) were collected at St. Joseph Bay, Florida, in October 2001. Eight sea urchins were held individually in 1-L containers within an 80-L aquarium with recirculated synthetic seawater at 32-ppt salinity. Three aquaria with the containers were each placed in three incubators at temperatures of 16, 22, or 28 °C for 8 weeks. Sea urchins held at 22 °C had the highest rate of feed intake. Feed intake in individuals held at 16 °C decreased significantly during the first 2 weeks of exposure and then increased to values not significantly different from those held at 28 °C by week 6. The dry matter absorption efficiency of individuals held at 28 °C was significantly higher than those held at 16 °C or 22 °C. The percentage of organic matter in the feces did not vary significantly with temperature. Individuals increased significantly in diameter, wet weight, and gonad weight at all temperatures. The wet weights of individuals held at 22 °C were significantly higher than those held at 28 °C or 16 °C, which did not differ significantly. The gut weight varied inversely with temperature. The wet weight of gonads of individuals held at 22 °C was significantly higher than those held at 28 °C, but neither differed significantly from those held at 16 °C. Production efficiencies, both organismal and gonadal, were inversely proportional to temperature, indicating that the overall metabolic cost of production increased with increasing temperatures. Organism production efficiencies were lower and gonad production efficiencies were higher than those found in small sea urchins, emphasizing that patterns of nutrient allocation vary between small and large sea urchins. Physiological processes associated with feed intake, absorption, and nutrient allocations vary with temperature, but allow the sea urchins to maintain growth and gonad production at a variety of temperatures. These data suggest that temperatures near the upper limits do not promote efficient use of resources, an important consideration for future commercial culture. Since gonad (roe) production is the ultimate goal of many aquaculture operations, gonad production efficiencies will provide a valuable tool for evaluating the efficacy of various feeds and feeding conditions on gonad production.

Effect of macrophyte diet and initial size on the survival and somatic growth of sub-adult Strongylocentrotus purpuratus: a laboratory experimental approach

Recently recruited urchins from the same brood, but with different initial sizes, may respond differently to similar environmental factors. The aim of this study was to assess and compare the effects of starvation and diet on the survival, growth rates in size and weight, and gonad index among small and large sub-adult purple sea urchins, Strongylocentrotus purpuratus. Small urchins ranged from 7.3 to 7.8 mm and large urchins from 11.8 to 14.1 mm (test diameters). Two independent experiments were performed. In the first experiment, sea urchins were fed during 22 weeks on Egregia menziesii (ad libitum) and for only 1 day month−1 (starved condition). Feeding regime significantly affected survival, somatic growth rate in size and weight, and gonad index, with higher means in the ad libitum treatments than in starving conditions. A recurrent cannibalism event by conspecifics occurred in small sea urchins under starving conditions. In the second experiment, sea urchins were fed during 13 weeks ad libitum with four diets: kelp (E. menziesii), coralline algae (Bossiella orbigniana), eelgrass (Phyllospadix scouleri) and a mixed diet of the three species. Survival was not affected by diet or urchin size, but diet significantly affected somatic growth rate in size and weight and gonad index. Kelp promoted the highest growth rate (2.23 ± 0.21 mm month−1), the mixed diet produced an intermediate growth (1.26 ± 0.21 mm month−1), while the lowest values corresponded to coralline algae and the eelgrass (0.30 ± 0.12 and 0.10 ± 0.03 mm month−1, respectively, means ± SE). The mean growth rate of small urchins (averaging all diets) was higher than in large specimens (1.17 ± 0.37 and 0.77 ± 0.28 mm month−1, respectively).

Growth assessment on three size classes of the purple sea urchin Paracentrotus lividus using continuous and intermittent feeding regimes

A 6 month experiment (March '05–September, '05) was conducted to assess the somatic and gonadal growth of three different size classes (small, medium and large) of the European sea urchin Paracentrotus lividus under two feeding regimes. For each size class, four replicate cages were fed continuously in which feed was replenished every seven days and four replicate cages were fed intermittently (an alternating weekly regime of feeding/fasting). Test diameter was measured on a monthly basis while gonad index was determined at the end of the experiment. Small sea urchins (10.0–12.9 mm) were significantly affected by intermittent feeding and showed high mortality rates across the experimental period (39.4 ± 3.32%). Continuous feeding supported somatic growth in small and medium sea urchins (10.0–23.0 mm). Feeding regimes did not affect somatic growth of large sea urchins (29.0–33.5 mm). However, large sea urchins on a continuous feeding regime had greater gonad indices (15.76 ± 1.13%) than those on an intermittent regime, which suggests that an intermittent feeding regime cannot support gonadal production.

Consumers of sea urchins, Paracentrotus lividus and Arbacia lixula, in shallow Mediterranean rocky reefs

Underwater observations on fish and asteroid consumers (i.e. predators and scavengers) of sea urchins, Paracentrotus lividus and Arbacia lixula, were carried out at several locations in shallow Mediterranean rocky reefs. Observations conducted in the marine reserve of Torre Guaceto (Adriatic Sea) revealed that sparid fishes, Diplodus sargus and D. vulgaris, are the main fish predators of small (<1 cm in test diameter) and medium (1–4 cm) sea urchins, whereas the labrids Coris julis and Thalassoma pavo preyed only upon small sea urchins. Large D. sargus were able to prey upon small and medium, and occasionally large (>4 cm) sea urchins, whereas medium and small Diplodus preyed mainly upon small sea urchins. The number of sea urchins preyed upon by fishes was negatively related to sea urchin size for both species. P. lividus appeared to be subject to higher predation levels than A. lixula. The scavenger guild comprised 11 fish species, with D. sargus, D. vulgaris, Coris julis and Chromis chromis accounting for about 80% of scavenger fishes. Observations performed at several locations in the Mediterranean on the predatory asteroid Marthasterias glacialis revealed that only 3% of the detected individuals were preying upon sea urchins. Due to the importance of sea urchins for assemblage structure and functioning of Mediterranean rocky reef ecosystems, these results may have also important implications for management of fishing activities.

Changes in sea urchins and kelp following a reduction in sea otter density as a result of the Exxon Valdez oil spill

Interactions between sea otters Enhydra lutris, sea urchins Strongylocentrotus droebachiensis, and kelp were investigated following the reduction in sea otter density in Prince William Sound, Alaska, after the Exxon Valdez oil spill in 1989. At northern Knight Island, a heavily oiled portion of the sound, sea otter abundance was reduced by a minimum of 50 % by the oil spill, and from 1995 through 1998 remained at an estimated 66 % lower than in 1973. Where sea otter densities were reduced, there were proportionally more large sea urchins. However, except in some widely scattered aggregations, both density and biomass of sea urchins were similar in an area of reduced sea otter density compared with an area where sea otters remained about 10 times more abundant. Furthermore, there was no change in kelp abundance in the area of reduced sea otter density. This is in contrast to greatly increased biomass of sea urchins and greatly reduced kelp density observed following an approximate 90 % decline in sea otter abundance in the western Aleutian Islands. The variation in community response to a reduction in sea otters may be related to the magnitude of the reduction and the non-linear response by sea urchins to changes in predator abundance. The number of surviving sea otters may have been high enough to suppress sea urchin populations in Prince William Sound, but not in the Aleutians. Alternatively, differences in response may have been due to differences in the frequency or magnitude of sea urchin recruitment. Densities of small sea urchins were much higher in the Aleutian system even prior to the reduction in sea otters, suggesting a higher rate of recruitment.

Settlement and recruitment of sea urchins (Strongylocentrotus spp.) in a sea-urchin barren ground and a kelp bed: are populations regulated by settlement or post-settlement processes?

I sampled recruitment of very small sea urchins (Strongylocentrotus spp.) by using the anesthetic magnesium chloride to remove individuals from substrata collected in sea-urchin barren grounds (barrens) and kelp beds at Naples Reef near Santa Barbara, California, USA. Preliminary sampling found low numbers of newly settled individuals(<0.6 mm test diam) from April–July in 1984 and 1985, and in April, 1986. In early May, 1986, I found many newly settled seaurchins (0.3 to 0.6 mm, 5 to 17 d old), and I compared the densities of the cohort on several types of natural substrata in barrens and kelp-bed habitats. Newly settled individuals of both purple sea urchins (S. purpuratus) and red sea urchins (S. franciscanus) were present in similar, high densities (1 000 S. purpuratus m-2) on foliose red algal turf, a dominant substratum ofthe kelp bed, and on crustose coralline algae, the dominant substratum of an adjacent barrens. Larvae of S. purpuratus reared and tested in the laboratory showed high rates of settlement on both red algal turf and on crustose coralline algae, but significantly lower rates on rock. Larvae also settled in response to a partiallypurified extract of coralline algae. The reduced settlement on natural rock surfaces relative to either algal treatment and the significant settlement in response to the extract of coralline algae indicate that larvae discriminate between natural substrata and probably respond to a settlement cue other than, or in addition to, a simple microbial (bacterial) film. The similar densities of young recruits of S. purpuratus on dominant substrata of barrens and kelp bed show that, at least in this case, differential settlement cannot explain the high densities of sea urchins in the barrens habitat. Movement between barrens and kelp bed is unlikely given the small sizes of the newly recruited sea urchins relative to the large distances often involved. Reduced post-settlement mortality of newly settled individuals in the barrens remains the most likely mechanism leading to the higher densities of sea urchins in barrens relative to kelp-bed habitats.

Respiration during the reproductive cycle in the sea urchin, Strongylocentrotus purpuratus.

1. The oxygen consumption of small sea urchins differing in gonad index, studied by Warburg-Barcroft manometry, is 26 to 31 µl. O2 per hour per g. wet weight, or between 3.0 to 5.6 µl. O2 per hour per mg. nitrogen.2. The rate of oxygen consumption per unit wet weight is approximately the same regardless of gonad index of the animals.3. The gonad index measures the size of gonad relative to the entire body weight. In a sea urchin the wet weight does not change appreciably within a reproductive cycle as the gonad index increases because the perivisceral fluid, of much the same specific gravity as the tissues, is displaced by the gonads.4. With increase in gonad index, however, the bulk of organic material as measured by nitrogen content in a sea urchin increases, ultimately doubling. Consequently, when the rate of respiration is measured per unit nitrogen content, it is found to fall with rise in gonad index.5. When gonads and other body components are removed from the body, and oxygen consumption measured ...

Food of Sea Otters and Harbor Seals at Amchitka Island

Studies of the food of sea otters (Enhydra lutris) have, because of the limited populations and stringent prohibitions against killing the animals, been necessarily restricted to scat examinations. Scat analyses published by Murie (1940), Williams (1938), and Jones (1951) were alike in showing sea urchin, ( Strongylocentrotus drobachiensis), mussel (Mytilidae), chiton (Polyplacophora), crab (Anomura and Brachyura), and limpet (Acmaeidae) as the dominant food items. Because of the method of feeding and the relative proportion of hard and soft parts, both scat and stomach examinations often do not accurately represent the relative importance of certain food species. For example, the shell of the rock oyster, Pododesmus macroschisma, is usually discarded, and the soft parts of limpets are also extracted from their shells. By contrast, small sea urchins and mussels are simply crunched, shell and all, and swallowed. Large sea urchins may be cracked by beating on a hard surface, but much of the rather fragile shell is swallowed. Miss Fisher (1939) stated that the internal structures of a sea urchin are sucked or licked out after a hole is bitten in the test. Such behavior is not usual when the otters are feeding on the small sea urchins that predominate on Amchitka Island. In captivity, sea otters crack clams by beating them on any available hard surface to make the soft parts accessible, and some fragments of the shell may be swallowed when the body is eaten. Clams do not appear to enter the diet of sea otters in the wild to any appreciable extent. It is not known whether or not sea otters have the ability or inclination to dig clams from a mud or gravel bottom. It appears that their feeding is confined to species that lie on the bottom or are attached to the surface of rocks and marine vegetation. In Table 1, giving the stomach contents of five otters, the same food species appear as in the scat analyses. Sea urchins and mussels are most important. The extent to which sea otters feed on fish is not fully known. A fringed greenling appeared in one of the five stomachs, and occasionally a collection of scats may be made up largely of fish bones. There is some evidence that fish are eaten more frequently in the winter and early spring, when feeding conditions are less favorable. In Alaska, the harbor seal (Phoca vitulina richardi) is the subject of more intensive control measures than any other predator. These measures are mostly confined to the mouths of important salmon rivers where the seals feed primarily on eulachon (Thaleichthys pacificus) in the spring months but take salmon as the season advances (Heintzlemann, et al., 1954). No stomach-examination results are TABLE 1. STOMACH CONTENTS OF FIVE SEA OTTERS FROM AMCHITKA ISLAND, MARCH 1954