Sea Urchin Larviculture

Echinoids have been successfully raised in the laboratory for over 100 years (MacBride 1903). The reproductive periodicity for many echinoid species is well described; temperate water species in culture tend to have one spawning period per year (Himmelman 1977; Byrne 1993; Kelly 2000; Fig. 1) and brood stock are collected locally. Gravid individuals are induced to spawn either by temperature shock or, commonly, by injection of 0.5 M KCl to the coelom via the peristomal membrane. The concentration of sperm allowed to mix with the eggs must be controlled to optimise fertilisation and development success rates. The fertilised eggs hatch in approximately 10-15 h, depending on the species, to release a ciliated blastula which develops to the four-armed, then six-armed, then eight-armed pluteus larvae (Fig. 2). To raise large numbers of larvae in a commercial context the culture techniques must be refined in terms of food quality and quantity, larval density and water quality; and then shown to be effective once scaled up to large batches of larvae (>100,000). Static (no through-flow) aerated systems with a variable number of complete or

Fig. 1. Annual reproductive cycles (Nov 1995 to Oct 1997) of the sea urchin Psammechinus miliaris in a Scottish sea loch as described by the gonad index (GI). GI was calculated as wet weight of gonad divided by drained wet weight of sea urchin expressed as a percentage (n=10) for two littoral (LA and LB) and two subtidal (SA and SB) populations. Seawater temperature (°C) and day length (h) a are illustrated. Error bars represent 95 % confidence limits. (Kelly 2000)

Fig. 1. Annual reproductive cycles (Nov 1995 to Oct 1997) of the sea urchin Psammechinus miliaris in a Scottish sea loch as described by the gonad index (GI). GI was calculated as wet weight of gonad divided by drained wet weight of sea urchin expressed as a percentage (n=10) for two littoral (LA and LB) and two subtidal (SA and SB) populations. Seawater temperature (°C) and day length (h) a are illustrated. Error bars represent 95 % confidence limits. (Kelly 2000)

partial water changes throughout the larval life have been widely used (Fenaux et al. 1988; Leighton 1995; Grosjean et al. 1998; Kelly et al. 2000). In large-scale culture in Japan, partial exchange systems (Sakai et al. 2004) and continuous flow systems are used, the water flow being increased as the larvae develop (Hagen 1996). Upwelling silos, of the type used for fragile halibut yolk-sac larvae, have been tested on a small scale (M. Russell, Villanova University, USA, pers. comm.) and may also prove suitable for the large-scale culture of sea urchin larvae.

The planktonic diatom Chaetoceros gracilis is widely used as larval food in sea urchin hatcheries in Japan (Sakai et al. 2004). Many studies have compared different species or combinations of species of microalgae as larval foods for other species of sea urchin. Some species of microalgae used regularly and with success include Isocrysis galbana (Gonzalez et al. 1987), Cricosphaera (Hymenomonas) elongata (Fenaux et al. 1988) and C. carterae (Leighton 1995), the diatom Phaeodactylum tricornutum (Grosjean et al. 1998) and Dunaliella tertiolecta (Kelly et al. 2000; Jimmy et al. 2003). One noteworthy observation from these studies is there is no one optimal larval diet; different sea urchin species are reported to respond best to different algae. Of course, the biochemical and therefore nutritional value of the same species of microalgae, grown in different laboratories, may not be identical. However, it seems likely that there are true species-specific differences in echinoid larval dietary requirements.

Echinoid larvae demonstrate considerable plasticity in their morphology in response to varying food ration and quality. Growing larvae must increase

Echinodermata Life Cycle

Fig. 2. Life cycle of the regular echinoid Psammechinus miliaris in culture. Gravid adults shed their gametes to seawater where fertilisation and first cleavage of the developing embryo occur within hours. Over the next 21 days, the planktonic larvae, maintained in aerated 250-l containers of seawater and fed microalgae, develop to a point where they are competent to settle. Newly metamorphosed juveniles are maintained on PVC wave plates coated with diatoms. At approximately 5-mm test diameter they are weaned to other foods (soft macroalgae or artificial diets) and transferred to a grow-out system where they mature to market size (40-50 mm test diameter)

Fig. 2. Life cycle of the regular echinoid Psammechinus miliaris in culture. Gravid adults shed their gametes to seawater where fertilisation and first cleavage of the developing embryo occur within hours. Over the next 21 days, the planktonic larvae, maintained in aerated 250-l containers of seawater and fed microalgae, develop to a point where they are competent to settle. Newly metamorphosed juveniles are maintained on PVC wave plates coated with diatoms. At approximately 5-mm test diameter they are weaned to other foods (soft macroalgae or artificial diets) and transferred to a grow-out system where they mature to market size (40-50 mm test diameter)

the ciliated band length in order to increase feeding capability (McEdward 1984; Strathmann et al. 1992). Ciliated band length is increased by increasing arm length and by developing additional pairs of larval arms. The relative proportions of the larval body, e.g. post-oral arm length to larval body length, can therefore be a useful indicator of the nutritional status of larvae in culture (Fig. 3). Underfeeding will increase arm length relative to body length and overfed larvae may show a reduction in the length of the larval and in particular post-oral arms (Kelly et al. 2000; Jimmy et al. 2003).

One labour-intensive aspect of larval culture is the need for the simultaneous production of microalgae as live feed. However, sea urchin larvae may prove suited to culture using artificial diets, as research on Lytechinus variega-tus (J.M. Lawrence, University of South Florida, USA, pers. comm.) has shown. It is the lipid or fatty acid component that is lost or destroyed in some forms of preserved algae. For example, their loss renders spray-dried microalgae a relatively poor food source for bivalve larvae which require poly- and highly unsaturated fatty acids (PUFAs and HUFAs) (Caers et al. 1998). Some species of sea urchin larvae have been shown to grow well when fed the green microalga Dunaliella tertiolecta (Kelly et al. 2000, Jimmy et al. 2003), which is

Fig. 3. Relative proportions of the echinoid larva that can be used as a measure of its nutritional status. a Larval length; b larval body length; c larval width; d post-oral arm length; e rudiment length; R echi-norudiment; PO post-oral arm c

Fig. 3. Relative proportions of the echinoid larva that can be used as a measure of its nutritional status. a Larval length; b larval body length; c larval width; d post-oral arm length; e rudiment length; R echi-norudiment; PO post-oral arm c known to be deficient in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Therefore, sea urchins may prove suited to culture using spray-dried or other preserved algal preparations.

Metamorphosis and the Post-Larval Stage

Sea urchin juveniles have been produced on a commercial or semi-commercial scale by hatcheries in Japan, South Korea, Ireland, Norway, Scotland and in British Columbia, Canada. When deemed competent to settle (judged by the size and state of development of the echinorudiment; Fig. 3), sea urchin larvae are presented with a substrate likely to induce metamorphosis, but which will subsequently serve as a food source for the early juvenile. Most cul-turists use a natural biofilm or a specially seeded diatom substrate created from species isolated locally and grown on a PVC wave plate. Optimising diets for the early juveniles and/or the replacement of diatom biofilms with artificial diets is probably one of the most challenging areas left to research. The variation in size and subsequent variation in growth rates of post-larvae remain a bottleneck in the supply of hatchery-reared juveniles. Hatchery-reared juveniles are robust enough to survive transfer to sea cages or other grow-out systems from a small size (5-mm test diameter) (Kelly 2002; Sakai et al. 2004). At this point they are weaned onto other diets, soft macroalgae or artificial diets, depending on the grow-out system.

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  • consolata marchesi
    What is the post oral arm on a larvea?
    29 days ago

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