Artificial rearing of Actinopyga lecanora (Holothuroidea: Holothuriida) with spawning induction using relaxin: Lecithotrophic short larval period

Artificial rearing techniques of larvae and juveniles of the tropical sea cucumber, Actinopyga lecanora were developed for the first time. Broodstock individuals were collected in Yaeyama Islands, Okinawa, subtropical Japan and sorted by the gonad maturity which was determined by biopsy. An artificially-synthesized relaxin peptide, originated from Holothuria (Metriatyla) scabra, was injected into the coelomic fluid of mature A. lecanora at amounts of 3 × 10⁻⁹ to 10 × 10⁻⁹ mol kg⁻¹ body weight, which successfully induced spawning in 10 of 14 females and 3 of 3 males after 26–70 min. The number of eggs spawned per one female individual ranged 2.2–9.5 × 10⁵. The mean diameter of oocytes of A. lecanora individuals which spawned by the induction was 324 ± 13 μm, much larger than those of sea cucumber species having a planktotrophic larval stage. The embryo hatched during gastrula, and the larva metamorphosed to planktonic auricularia followed by doliolaria and finally settled into the benthic stage as pentactula. Time after fertilization to settlement was 4–8 days when larvae were reared in 5–500 L tanks at 26–28 °C, which is one of the shortest development periods among sea cucumbers. Although larvae were supplied with Chaetoceros gracilis cells in most rearing batches, the larvae were later confirmed as lecithotrophic (non-feeding) as (1) larvae were successfully reared until settlement without feeding, and (2) chlorophyll a fluorescence of C. gracilis was not incorporated into the larval intestine as observed under fluorescence microscopy. Auricularia larvae showed poor development of lateral arm protrusion and did not form hyaline spheres. Initial rearing densities under 0.4 larva mL⁻¹ resulted in relatively good survival. Settling larvae were retrieved using polycarbonate corrugated sheets on which periphytic diatoms had been grown beforehand. Survival from hatching to 2 months after settlement was 1.0–1.8%, comparable with values reported for other sea cucumbers. Postsettled juveniles became orange-colored with black dorsal papillae in 50 days after settlement and subsequently gradually obtained a dark green-brown dorsal pigment. Juveniles having anal teeth and adult-like color pattern appeared from 4 months after settlement. Growth of the settled juveniles was 91.8 ± 1.5 μm day⁻¹ from Jun to Dec, reaching 15.11 ± 5.63 mm long in 170 days, while the juveniles negatively grew during winter from Dec to Feb. The lecithotrophic short larval period of A. lecanora has potential for low-cost seed production, which is advantageous for development of aquaculture and restocking.