ABSTRACT
In the Lower St. Lawrence Estuary, eastern Canada, the 1991, 1992 and 1993 spawnings of the sea cucumber Cucumaria frondosa (Gunnerus) occurred at the beginning of June, induced by the first significant increase of phytoplanktonic cells. The study of developmental biology indicated that the buoyant embryos develops into a pentactula 9 d after the fertilization, near the surface of the water column. Settlement 48 d after fertilization, is helped by the progressive loss of buoyancy by the embryo. The 5 unbranched tentacles are used for the first settlement. Experiments showed that gravel or rock cervices are favorised by the larvae during settlement. Under lighting conditions the newly settled larvae migrate to the shady side of the rock helped by the tentacles. Under dark conditions the migrations are delayed as long as light is not provided. Final settlement occurs when ambulacral podia come in contact with the substrate, when the embryos lose buoyancy (3 to 40 h after the first settlement). After reaching about 2.8 mm in length (4–5 months later), the embryo progressively moves to the illuminated crevice always on rocky surfaces. Laboratory experiments indicate that the growth rate is slow until individuals reach 34 mm long (the first two years). In the field when individuals reach 40 mm until 50 mm long they accelerate their growth rate to attain 2 mm a month. Furthermore the growth rate always attains a peak up to 5 mm.mo−1, during spring and summer with the phytoplanktonic blooms and temperature increase. The maximum growth rate and the minimum time to attained the sexual maturity increase with depth, attaining a peak at the deepest waters studied (15–20 m). The sea cucumbers migrate from the protected area to the exposed side of the substrate only when the minimum safe size of 35 mm is attained, in order to escape the predation. The major steps of embryos mortality during the development occur during fertilization, hatching and metamorphosis when 80% of individuals die. Field observations suggest that small animals under 150 mm are abundant mostly in the shallow areas (<15 m, photic zone) and migrate progressively to deeper water when reaching ≈130 mm long, especially during late fall. The majority of larger animals (350 mm equivalent) can therefore be found at depths between 20–60 m. Laboratory experiments showed that the best culture conditions for the embryos of C. frondosa were at 12 °C, with a pH around 8, and a salinity of 26. Our results indicated that gonads began to develop when individuals reached 9 mm (24 mo old), remained immature at 45 mm (36 mo old) and attained the size at sexual maturity at 90 mm (47 mo old sea cucumber). We estimate, using the Brody-Bertalanffy equation, that around 18 years of growth are necessary before individuals reach 350 mm (≈950 g) under natural environmental conditions. Very abundant in nature (10–20 ind.m−2), C. frondosa is of interest as a potential species for commercial fisheries in the North Eastern coast of America.
