Development of Numerical Larval Model Based on Swimming Behavior for Larval Snow Crab Reared in the Laboratory

Abstract

The planktonic larval snow crab Chionoecetes opilio come up to near the sea surface induced by underwater light after hatching, and start descending when their weight, which increases with growth, exceeds the upward force produced by their swimming behavior towards the surface. Based on these activities, we developed a model representing the swimming pattern of the planktonic larvae until they acquire settlement. The larvae were put in the experimental tank with the same water conditions, and a monochromatic light of different wavelength, from 430 to 645 nm, was irradiated from above. The planktonic larvae in the zoea stage responded sensitively to blue light, showing positive phototaxis. Furthermore, experiments were made under various conditions with water temperatures from 3 to 24℃ and with salinities from 30.0 to 35.0 to examine the geotaxis of the larvae. A strong swimming inhibition was observed in a condition with a water temperature of 21℃ or more, but no influence by salinity was found. Based on these findings, we calculated the average upward swimming force of the larvae and compared it with their weight that produced a downward force. As a result, it was found that the sedimentation of the larvae, caused by their weight, starts at the megalopa stage.

The moulting cycle and changes in body density in larvae of the snow crabChionoecetes opilio(Brachyura: Majoidea) under laboratory conditions

“…Laboratory experiments have shown that larval behaviour changes in response to abiotic and biotic factors, including light intensity and wavelength, barometric pressure, gravity, temperature, salinity and chemical cues from predators (Sulkin 1984;Forward 1988Forward , 2009). Moreover, the direction of larval swimming can be modelled in terms of drag forces, the downward gravitational force and the upward buoyant force (Konishi et al 2011); body density is an important dimension for calculating these parameters. However, larval body density has been examined for only a few decapod crustacean species (Mizumoto et al 2012;Hamasaki et al 2013;Ichikawa et al 2014), and no study has investigated the ontogeny of larval body density in relation to the moulting cycle.…”

“…A number of studies have elucidated the general biology of snow crab larvae, focused on managing stocks or on the mechanisms of fluctuations in stock abundance (e.g., Kon 1980;Incze et al 1984;Davidson & Chin 1991;Lovrich & Ouellet 1994). Their phototactic and geotactic behaviours have been studied to better understand the mechanisms of their spatiotemporal distribution (Kogane et al 2007;Konishi et al 2011). However, their body densities were measured using specimens preserved in formalin solutions (Konishi et al 2011) and might not reflect values in living animals.…”

“…Their phototactic and geotactic behaviours have been studied to better understand the mechanisms of their spatiotemporal distribution (Kogane et al 2007;Konishi et al 2011). However, their body densities were measured using specimens preserved in formalin solutions (Konishi et al 2011) and might not reflect values in living animals. Moreover, moult-cycle stages have not been fully defined for snow crab larvae; Incze et al (1984) described the epidermal retraction process in first and second zoeae caught from the plankton.…”

“…Therefore, tissue growth might be related to the rapid increase in the body density during the premoult period in snow crab larvae. Konishi et al (2011) measured the body densities of snow crab larvae using formalin-preserved specimens and reported that their body densities increased with larval development. These observations differ from values obtained using living animals in our study, which presumably are more reliable indicators of the natural state.…”

“…Kogane et al (2007) also observed that first zoeae floated in the upper layer of the rearing water, but almost all of the later larvae sank to the bottom of the rearing tank. Moreover, Konishi et al (2011) reported that zoeae swam more frequently than megalopae. Snow crab larvae might sink with larval development because of decreasing upward swimming activity in the natural habitat.…”