Effects of water temperature and thermoclines on larval behaviour and development in the giant crab Pseudocarcinus giga (Lamarck)

Pseudocarcinus gigas is a deep-water commercial species. To date, only three larvae have been collected, despite extensive sampling. Therefore, to assist the targeting of field sampling of larvae, laboratory-based research was undertaken to identify temperature preferences. Two approaches were taken. The first investigated the behavioural responses of P. gigas larvae to temperature. Experimental columns with discontinuous temperature gradients were used to test the ability of P. gigas larvae to penetrate thermoclines vertically. Each trial also presented the larvae with a choice of two temperature options so results were used to infer preferred temperature ranges. In the second approach, we measured instar duration, somatic growth and survival of larvae reared at 12 temperatures between 10.5 and 21.1°C. Behavioural experiments indicated that zoeas I and II could readily penetrate experimental thermoclines of ∼2, 5 and 10°C. Upward swimming was induced in water temperatures ≤12.7°C and downward swimming or passive sinking induced by temperatures ≥16.2°C. Intermoult duration decreased with increasing temperature, although more rapid development was at the expense of somatic growth. Apparent temperature preferences of larvae from behavioural trials were also optimal for survival and growth. No larvae survived to megalopa in those treatments below the threshold temperature where upward swimming was induced in behaviour trials. Many megalopas died shortly after moulting from zoea V, particularly in treatments with rapid development (>16.8°C) and above those temperatures where downward migration of zoeas was induced. Highest abundance of P. gigas larvae in the field is predicted to be in water of 13-16°C.