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

Pseudocarcinus gigas is a deep-water commercial species. Todate, only three larvae have been collected, despite extensivesampling. Therefore, to assist the targeting of field samplingof larvae, laboratory-based research was undertaken to identifytemperature preferences. Two approaches were taken. The firstinvestigated the behavioural responses of P. gigas larvae totemperature. Experimental columns with discontinuous temperaturegradients were used to test the ability of P. gigas larvae topenetrate thermoclines vertically. Each trial also presentedthe larvae with a choice of two temperature options so resultswere used to infer preferred temperature ranges. In the secondapproach, we measured instar duration, somatic growth and survivalof larvae reared at 12 temperatures between 10.5 and 21.1°C.Behavioural experiments indicated that zoeas I and II couldreadily penetrate experimental thermoclines of 2, 5 and 10°C.Upward swimming was induced in water temperatures 12.7°Cand downward swimming or passive sinking induced by temperatures16.2°C. Intermoult duration decreased withincreasing temperature, although more rapid development wasat the expense of somatic growth. Apparent temperature preferencesof larvae from behavioural trials were also optimal for survivaland growth. No larvae survived to megalopa in those treatmentsbelow the threshold temperature where upward swimming was inducedin behaviour trials. Many megalopas died shortly after moultingfrom zoea V, particularly in treatments with rapid development(>16.8°C) and above those temperatures where downwardmigration of zoeas was induced. Highest abundance of P. gigaslarvae in the field is predicted to be in water of 13–16°C.