Aspects of the population biology and ecology of the New Zealand cancer crab, Cancer novaezelandiae (Jacquinot, 1853) :

Abstract

Aspects of the population biology, ecology, life history, and behaviour of Cancer novaezelandiae (Jacquinot, 1853) from Otago coastal waters were described.

Populations of C. novaezelandiae in the Otago region were sampled over a 24 month period to determine relative abundance, patterns of distribution, and seasonal changes in population and moult stage composition. In Blueskin Bay crabs showed marked seasonal cycles in abundance as well as considerable inter-annual variation in peak abundance in beam trawl samples. Seasonal abundances were highest in winter/spring. Crabs were virtually absent in summer/autumn. Moulting peaks for females in June/July and for males in September. It is suggested that there is a winter/spring moulting/mating aggregation of adult crabs within the bay. Seasonal changes of abundance in Otago harbour were not as marked and there appears to be a large resident population present. Juvenile and ovigerous crabs were rarely sampled. Crabs were rare at Victory Beach. Changes in size-frequency distributions and sex ratios of crabs in pot samples are related to changes in crab catchability.

Reproductive periodicity was determined using visual observation of gonads and the gonad index method in conjunction with histological examination of reproductive cells.

Males from Blueskin Bay show continuous testicular activity. Peak gonad development in females occurred in summer. The spawning period extends from October/November through to May/June. Brood size / carapace relationship was described by the power curve y =5.42x(2.6414). Females are capable of producing successive broods without the need for an intervening moult/mate. Females are predicted to be capable of producing 3 broods during a 2 year period.

Size at onset of sexual maturity (SOM), as indicated by reproductive condition and gonad development, was between 65.0 and 69.9 mm CW for males and 70.0 and 74.9 mm CW for females. From analysis of the patterns of relative growth of various secondary sexual characters (chelae, abdomen, gonopods) the size at which moult to puberty occurs was determined at between 40.0 and 45.0 mm CW for both sexes. The discrepancy between SOM and size at moult to puberty indicated that commencement of sexual activity is delayed until the first adult instar is reached. Laboratory growth experiments provided data on moult increment, moult frequency, and intermoult period for juvenile and adult crabs. Approximately 8 juvenile and 5 to 6 adult instar were predicted. A generalised growth curve indicated that growth in juvenile instars is rapid, with crabs able to reach sexual maturity within 1 year of settlement. Growth slows in adults, the result of decreasing percentage moult increments, lengthening intermoult periods, and commitment of energy to reproduction. Maximum predicted age for C. novaezelandiae was 3 to 4 years.

The effects of the diel cycle and tides on the locomotor activity rhythms of C. novaezelandiae from Otago Harbour were investigated in the laboratory and in the field. Subtidal and intertidal potting experiments showed that timing of activity in the field is influenced by daily and tidal variables. Under constant conditions (DD) in the laboratory C. novaezelandiae exhibits an endogenous, circadian, nocturnal locomotor rhythm, with no evidence of a circatidal component within the pattern of activity. Lack of an endogenous tidal oscillator was indicated by (1) the stable phase-angle relationship between initial onset of activity to time of expected sunset, regardless of daily timing of high tides, and (2) the fact that the characteristics of the rhythm in animals entrained by a natural LD cycle after 50 - 60 days in non-tidal conditions, were identical to those of the rhythm seen in freshly collected crabs. Crabs in a light/dark cycle showed a direct responsiveness to tidally-timed pulses of vigorous aeration. Results illustrated the importance of exogenous factors in regulation of timing of activity of epifaunal decapod crustaceans.

Laboratory studies on the foraging behaviour of C. novaezelandiae on the New Zealand cockle Chione stutchburyi (Bivalvia, Veneridae) have characterised the attack methods used by crabs on cockles, the relationship between prey profitability and prey size as measured by prey handling times, and the behavioural patterns which determine prey-size selection. Crabs opened cockles by either crushing or wedging, these attack methods allowed a wide range of prey sizes to be opened including very small and large cockles. Handling times increased exponentially with prey size. Profitability curves calculated from prey handling times indicated that medium sized cockles (18 - 30 mm shell length) were the most profitable (optimal) prey sizes. Prey-size selection experiments showed that crabs actively selected small to medium (optimal) sized cockles that could be easily crushed. Preference for optimal sized cockles remained relatively inflexible despite increasing relative abundances of large suboptimal cockles. This finding differed from that predicted from optimal diet models predicted from the foraging behaviour of portunid crabs. The foraging strategy of C. novaezelandiae simultaneously maximises energy intake and minimises prey handling time.