Growth of spanner crabs, Ranina ranina, off the east coast of Australia

Spanner crab, Ranina ranina, is the target of an important fishery off the east coast of Australia, yet its rate of growth is unknown. Descriptions of growth rates for spanner crabs, like those for other crustaceans, are difficult because during each moult all hard parts that might be used for ageing are lost, and because, as a result of moulting, the growth process is discontinuous. Tag-recapture data were used to model the relationships between the probability of moulting and the number of days at large and between the size increments per moult and the size at tagging. For crabs in similar size classes, males moulted about twice as often as females and had larger increments in size per moult. The relationship between increment per moult and size at tagging was not strong (small r²), but statistically significant. Size increments per moult tended to increase with the pre-moult sizes, indicating that larger crabs tended to grow more in a moult than smaller crabs. However, small crabs moulted more often than large crabs. A probabilistic stepwise growth simulation was used to generate a distribution of growth curves that mimics discontinuous growth patterns and samples the variation in the data.