The barnacle Notobalanus flosculus as an indicator of the intermolt period of the male lithodid crab Paralomis granulosa

Abstract

The exoskeleton of the stone crab Paralomis granulosa (Decapoda: Lithodidae) is one of the sub- strates on which the barnacle Notobalanus flosculus set- tles in the Beagle Channel of Tierra del Fuego, Argentina (55� S6 7� W). The presence of such an epi- biont can be useful for estimating the intermolt period of pre-recruit and recruited crabs to the legal, fishable stock of crab. We undertook an investigation of the timing of N. flosculus settlement on P. granulosa exoskeletons, and their subsequent loss during molting, as a step toward forecasting fishable stock biomass. Between December 1996 and December 1998, 10-15 male P. granulosa with the epibiont N. flosculus were sampled each month in the fishery catches. Almost every month from 1995 to 1998 we evaluated the carapace age of 30,051 crabs, as judged from the degree and type of epibionts. We supplemented our fishery-based investigation with an overlapping investigation of the timing of N. flosculus settlement on artificial collectors. In September 1996, we deployed 240 clay tiles as collectors which were checked monthly for the presence of newly settled N. flosculus. From 1995 to 1997 the prevalence of N. flosculus as an epibiont of P. granulosa was 20% in male crabs >80 mm carapace length (CL). P. granulosa molted in December, yet the frequency of molted crabs >80 mm CL de- creased with size and the prevalence of barnacles was significantly greater than on crabs <80 mm CL. N. flosculus grew well as an epibiont on P. granulosa and remained on crab exoskeletons for 13 months. Our observation that the densest settlements of barnacle cohorts on the artificial collectors occurred previously or simultaneously with the molting period of male P. granulosa provides evidence that P. granulosa carry- ing N. flosculus between January and November skipped their molt. Moreover, barnacle cohorts of December, February and March seemed not to settle onto crab carapaces. Our conclusion can help us calculate the proportion of skip-molters in the population, and sub- sequently explain or forecast changes in the fishable stock biomass of crab after each molting season.