Aspects of the life-cycle of Loxothylacus panopaei (Gissler), a sacculinid parasite of the mud crab Rhithropanopeus harrisii (Gould): a laboratory study

Abstract

The life cycle of Loxothylacus panopaei, a sacculinid barnacle parasite, has been investigated. At 20% salinity and 25°C larval broods are released every 5–6 days. The free-swimming larval phase involves four nauplius stages and a cypris stage. Development time to the cyprid is rapid -≈48 h. The lecithotrophic larvae occur in two forms - male and female, most easily distinguished at the cypris stage. Size frequency data of cyprids shows only slight overlap of the sexes, but the unequivocal distinguishing feature is the extra sac on the antennule of the male. Female cyprids settle on the megalopa larva, early crab stages and adults of the host Rhithropanopeus harrisii but only within the 24-h post-moult period. Female cyprids do not settle on the zoeal stages of the crab. Male cyprids settle only on virgin externae. R. harrisii is an estuarine mud crab so its larvae need to be retained in an estuary for successful recruitment. It is argued that L. panopaei larvae most probably use the same retention strategy of remaining at the depth of no net flow, although their salinity tolerance is somewhat more restrictive. The parasite interna phase is particularly rapid (25–27 days) and maturation of the externa is also rapid, the first larval brood being released 14–16 days following male inoculation.