Developmental trade-offs in Subantarctic meroplankton communities and the enigma of low decapod diversity in high southern latitudes

Developmental modes, occurrence and distribution patterns of invertebrate larvae were studied in the Subantarctic Magellan region of South America on the basis of quantitative plankton hauls obtained during the 'Victor Hensen' campaign in November 1994. The meroplankton community was found to be numerically dominated by decapod crustacean larvae (47%), followed by polychaetes (20%), echinoderms (16%), cirripedes (8%) and molluscs (7%). A rich decapod community was detected, with 2 thalassinid, 5 brachyuran, 4 anomuran, 6 caridean, 1 astacid and 1 palinurid species/morphotypes identified. Cluster analyses clearly distinguished deep-water stations (250 to 400 m) south of the Straits of Magellan from shallow-water stations (30 to 100 m) in the Beagle Channel, where meroplankton was dominated by decapod larvae (>90%). Three main larval developmental modes, characterised by morphogenesis, mode of larval nutrition and site of larval development, were observed in Magellan decapods: (1) Extended, planktotrophic development of planktonic larvae; (2) abbreviated, planktotrophic development of planktonic larvae; and (3) abbreviated, endotrophic (lecithotrophic) development of demersally living larvae. Several caridean shrimps with abbreviated larval development, which have congeners in the Antarctic, suggest a strong synchronisation between abbreviated planktotrophic larval development and short periods of primary production. This seems to be an essential factor in early life history adaptation for the colonisation of the Antarctic environment. The impoverished Antarctic decapod fauna, with only a few representatives of caridean shrimp species left, may be related to the lack in flexibility of reptant decapods in distributing energy resources between adults and their offspring, which would allow abbreviated planktotrophic larval development.