Feeding ecology of the mysid, Mesopodopsis wooldridgei, in a temperate estuary along the eastern seaboard of South Africa

Abstract

Feeding ecology of the mysid, Mesopodopsis wooldridgei, was investigated at six stations in the Kariega estuary during summer (November) 1999 using in vitro incubations and the gut fluorescent technique. Three functional feeding groups were examined, adults (>15 mm), immatures (6-8 mm) and juveniles (<4 mm). Individual levels of gut pigment concentration for adults and immatures ranged from 0.8 to 1.4 ng pigment per individual (ind.-1) and between 0.3 and 1.2 ng pigment ind.-1, respectively. Among the juveniles, gut pigment concentrations ranged from 0.1 to 0.3 ng pigment ind.-1. Gut evacuation rates for adults and immatures were 0.90 and 0.98 h-1 respectively. Juvenile gut evacuation rates were equivalent to 1.38 h-1. Losses of pigment during digestion were 53, 67 and 78% for adults, immatures and juveniles. Carbon derived from the consumption of phytoplankton for adults, immatures and juveniles ranged from 1.06 to 1.77, from 1.40 to 3.39 and from 0.19 to 0.56 μg C ind.-1 day-1. Clearance rates of the three size classes feeding on microzooplankton were on average 29, 12 and 6 ml ind.-1 h-1 for adults, immatures and juveniles. These rates correspond to an ingestion rate of between 0.7 and 1.66 μg C ind.-1 day-1 for adults and between 0.21 and 0.63 μg C ind.-1 day-1 for immatures. Total carbon ingested by juveniles feeding on microzooplankton corresponded to between 0.12 and 0.31 μg C ind.-1 day-1. Carbon derived from the consumption of copepod nauplii and copepodids were equivalent to between 0.8 and 3.5 μg C ind.-1 day-1 for adults and between 0.4 and 1.05 μg C ind.-1 day-1 for immatures. Juveniles did not feed on copepod nauplii. Results of the investigation suggest that the diet of M. wooldridgei in the Kariega estuary changes with development. This is probably due to the inability of larger individuals to feed on the small phytoplankton cells (<10 μm) which dominate total chlorophyll a in the estuary.