Population biology of Dyopedos monacanthus (Crustacea: Amphipoda) on estuarine soft-bottoms: importance of extended parental care and pelagic movements

Abstract

The suspension-feeding amphipod Dyopedos monacanthus (Metzger, 1875) is a common epibenthic amphipod that lives on self-constructed ''mud whips'' (built from filamentous algae, detritus and sediment particles) in estuaries of the northern North Atlantic Ocean. The population biology of D. monacanthus at a shallow subtidal site in the Damariscotta River Estuary (Maine, USA) was examined between July 1995 and July 1997. The resident population at the study site was dominated by adult females during most months of the year. High percentages of subadults were found in late summer/early fall. Often, between 10 and 20% of the adult females were paired with males, and the percentage of ovigerous females varied between 40 and 100%, in- dicating continuous reproduction. The percentage of parental females varied between 40 and 80% during most months, but dropped to levels below 20% during summer/early fall. The average size of amphipods on their own mud whips was4 mm during the summer/ early fall, after which it increased continuously to >7.0 mm in March or April, and then dropped again. In March and April, the average number of eggs and juveniles female )1 was100 eggs and 55 juveniles, while during the summer/early fall the average number of eggs female )1 was 1.4 mm) on their mothers' whips in winter/early spring but not in the summer/fall. The average number of amphipods at the study site was low in late summer/early fall ( 3000 individuals m )2 in April 1996 (>1600 individuals m )2 in May 1997), after which densities decreased again. The decrease of the D. monacanthus population at the study site coincided with a strong increase of amphipods found pelagic in the water column. This behavioural shift occurred when temperatures increased and benthic predators became more abundant and active on shallow soft-bottoms, suggesting that D. monacanthus at the study site is strongly aAected by predation. The eAects are direct (by predation on amphipods) and indirect (by reducing du- ration of extended parental care and enhancing pelagic movements). Both extended parental care and pelagic movements are important behavioural traits of D. mon- acanthus (and other marine amphipods), and signifi- cantly aAect its population dynamics.