DIFFERENCES IN LIFE HISTORY AND SUCCESS BETWEEN SUPRABENTHIC SHELF POPULATIONS OF ARRHIS PHYLLONYX (AMPHIPODA GAMMARIDEA) IN TWO ECOSYSTEMS OF THE GULF OF ST. LAWRENCE

ABSTRACT Shelf populations of the high-arctic deposit-feeding amphipod Arrhis phyllonyx were quantitatively sampled (May-October, day and night) with the Macer-GIROQ suprabenthic sled, mainly at two seasonally constant (0.4–4°C, silt-clay plateau) monitoring stations. One station was in the Lower St. Lawrence Estuary (139 samples in 1970-73 and 35 in 1978-80), the other in the Baie des Chaleurs (49 samples in 1969). Spatial distribution in these two ecosystems was assessed by combined suprabenthic and endobenthic sampling at 99 survey stations. Cold temperature was a more important spatial distributional factor than mud. The species rank among Gammaridea and mean suprabenthic density were higher at the Bay station (third rank, 77.0 individuals/100 m3) than at the Estuary station (12th rank, 2.2 individuals/ 100 m3). Vertical migrations, more extensive in the Bay than in the Estuary and in the equinox than in the solstice period, largely accounted for seasonal changes in observed population density. In the Bay, males and females typically reach maturity in their third year, after a maximum of 14 postmarsupial moults; in the Estuary, an additional subpopulation of small-size individuals matures precociously in its second year after fewer moults. Adults breed only once, in October-November in the Bay and in November-December in the Estuary. In both ecosystems, brooding lasts 7-8 months. Individuals from both subpopulations in the Estuary were of smaller size than in the Bay, at the monitoring stations or at some other depths, regardless of bottom type or water temperature. Low density, small size, dichotomic population structure and delayed breeding of the Estuary population are probably adaptations to poorer (tenfold) and delayed primary production. Populations of Arrhis phyllonyx from the Estuary station, much closer to the continental slope, may also be exposed to competition from species better adapted to a low food supply.

Affiliations: 1: Département de Sciences biologiques, Université de Montreal, C.P. 6128, Montreal, Québec H3C 3J7, Canada; (present address of BS-M) Department of Biology, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada. (Reprint requests to PB.)