Periodic swimming behaviour of amphipods in Wellington Harbour

Abstract

Twenty‐four hour, lunar‐month, and seasonal, samples of amphipods were taken using a light trap from Karaka Wharf, Wellington Harbour, New Zealand (41° 16'S, 174° 51'E). Average number of amphipods for the 31 night samples was 255 per 30‐min sample; few amphipods were caught in daylight samples. Algae‐dwelling species formed 80.6%, infaunal species 16.6%, and pelagic species 2.8% of the total amphipods caught. Infaunal species appear early in the dark period and numbers decline during the night; numbers of algae‐dwelling species build up during the night and reach a peak before dawn. Numbers of infaunal species increase on days 1 and 20 of the lunar month, whereas algae‐dwelling species show no lunar rhythmicity but numbers increase in turbulent sea conditions. Seasonal sample numbers were erratic, greatest numbers being taken in early August and small peaks coinciding with spring and autumn sea temperature changes. Peak, swimming activity by adults of the infaunal lysianassid Parawaldeckia stephenseni was on the early ebb tide; juveniles continued to swim until dawn. Ovigerous females were present in the swimming population from days 1 to 12 of the lunar month; non‐ovigerous females are mated by day 24 and disappear from the swimming population. Males swim throughout the lunar month. Average number of eggs carried is 12, females mature at 3.6 mm, and numbers of eggs carried increase after each successive moult.