Biology of Nereis spp. and Arenicola marina
Nereis spp
The closely related polychaetes Nereis diversicolor (Fig. 1) and N. virens (Fig. 2) are widely distributed inhabitants of shallow silty and sandy sediments along Northern European coasts. Adult individuals of N. diversicolor are rarely longer than 15 cm whereas adult N. virens may occasionally exceed 50 cm in length. In accordance, the normal density ranges from 500 to 1000 individuals m-2 for N. virens and 3000 to 4000 individuals m-2 for N. diversicolor.[10] The small and opportunistic N. diversicolor is usually confined to a refuge in the intertidal zone by interspecific aggressions from the larger and competitively superior N. virens. Thus, fluctuations in salinity and temperature restrict the landward distribution of the relatively stenohaline and stenothermic N. virens, while competition restricts the seaward distribution of the euryhaline and eurythermic N. diversicolor.[10]
They both live in semi permanent U- or Y-shaped burrows in the sediment (Fig. 3). Through time, burrows may develop into multibranched structures extending to 20 cm depth for N. diversicolor and > 40 cm depth for N. virens with numerous openings at the sediment surface.[12,13] The inhabitants ventilate their burrows vigorously from head to tail by undulatory body movements.
The two Nereis species have been described as omnivores and detritivores feeding by swallowing surface sediment as well as plant and animal remains around the burrow opening.[14] However, N. diversicolor also has the ability to live as a suspension-feeder.[15] The ventilatory water current is then driven through a mucus net spun by the worm near the entrance. Suspended food particles are retained in the net, which is subsequently eaten. No such suspension feeding has been observed in N. virens.
Arenicola marina
The lugworm Arenicola marina (Fig. 4) is abundant on the lower intertidal zone in sandy to muddy-sand sediments along most Western European coasts. It is particularly abundant in the Wadden Sea where the density of lugworms frequently exceeds 50 individuals m-2.[16,17] Adults have a body length of 15 to 25 cm, but individuals as long as 36 cm have been recorded.[18]
A. marina lives head-down in 20 to 40 cm deep J-shaped burrows in the sediment (Fig. 3). It ingests deep subsurface sediment and defecates at the surface.[19] As a result, the sand above the head sinks downward forming a feeding funnel. During defecation, the worm moves backwards until the tail reaches the sediment surface where it ejects a characteristic "worm-like" faecal cast (Fig. 5). Since A. marina ingests sediment of low nutritive value, it must handle large amounts to satisfy its metabolic needs. It is generally believed that A. marina assimilates living bacteria, microphytobenthos, microfauna, meiofauna as well as digestible detritus associated with the sediment sinking down the feeding funnel.[18,20]
Burrow ventilation by A. marina is driven from tail to head by peristaltic movements of the body. Water enters the burrow through the tail opening to the surface and exits by percolation into the sediment in front of the head and up through the feeding funnel.[21]
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