Observations on the Settling Behaviour of Larvae of the Tubeworm Spirorbis borealis Daudin (Polychaeta)

Abstract

Aspects of natural and experimental settling of Spirorbis borealis on the seaweed Fucus serratus L. were compared. Under natural conditions S. borealis appeared to settle preferentially in the concave grooves flanking the midribs, and less commonly on the flatter parts of the fronds. Settlement seldom took place on the convex midrib. The centres of individuals of dense populations, which were spaced along the grooves, were 1-2 mm from those of their nearest neighbours; none of the 2582 examined were closer than 0.5 mm. Experiments showed that larvae tended to avoid settling on P. serratus bearing adults of their own species, or similar shaped objects, when the density of these exceeded 10 per cm² and if alternative settling surfaces of Fucus were provided. Direct observations showed that larvae tended to explore concave areas of the fronds more intensively than other areas and turned away from nearby round, protruding objects during their final settling movements. However, larvae withheld from suitable settling surfaces for increasing periods of time showed a successive abbreviation of the pattern of searching behaviour. When they had been prevented from settling for 8 hours or more they settled on or very close to the point where they first alighted, apparently regardless of the surface contour or the presence of nearby objects. The most densely settled populations encountered naturally in the area (Menai Straits, North Wales) ranged from 12 to 33 individuals per cm² and averaged 21, but experimental densities of 64 to 322 individuals per cm² were obtained readily when larvae competed for limited settling spaces on F. serratus under laboratory conditions. Analysis of the settlement behaviour and its modification with the passage of time indicated how the pattern of settlement observed in nature could result.