Pattern analysis in bryozoan and spirorbid communities. II. Distance sampling methods

Abstract

The spatial pattern of settlements of the bryozoan Alcyonidium polyoum (Hassall) and of Spirorbis spirorbis (L.) on plastic (Perspex) disks has been examined using nearest-neighbour analysis. Cumulative frequency distributions of ω = r 2, where r is the distance from each individual to its nearest neighbour, have been set up and compared with those expected assuming random settlement on each disk. Deviation above the expected line indicates aggregation and deviation below it indicates regularity or spacing apart. At low densities (< 7.4 cm −2) Spirorbis settlements were demonstrably aggregated and at high densities (> 18.2 cm −2) they displayed spacing apart. Alcyonidium ancestrulae showed aggregation at all densities (1.4–45.6 cm −2) under the null hypothesis that when a larva encounters an ancestrula it moves away and settles randomly elsewhere. Truncating the cumulative distributions to consider only the settlement within aggregations still showed no evidence of spacing apart. Under a different null hypothesis, that a larva encountering an ancestrula settles contiguously to it, spacing apart was demonstrated only at unnaturally high densities (> 13 cm −2). It is concluded, on the basis of observed larval exploratory behaviour and naturally occurring settlement densities, that settling Alcyonidium larvae do not space apart. The biological significance of this finding is discussed.