Summary1. As many species of marine benthic invertebrates have a limited capacity for movement as adults, dispersal mode is often considered as a determinant of geographical ranges, genetic structure and evolutionary history. Species that reproduce without a larval stage can only disperse by floating or rafting. It is proposed that the colonization processes associated with such direct developing species result in spatial distributions that show relatively greater fine scale patchiness than the distributions of species with a larval dispersal stage. This hypothesis was tested by collecting molluscs at different spatial scales in the Isle of Man. 2. Spatial distribution patterns supported the predictions based on dispersal mode. Estimated variance components for species with larval dispersal suggested that the majority of the spatial variation was associated with variation between shores. In comparison, there was relatively more variability within shores for abundance counts of species with direct development. 3. Multivariate analyses reflected the univariate results. An assemblage of direct developers provided a better discrimination between sites (100 m separation) but the group of species with larval dispersal gave a clearer separation of shores (separated by several km). 4. The fine scale spatial structure of direct developing species was reflected in higher average species diversity within quadrats. Species richness also reflected dispersal mode, with a higher fraction of the regional species pool present for direct developers in comparison to species with larval dispersal. This may reflect the improved local persistence of taxa that avoid the larval dispersal stage.

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A long-finned specimen of Beryx decadactylus from off Rockall is reported and the Northeastern Atlantic persistence of juvenile characters in subadult and adult fish is discussed. 1999 The Fisheries Society of the British Isles A specimen of Beryx decadactylus Cuvier, 1829 (Figs 1 and 2) with elongate rays in the pelvic fins and with the spines on the head, especially the infraorbital spine, exceptionally strongly developed, was captured from 200 fathoms (c. 366 m) at 56 50 N 15 45 W, c. 90 miles SW of Rockall, on 7 May 1997, by the Shetland registered vessel Alison Kay (LK 57), skipper J. Anderson, whilst fishing for haddock, Melanogrammus aeglefinus (Linnaeus, 1758). The specimen was frozen for some months before being fixed in 4% formaldehyde solution and transferred to 70% industrial methylated spirits for long-term preservation. The specimen is registered in the collections of the National Museums of Scotland as NMSZ 1997.56.Morphometric data (measurements in mm, rounded down to the nearest 0·5 mm; those c100 mm taken using dial calipers, >100 mm taken using a steel rule): L T 245·0; L F 230·0; L S 205·0; L H 66·0; orbit 27.0; body depth max 109; first infraorbital spine 6·5; intestine (from insertion of pyloric caeca to anus-measured in situ against a length of twine) 77; pectoral fin max 61; pelvic fin-rays V 1 136, V 2 138, V 3 118, V 4 130, V 5 113, V 6 92, V 7 60, V 8 58, V 9 22, V 10 7.Meristic data: D IV+18; A IV+25; P 15; V 1+10; Ll 60 (63 inclusive of scales on caudal base).The genus Beryx comprises two species, B. decadactylus and B. splendens Lowe 1833.