Surveys of marine benthic distributions are usually performed by sampling from a vessel capable of handling the heavy sampling devices necessary. Because of the inability of such vessels to work in very shallow water or close to the shore the area just beyond the intertidal zone is inaccessible to investigation except by a special vessel (e.g. Mclntyre & Eleftheriou, 1968) or by using diving techniques (e.g. Hughes & Thomas, 1971). This has created a gap in our knowledge of this area, which may be of some importance in reflecting adaptation to effects of land on sea which are not complicated by the effects of tidal exposure.
Three species of leptostracan malacostracous Crustacea occur in the Rockall Trough. The epibenthic Nebalia typhlops, previously recorded at depths of 200–500 m in the north Atlantic but to 1100 m in the Mediterranean, was found at depths of 1990–2925 m. The epibenthic Nebaliella caboti, previously recorded in the northwestern Atlantic at depths of 378 and 2085 m, was caught in the Rockall Trough at 1390–29000 m. The pelagic Nebaliopsis typica only occurred in samples from depths greater than 1500 m. A comparative study of the previously recognized species within the Nebaliacea is required.
Specimens were examined from a number of stations (960 to 2120 m depth) in the Porcupine Seabight and Rockall Trough, N. E. Atlantic Ocean. Both species are gonochoric and show an equal division of the sexes. The ovary of Laetrnogone violacea is a compact nodose structure containing eggs up to 400 pm diameter. The testis is highly digitate, with the wall of each tubule containing numerous infoldings lined with spermatogonia and spermatocytes; spermatozoa reach a maximum size of 2 pm (head diameter). The ovary of Benthogone rosea consists of thin-walled nodose tubules through which the large egg of 750 pm diameter can be clearly seen. The testis of B. rosea is a small digitate structure, the walls of which lack the infoldings found in L. v~olacea; spermatozoa have a head diameter of 5 to 7 pm. In neither species is there any evidence for reproductive seasonality.
Variation in morphological characters seen in a large collection of specimens of the genus Echinosigra from the Rockall Trough can be arranged in a continuous ontological series. This growth series embodied forms agreeing with Mortensen's (1907) descriptions of both E. phiale and E. paradoxa. Bivariate biometrical study showed a well-defined allometric relationship of test length to other body dimensions, particularly the width of the ‘head’ and the width and height of the ‘body’. Immature specimens corresponded to E. phiale whilst the typically elongated ‘neck’ of adults corresponding to E. paradoxa was developed, along with the opening of the genital pores, at around a test length of 20–24 mm.
The vertical distribution of the benthic fauna, including Nematoda, of soft sediments in shallow water was studied along nearshore transects at two sites in Loch Etive and compared with transects worked there previously. The mathematical techniques of hierarchical classification and reciprocal averaging ordination were applied to the species abundances along the transects in an attempt to summarize the data and describe the faunal zonation in a clear and objective way. The inverse reciprocal averaging ordination of pooled transect data described the zonation for all species at each site as a faunal gradient with a single large step at a depth where hydrographic conditions were most variable. This was the most satisfactory description of zonation since it indicated the pattern of sharp faunal change at these critical depths better than previously and allowed direct comparison between transects.The vertical distribution of the Nematoda alone could be simplified to a pattern which paralleled that of the other fauna. A single step or break demarcated a shallow brackish assemblage from a typically marine assemblage found in deeper water. Comparison with brackish fauna of the Baltic and from British estuaries illustrate the coherent nature of the brackish component. Zonation of many nematode species paralleled closely previously published descriptions of zonation in the Exe estuary, indicating the dominant effect of salinity governing distribution along the Etive transects.Classification techniques based on faunal indices of affinity between stations along the transects were less satisfactory. Different indices of faunal similarity yielded different patterns of hierarchy, although an index based on presence/absence data yielded results similar to reciprocal averaging.A group of species intermediate between the brackish and marine elements was isolated from the non-nematode component, but this was not reflected in the nematodes.
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