A photographic survey of certain areas of sea floor near Plymouth

Abstract

An improved underwater photographic apparatus has been used to take further series of photographs of the sea bottom near Plymouth. The photographs in the present series are each ¼ m.² in area instead of 1 m.², and at this scale definition is much better.Photographs of the bottom in the Rame Mud area showed ripple marks but no living epifaunal animals. In an area south of the Rame Mud, on a muddy sand and gravel bottom and in a similar area 6 miles south of Looe, dense populations of the brittle-star, Ophiothrix fragilis were photographed. In the area south of Looe this type of population (density more than 100 individuals per m.²) ha s been photographed on three different occasions in 1950–1. In a similar but still denser Ophiothrix population found ¾ mile north-west of Eddystone, there were about 340 individuals per m.², and this aggregation was apparently of long standing, since exceptionally large dredge hauls of Ophiothrix were taken there in the last decade of the nineteenth century. It is suggested that the food supply for these populations, in the form of suspended material, is brought to them largely by the tidal streams and that the crowded beds are in localities where this is likely to happen.