An area in the English Channel of 20 × 40 nautical miles, centred on the Plymouth routine stations L, L and E, was sampled at almost monthly intervals from August 1969 to November 1970, using an improved version of the 1 m net towed at 4 knots on an oblique path. The area includes an inshore region where depths are about 20 fm (37 m) and salinities 35·0‰ or less, and an offshore region with depths over 40 fm (74 m), where salinities can exceed 35·3·. During the first year the offshore region was characterized by water of south-western origin, and temperatures were higher than in the following year when north-western plankton was more in evidence.

The hydrographic conditions in the two sea-lochs is briefly described where relevant to the survey of the benthos. Seasonal changes in temperature and salinity in Loch Etive appear to follow roughly the local coastal pattern but usually with a lag in timing that is considered to be because of the restricting effect of the twin sills. Replenishment of deep-basin water by ventilation over the sills is spasmodic and controlled by surface stratification; the latter being due primarily to freshwater runoff. This situation contrasts with Loch Creran which has only a small catchment and where the seasonal hydrography closely follows the pattern outside. Other possible reasons for the differences observed between the two lochs are discussed.

The bottom fauna of both lochs was sampled semi-quantitatively with an anchor dredge at 22 stations in Loch Etive and 8 in Loch Creran at depths from 7–117 m. A detailed list of the fully marine fauna present is given. The species occurrences in the single-haul samples are compared objectively using a coefficient of similarity of the presence/absence records. An attempt is made to relate the groupings shown up to the classical ‘bottom community’ concept, to the feeding-type categories of the bottom animals and to analysis of particle size of sediment samples taken at the same stations. A correlation to sediment type was apparent. This is considered to owe as much to local hydrographic conditions which the sediment reflect as to the sediment itself.

The development of Callionymus reticulatus Valenciennes, 1837, from the early postlarval stage (2·7 mm) up to about the end of metamorphosis (11·4 mm) is described. The main differences between its postlarvae and the postlarvae of C. lyra Linnaeus, 1758 and C. maculatus Rafinesque-Schmaltz, 1810 are described and illustrated. The spawning periods of the three Callionymus species are briefly discussed and the seasonal occurrence and distribution of their postlarvae off Plymouth are given.

The breeding cycle and growth of a population of Littorina littorea (L.)from an exposed rocky shore is compared with a population from a neighbouring sheltered estuarine habitat. The estuarine population matures earlier in the year, and maximum spawning activity is recorded in January compared with March on the open coast. This is considered to be the result of the higher nutritional status of the estuary. Egg capsules and veliger larvae have been found amongst the detritus on the estuary, and the establishment of an earlier settlement in this population is explained by concluding that a substantial proportion of the egg capsules do not leave the estuary, but undergo their full development in the moist detritus. At both localities individuals are sexually mature at a shell height of about 11–12 mm. Snails from the estuary reproduce for the first time during their second winter compared with the second or third winter for the open coast population.

The vertical distributions and diurnal migrations are analysed for nineteen species of calanoid copepods in the families Lucicutiidae up to and including the Candaciidae. The relative abundance, both vertical and horizontal, is given for each species.

With few exceptions most of these species show little or no vertical migration. There is some suggestion of a reversed (downward) migration for Paracandacia bispinosa andP. simplex but since there are difficulties in analysing the surface samples the evidence is inconclusive. The species which are abundant in the family Heterorhabdidae all have little or no vertical migration and are at least partially vertically segregated by both day and night. Phyllopus impar and P. helgae are clearly vertically segregated and Haloptilus acutifrons and, to a lesser extent, H. ornatus each have two vertically separated populations by both day and night. Two size groups are present in each sex of Lucicutia flavicornis.

The vertical distributions and numbers throughout the water column are analysed for 203 and 176 species in the day and night series respectively; it is concluded that while many species are relatively abundant at one or more particular depth most of them are numerically relatively insignificant in the water column as a whole. Only two species (day) and four species (night) formed more than 5 % of the total numbers caught in their respective series.

The relative abundance, both vertical and horizontal, of the commonest genera are analysed. In the whole water column the relative abundance of most of the genera is constant by both day and night, and, as with the individual species, most of them are relatively scarce.

A new method is proposed for the determination of total dissolved free fatty acids (FFA) in sea water. After preliminary extraction by chloroform, a copper complex is formed which is then extracted by a chloroform-heptane solution. Copper concentration is measured by atomic absorption spectrophotometry. The method is particularly adapted to the determination of long chain FFA (longer than C10) and dissolved organic matter usually present in sea water gives no interference. At the 30 μ/l. level the standard deviation is 1 μ/l. for seven determinations.

Several examples of intermediates between the closely related hermit crabs Pagurus bernhardus (L.) and P. pubescens Krayer are reported. Selected critical characters were examined in the two species and the intermediates and the results accord with the belief that the latter are the result of hybridization. The implied closeness of the relationship between the two species agrees with the evidence presented by the larvae, as pointed out by MacDonald, Pike & Williamson (1957).