INTRODUCTION

The nauplius stages of many barnacle species have been described at the light microscope level (Bassindale, 1936; Knight-Jones & Waugh, 1949; Costlow & Bookhout, 1957; Karande, 1974), including those of the British barnacles Balanus balanoides (L.) andBalanus hameri (Ascanius) (Crisp, 1962a, b). Although these studies have included full descriptions of limb structure, it is not possible with the light microscope to resolve the topographical relationship of the limbs and other structures associated with feeding. Attempts to describe the feeding process of barnacle nauplii (Lochhead, 1936; Norris & Crisp, 1953; Gauld, 1959; Moyse & Knight-Jones, 1967) have necessarily been limited for this reason.

Rainbow & Walker (1977) have described the histology, histochemistry and ultrastructure of the barnacle alimentary tract and this study continues the overall investigation into barnacle digestive physiology. Possible sites of alimentary absorption in the barnacle tract have been proposed from histological (see Törnävä, 1948) and histochemical and ultrastructural evidence (Rainbow & Walker, 1977) but no direct studies on sites of absorption have previously been made. Similarly, although it has long been known that barnacles produce discrete faecal pellets (Darwin, 1851), the time interval between ingestion and defaecation has not been determined.

The littoral prawn Palaetnon elegans Rathke regulates the body concentration of zinc at higher external zinc concentrations in the presence of EDTA. The dissolved zinc concentration in artificial sea water corresponding to the threshold of regulation breakdown changed from ca. 100 μg Zn 1-1 (ca. 1.53 μmol Zn 1-1) without EDTA, to ca. 316 μg Zn 1-1 (ca. 4.8 μmo1 Zn 1-1) in the presence of 8.6μmol EDTA 1-1 at 10 °C. The regulated body Zn concentration remained unchanged at 77–79 μg Zn g-1 dry wt with or without EDTA. Increased levels of EDTA in the medium decreased the rate of uptake of labelled zinc by P. elegans. The presence of 3 μmol EDTA 1-1 decreased the mean Zn uptake rate of prawns in 100 fig Zn 1-1 (ca. 1.53 μmol Zn 1-1) from 2.9 to 0.25% of total body Zn g-1 day"1 at 10 °C. The increased ability of P. elegans to regulate zinc in the presence of EDTA may be explained by the reduced bioavailability of the zinc-EDTA complex for uptake. There is marked individual variation in the rate of uptake of labelled zinc in prawns, even in the presence of the same concentration of zinc or zinc-EDTA.

The exposure of two species of talitrid amphipods (Orchestia gammarellus and Orchestia mediterranea) to a range of zinc and copper concentrations in solution has enabled the categorization of the mode of metal accumulation adopted by these supralittoral amphipods. Both species showed net accumulation of dissolved zinc and copper at all experimental exposures between 20 and 1000 μig Zng 1 and 13 and 1000 μg Cu 1 in artificial sea-water. Studies using Zn have shown that the zinc uptake rates of both species (0·430 μg Zn g d, SE 0·032 and 0·408 µg Zn g d, SE 0·073 for O. gammarellus and O. mediterranea respectively at 10°C in 31·6 μg Zn 1) are not significantly different, and are low in comparison with other crustaceans. Laboratory results are related to field conditions, and it is concluded that Firth of Clyde O. gammarellus are more dependent upon a food source in order to meet both their copper and zinc requirements than are O. mediterranea which are able to obtain sufficient metabolic copper from solution. The use of talitrid amphipods in zinc and copper biomonitoring programmes is also discussed.

Barnacle cypris larvae, the specialized settlement stage of the life-cycle, are classical examples of larvae showing gregarious settlement. Gregarious settlement is shown when the larva is induced to settle by established individuals of its own or of a closely related species, reflecting the ability of the larva to recognize its own type, usually by chemo-tactile detection of a particular settlement inducing factor (see Crisp (1974) for review). Sessile barnacles such as species of Balanus or Chthamalus are typically hermaphrodite and gregarious settlement is of selective advantage in promoting the possibility of cross-fertilization, in addition to allowing a settling larva to identify a habitat providing all the necessary conditions for barnacle growth and development.