Luminescence in decapod crustaceans may be of two kinds. Either a secretion is ejected into the surrounding water as a luminous cloud, or specific light organs, which may be internal or superficial, glow steadily (Balss, 1944). There may be more than 150 photophores in some species of decapod, and the number, position and development of these organs in Systellaspis debilis and species of Sergia have been described by Coutière (1906), Kemp (1910a), Terao (1917) and Gordon (1935). The production of luminous clouds has been observed in the Oplophoridae, Pandalidae and Thalassocarididae of the Caridea, and in the Penaeidae of the Penaeidea (Table 1). Observations of the luminescence of specific light organs or tissues are much less numerous. Luminescence has been observed among the Oplophoridae from the photophores of species of Systellaspis and Oplophorus and from the liver of Hymenodora. The hepatic organs of Parapandalus of the Pandalidae have been seen to luminesce, and among the Penaeidea both the hepatic organs of Pesta of Sergestes and the superficial organs of Sergia have been seen to emit light (for references see Table 1). Undoubted hepatic light organs also occur in Chlorotocoides and Thalassocaris of the Thalassocarididae, and superficial organs in Hymenopenaeus debilis of the Penaeidae. Additional organs of doubtful function which have been described as possible photophores occur on the chelae of the homolid crab Hypsophrys, in the branchial chamber of Sergestes, the pleopods of Parapandalus and the uropods of Acetes (Table 2). There are unconfirmed reports of luminescence in the eye of Pasiphaea etarda (Kiernik, 1908) and in Polycheles (Alcock, 1902) and Leptochela (Chace, 1940), neither of the latter two cases being ascribed to a particular organ, though ‘a greasy glandular patch’ near the oviduct was implicated in Polycheles.

INTRODUCTION

In earlier papers (Allen, Blaxter & Denton, 1976; Blaxter & Denton, 1976) an account was given of the development and structure of the swimbladder-bulla-lateral line system of the herring Clupea harengus L. and sprat Clupea sprattus (L.) and its function in the larval stage. In this paper we describe experiments on juveniles of these species in which the system is fully developed.

An oceanographic survey of Galway Bay was initiated in 1972. Plankton samples were collected on monthly cruises at selected areas in the bay.

Sagitta elegans and Sagitta setosa were found not to be entirely resident in the bay and recruitment of all stages occurred from outside the bay several times during the study. It has also been suggested that the younger stages, especially of S. elegans, were washed out of the bay in the upper layers.

S. elegans was breeding in Galway Bay in July, August and September 1972 and possibly April and May 1973. Breeding was observed in the S. setosa population in September 1972 and from August to November 1973.

Much valuable information on aquarium systems was assembled in the published papers of the 1960 1st International Congress of Aquarology held in Monaco. To date no further congress has been held but Clark & Clark (1964) have collected together a number of papers on sea-water systems and a number of institutes have published details of their aquarium designs, e.g. for the Marine Biological Association at Plymouth (Wilson, 1960), Biologische Anstalt on Helgoland (Anon, 1959), Fishery-Oceanography Center at La Jolla (Lasker & Vlymen, 1969), Marine Laboratory, Aberdeen (Hawkins, 1971) and Sea Research Institute on Texel (de Blok, 1975). A very ambitious aquarium at Dalhousie University, Halifax - the Aquatron Laboratory - has not been fully described although a pamphlet giving some information is available from the curator, Dr Carl Boyd.

The Galway Bay area was sampled quantitatively for plankton during 1972–3. The larvae and/or post-larvae of 67 species of fish were identified from the plankton. The recorded occurrence per m3 and the percentage occurrence of the individual species is presented and discussed, and reference is made to the concurrent occurrence of various chaetognath species and copepod species. The results of previous investigations of the plankton of the Galway Bay area are mentioned.

The histology and histochemistry of whiting skin was studied in larval, juvenile and adult fish. The adult specimens were obtained throughout the year but there was no evidence of seasonal variation in the areas of the body sampled. The skin of the whiting is considerably more complex than that of any other teleost fish which has been described in detail. The epidermis of the mature fish comprises five layers, consisting of Malpighian cells and a variety of other structures including large cystic bullae, which ultrastructurally and histochemically are relatively amorphous but comprise a major proportion of the epidermis. A reticulum of epidermal melanocytes occurs above these structures and two types of mucous cell are described. The major histochemical feature of the epidermis was the alkaline phosphatase reaction, similar to that found in higher animals. Succinic dehydrogenase activity was found at all levels. The epithelial mucins were mainly acid mucopolysaccharide with a small sialic acid component and a greater proportion of sulphated muco-substance. The dermis was morphologically similar to that of other scaled fishes. Mast cells were particularly prominent and occasional non-pigmented ‘melanophores’ were seen.

The mesopelagic alepocephalid fishes Xenodermichthys and Photostylus have unusual compartmented sub-dermal spaces filled with a fluid more dilute than sea water. The fluid is probably lymph, and contributes to the low density of the fish. The external layer superficial to these subdermal spaces consists of cells containing large intracellular paracrystals which form tubular arrays. This external layer is interrupted by numerous photophores (in Photostylus borne on stalks) which have a reddish-violet central core, and are backed by a reflecting layer. The reflector does not contain guanine, it is made up of a series of flattened cells, and reflects blue-green light. The cells of the central core are assumed to be photocytes and are in connexion with apparent nerve terminals, but in neither fish has luminescence of the photophores been observed.

The haematocrit for Prionace was found to be 22.3%, which is similar to values reported for other carcharhinid and sphyrnid species. Haemoglobin content and mean haemoglobin concentration were respectively 57 g/100 ml and 257%. Erythrocytes were generally larger than reported for other carcharhinids.