Eels, elopids, notacanths and other elopomorph fishes spawn in the ocean and the hatchlings spend their larval life as pelagic, planktonic organisms. The larvae are known as leptocephali and their transparent, leaf-like body characteristically bears little resemblance to the respective adult form. Planktonic life in the ocean may last for years before the leptocephalus undergoes a morphological transformation and takes on a recognizable fish-like appearance. A prolonged larval life suggests a delay in structural and functional development and this premise was the basis for the present study. Before this work on morphology, chemical analysis of the whole leptocephalous body indicated an internal salinity and osmolality far and above the known values for marine teleost fishes. For that reason, particular attention was given to the structural maturation of the gills, gut and kidney, all of which are intimately concerned with osmoregulation. The leptocephalus of the bandtooth conger Ariosoma balearicum was chosen for morphological studies because of its abundance in the tropical western Atlantic and because the Bathymyrinae, which includes , is a reasonably generalized subfamily of congrid eels. The congrids are primitive and representative of the basic eel plan and, as such, are accorded a rather ancestral position. In order to grasp the fundamental organization of a leptocephalus it was soon discovered that all parts of the body needed to be examined. One by one, a long list of structural peculiarities came to light, each of which seemed to have a direct relation to the functional adaptation of the leptocephalus for a prolonged oceanic existence. The section describing the chondrocranium is particularly detailed and permits comparison with an examination of the leptocephalous skull in Anguilla performed by Norman some fifty years ago. The advantages of current methods for processing tissue and the electron microscope are evident in the micrographs of cell surfaces and intracellular organelles. Gill function in the premetamorphic leptocephalus is limited by structure to a mandibular hemibranch. The four gill arches are present, but the vasculature is undeveloped and gill filaments are absent. The gastrointestinal tract is always devoid of food material and some sections of the intestine do not have a discernible lumen. The size, shape and appearance of the teeth seem unrelated to the source of nutrition. All kidney tubules are aglomerular. In summary, the leptocephalus demonstrates many features of structural and functional immaturity and, until transformed by metamorphosis, appears to be an obligatory inhabitant of the open ocean.

Over 1200 squids were captured by night lighting, trawling, or seining in the northern Gulf of Mexico for laboratory maintenance. Two types of recirculating sea water systems were designed and evaluated: a 2 m circular tank (1500 liter capacity) and a 10 rn long raceway (10,000 liters). Mean laboratory survival was: Loligo plei (12 to 252 mm mantle length, ML) 11 days, maximum 84 days; Loligo pealei (109 to 285 mm ML) 28 days, maximum 7 1 days; Loiiguncula brevis (27 to 99 mm ML) species-specific and were useful indices of the squids' condition. Key factors for lab oratory survival were (1) prevention ofskin damage, (2) tank systems with sufficiently large horizontal dimensions, (3) high quality water, (4) ample food supply, (5) no crowding, (6) maintaining squids ofsimilar size to reduce aggression and cannibalism, and (7) segregating sexes to reduce aggression associated with courtship, mating, and egg laying.

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Individuals of seven species of coleoid cephalopods (three species of octopus, three of squid, and one of cuttlefish), that were cultured and reared in laboratory aquarium systems, had a behavioral defect at hatching which was characterized by an inability to control orientation while swimming. These defective animals were designated as "spinners." An examination of statocysts from individuals of five of the affected species revealed abnormalities of the neuro-epithelial suprastructures: absence or malformation of the statolith of the gravity receptor system and absence of the cupulae of the angular acceleration receptor systems. The sensory epithelia did not differ from those of normal animals, nor did the synaptic structures and relationships, when examined both with scanning and transmission electron microscopy. The abnormalities were compared with congenital defects of the neuropeithelial suprastructures of the vestibular apparatus (especially in mammals). The defects observed in statocysts of spinner animals are thought to be the result of environmental causes, such as the temperature or chemistry of the seawater in the transportation vessels or rearing systems, rather than genetic causes.