Microstomus

Abstract: Salinity tolerance and osmoregulation of a population of Taeniomembras microstomus were studied. Results indicated that this atherinid fish is extremely euryhaline. Probit analysis revealed that the upper and lower L.D.50 values for salinity were 108‰ and 3.3‰, respectively. Salinity tolerance was apparently independent of acclimation, Taeniomembras microstomus has remarkable hypo-osmoregulatory abilities. The freezing point depression of body fluid varied from 0.558o to 2.729o when fish were exposed to a salinity range of 5-120‰.

Abstract: Etropus cyclosquamus new species, the shelf flounder and three sympatric congeners from the eastern coast of the United States-E. rimosus, E. microstomus and E. crossotus-are diagnosed and a key presented. Patterns and microstructure of the scales are sufficient to identify adults of these four species, but gill-raker counts, shapes of the body and jaws and color patterns are also useful distinguishing characters. E. cyclosquamus is distinguished from all known bothids except E. rimosus and E. microstomus by having small accessory scales on the exposed surfaces of scales on the eyed and blind sides; E. microstomus is distinguished from E. cyclosquamus and E. rimosus by having a symmetrical mandible (vs asymmetrical with upturned symphyseal knob) and less developed accessory scales; E. cyclosquamus is distinguished from E. rimosus by having cycloid scales on the blind side (ctenoid in rimosus) and relatively simple ctenoid scales on the snout that rarely extend forward beyond the nostril (vs specialized, heavily ctenoid scales extending forward beyond nostril, often to premaxillary groove). E. cyclosquamus is distributed from North Carolina to Mississippi and is most abundant at depths of 10-30 m, while E. rimosus occurs from North Carolina to northern Florida in the Gulf of Mexico with greatest abundance at depths of 30-60 m. E. microstomus is most abundant on the continental shelf from New York to North Carolina; during warmer months of the year, it interfaces sharply with E. cyclosquamus and E. rimosus near the 17 C isotherm in the Gulf Stream edge at approximately 35040'N latitude (just north of Cape Hatteras). In the winter, E. microstomus disperses southward along shore, occasionally reaching South Carolina or slightly farther; no specimens have been found from the Gulf of Mexico or the Caribbean. E. crosostus occurs from Virginia to northern South America and in the eastern tropical Pacific; between North Carolina and Texas, it is most common inshore and regularly enters estuaries.

Abstract: This histological study is concerned with demonstrating the existence of retinomotor changes in flatfishes. In three species, selected as representatives (sole, Solea solea; plaice, Pleuronectes platessa; and merry sole, Microstomus kitt), it is shown that normal radial displacements of rods, cones, and retinal pigment take place in expected directions in light and darkness, with exceptions which may be important but whose implications have still to be revealed. In the plaice the cones of the dorsal retina are permanently retracted against the basement membrane, and mobile elsewhere. The eyes of flatfishes, therefore, conform to those of other teleosts in photomechan-ical responses to illumination.

Abstract: Patterns of scale formation (onset, completion and spatial pattern) were examined for five species of flatfishes in four families (Paralichthyidae: summer flounder Paralichthys dentatus, smallmouth flounder Etropus microstomus, Scophthalmidae: windowpane Scophthalmus aquosus, Pleuronectidae: winter flounder Pseudopleuronectes americanus and Soleidae: hogchoker Trinectes maculatus, to determine if the patterns are a useful indicator for the transition from the larval to the juvenile periods. In all species (except T. maculatus in which samples were limited), the ontogenetic pattern was very similar with onset of scale formation occurring on the lateral surface of the caudal peduncle, then spreading anteriorly along the presumptive lateral line, then laterally over the body, on to the head, and eventually on to the median fins. The timing of scale formation, relative to fish size, was late relative to other morphological and behavioural characters (i.e. fin ray formation, eye migration and settlement). The onset of scale formation, across all species, occurred at 9·0–27·0 mm total length (LT), at the same approximate size as eye migration and settlement. Completion of scale formation on the body occurred at 22–54 mm LT but completion of scale formation on the fins did not occur until 44–88 mm LT. Thus completion of scale formation in these flatfishes is apparently the last external morphological change to occur during the larval to juvenile transition and, as a result, is not completed until approximately one third (S. aquosus and P. dentatus) to one fourth (P. americanus) or about the same time (E. microstomus and T. maculatus) as the size at first reproduction. This character may have relevance to defining the end of the larval period and the beginning of the juvenile period in flatfishes and other fishes. In addition, the pattern of scale formation may be useful in enhancing understanding of systematics, functional morphology and habitat use.