Ecology, Osmoregulation and Reproductive Biology of the White Steenbras, Lithognathus Lithognathus (Teleostei: Sparidae)

Abstract

Over a one-year period 437 steenbras, Lithognathus lithognathus, ranging from 8-39 em fork length were sampled from the Heuningnes River Estuary. The length-weight relationship was linear and there was no fluctuation in the modal size range throughout the year. Steenbras up to the age of six years and over inhabited the estuary, adapting to large salinity fluctuations. Abundance of food items, mainly Crustacea and Annelida, and virtual absence of predators made the estuary an ideal nursery ground. Ectoparasitic infestation by leeches and copepods was moderately intense without causing any apparent deleterious effects. In a series of experiments designed to study osmoregulation in steenbras, it was found that haematocrits from fish sampled after 48 hours in freshwater were significantly (p<0,01) higher than the seawater controls. Two of the five protein fractions, however, as well as total protein, chloride, sodium, potassium and osmolality were all significantly (p<0,01) decreased in freshwater. Steenbras were unable to survive more than one week in freshwater. Due to capture diuresis the plasma constituents from a freshly captured sample were all significantly (p<0.01) higher when compared to steenbras acclimated for 48 hours in seawater. Gonads from the entire estuarine sample were all infantile, virtually impossible to sex and showed no macroscopic signs of development. Histology of a representative sample showed them to be all hermaphroditic, with mainly testicular-dominant ovotestes. Maturity indexes assigned on the basis of ovarian and testicular development gave an indication of a cyclic rhythm in activity during prematuration, with a June peak. Marine steenbras ovotestes, on the other hand, were large and could be accurately sexed macroscopically. Histology of 42 gonads again revealed a majority of testiculardominant hermaphroditic ovotestes. Maturity indexes indicated peak activity in June and a minimum in September, suggesting a spawning period in July and August. Sex-chromatin (Barr) bodies were detected in liver nuclei of ovarian-dominant ovotestes, but were almost entirely absent from testicular-dominant ovotestes. Prediction of the genetic sex against the actual dominant macroscopic sex, based on the presence or absence of Barr bodies in a sample of steenbras liver sections proved very accurate. This is the first record of sex-chromatin occurring in teleost tissue. There was no indication of sex-reversal, the steenbras remaining permanently hermaphroditic throughout the study year. It is postulated that the steenbras is a rudimentary hermaphrodite with only one of the two sexes functioning throughout the individual's life span. This is possibly a transitionary stage towards eventual gonochorism.