Survival of teleost larvae requires growth and development which depend upon endocrine processes. In this dissertation I have examined the ontogeny of elements of the thyroid hormone (TH) and corticosteroid (CS) systems in red drum (Sciaenops ocellatus). Basal cortisol production was first detected 3 days post-hatch (DPH) and a cortisol stress response was present from 6 DPH forward. Changes in steroidogenic enzyme mRNA (CYP11B and CYP21) levels did not correlate with these events. The time necessary to reach peak cortisol levels as well as return to basal levels declined as larvae developed. A second set of studies examined ontogenetic patterns in levels of mRNAs encoding thyroid (soTR[alpha] and soTR[beta]) and corticosteroid (soGR) hormone receptors and assessed the regulation of these mRNAs by exogenous triiodothyronine (T3) and cortisol. soTR[alpha], soTR[beta] and soGR were expressed in all stages of red drum examined. soTR[alpha] levels increased during the time when surging TH levels have been reported. soTR[beta] levels did not differ significantly during development. soGR levels were strongly correlated with those of soTR[alpha]. T3 up-regulated soTR[alpha] and soTR[beta] levels in 7 DPH, but not older larvae. Cross-regulation of receptor mRNAs by exogenous treatment with T3 or cortisol was not observed. Finally, I assessed the influence of a common herbicide, atrazine, on receptor mRNA, TH levels and growth of red drum. In two experiments, red drum exposed to environmentally relevant levels of atrazine did not alter hormone receptor mRNA levels, or TH content. However, atrazine did depress growth in some instances. In conclusion, the expression patterns of hormone receptor mRNA in embryos suggests receptor proteins could be activated by maternal hormones prior to the onset of endogenous hormone production. A correlation between soTR[alpha] and soGR mRNA levels suggests coordinated function of TH and CS systems, although regulatory interactions between these systems were not evident under the conditions in this study. Patterns in soTR[alpha] and soTR[beta] mRNA levels support an important role for TH in the larval to juvenile transformation of red drum larvae. The results also support growing evidence indicating atrazine exposure effects larval growth and may impact their survival in the wild. / text

This collection of studies was designed to understand the mechanisms and consequences of competition among early life stages of red drum (Sciaenops ocellatus), with a special attention to asymmetric competition and multiple-breeding strategy of parents. The overall hypotheses were that (1) red drum larvae show behavioral interactions, and the magnitude of these interactions is explained by the sizes of the competing individuals, (2) red drum larvae compete for food and habitat use, and the competition is asymmetric as determined by size and behavioral interactions, and (3) the parents can reduce negative effects of competition among larvae and increase larval survival by altering their multiple-spawning traits. The laboratory experiments showed the existence of sociality in red drum larvae. The magnitudes of aggressive behavior, vigilant behavior and shoaling behavior were explained by a combination of absolute and relative phenotypes of competing individuals, such as body lengths and body condition. A new method of combining the laboratory experiments and the foraging theory disentangled interference competition for food from exploitative competition, revealing that both absolute and relative body lengths of competing individuals influence feeding efficiencies. In addition, vigilant behavior decreased feeding efficiency regardless of body sizes. The competitor size and behavioral interactions between individuals had different effects on competition for habitat use: the existence of larger individuals prevented the newly settled larvae from entering preferred habitats, whereas the aggressive behavior from the occupants of preferred habitats moved newly settled larvae to the unpreferred habitats. Finally, computer simulations using an individual-based model revealed that as interference competition among larvae increased, the number of spawning events and the spawning interval of the parents increased to lessen competition and maximize total survival of offspring. / text

While substantial progress has been made in replacing live prey with artificial diets in the feeding of marine fish larvae, it still remains impossible to successfully rear larvae on artificial diets without some period of co-feeding live prey or algae. This study investigated the presence and role of the gastrointestinal hormone cholecystokinin (CCK) in the red drum (Sciaenops ocellatus) to gain a better understanding of the factors limiting the utilization of artificial diets by red drum larvae. Work with other fish species has shown that CCK is the principal hormone which regulates the release of pancreatic enzymes into the gut lumen and emphasizes the potential importance of CCK in early red drum larvae. This work investigated the hypothesis that some signal present in the live prey or algae stimulates CCK and thereby initiates the digestive process in the larvae. First, the nucleotide and amino acid sequence of the putative red drum cholecystokinin precursor was determined and the development of CCK immunoreactive cells was examined. This work showed that red drum CCK is highly similar to CCK in other vertebrates and can be detected in the digestive tract of larval red drum within three days after the initiation of exogenous feeding. Next; postprandial trypsin, CCK, and CCK mRNA responses were quantified in red drum juveniles and larvae over a three hour period. Both CCK and trypsin were increased within thirty minutes following feeding while CCK mRNA levels were increased within the next two to three hours. Finally, the trypsin, CCK, and CCK mRNA responses of red drum larvae to homogenates of live prey and algae were examined. Homogenized rotifers appeared to be sufficient to induce both the CCK and trypsin responses in larval red drum. These results suggest that in addition to other factors, some component of live prey may initiate the release of CCK and prime the digestive process. Understanding these factors and their effects in early larvae may allow us to formulate and produce a prepared diet which will support growth and survival to metamorphosis equal to that provided by live feeds. / text