Cynthia K. Faulk

Batch spawning facilitates transfer of an essential nutrient from diet to eggs in a marine fish

Fatty acid composition of eggs affects development, growth and ecological performance of fish embryos and larvae, with potential consequences for recruitment success. Essential fatty acids in eggs derive from the maternal diet, and the time between ingestion and deposition in eggs is ecologically important but unknown. We examined the dynamics of diet–egg transfer of arachidonic acid (ARA) in the batch-spawning fish, red drum (Sciaenops ocellatus), by measuring ARA concentrations in eggs after a single diet shift and during a period of irregular variations in diet. ARA concentrations in eggs changed within 2–16 days of a diet shift. The rate of change was proportional to the magnitude of the shift, with no evidence of equilibration. These results are not consistent with eggs being assembled entirely from accumulated body stores. The immediate source of ARA in eggs appears to be the recent diet. We propose that batch spawning produces rapid diet–egg transfer of ARA because it removes large amounts of fatty acids from the body and prevents equilibration. The immediacy of the diet–egg connection suggests that spawning migration combined with short-interval batch spawning may have evolved to take advantage of nutrients critical for offspring survival that are available at the spawning site.

Ocean acidification increases fatty acids levels of larval fish

Rising levels of anthropogenic carbon dioxide in the atmosphere are acidifying the oceans and producing diverse and important effects on marine ecosystems, including the production of fatty acids (FAs) by primary producers and their transfer through food webs. FAs, particularly essential FAs, are necessary for normal structure and function in animals and influence composition and trophic structure of marine food webs. To test the effect of ocean acidification (OA) on the FA composition of fish, we conducted a replicated experiment in which larvae of the marine fish red drum (Sciaenops ocellatus) were reared under a climate change scenario of elevated CO2 levels (2100 µatm) and under current control levels (400 µatm). We found significantly higher whole-body levels of FAs, including nine of the 11 essential FAs, and altered relative proportions of FAs in the larvae reared under higher levels of CO2. Consequences of this effect of OA could include alterations in performance and survival of fish larvae and transfer of FAs through food webs.

Developmental expression, differential hormonal regulation and evolution of thyroid and glucocorticoid receptor variants in a marine acanthomorph teleost (Sciaenops ocellatus)

Interactions between the thyroid hormone (TH) and corticosteroid (CS) hormone axes are suggested to regulate developmental processes in vertebrates with a larval phase. To investigate this hypothesis, we isolated three nuclear receptors from a larval acanthomorph teleost, the red drum (Sciaenops ocellatus), and established their orthologies as thraa, thrb-L and gra-L using phylogenomic and functional analyses. Functional characterization of the TH receptors in COS-1 cells revealed that Thraa and Thrb-L exhibit dose-dependent transactivation of a luciferase reporter in response to T3, while SoThraa is constitutively active at a low level in the absence of ligand. To test whether interactions between the TH and CS systems occur during development, we initially quantified the in vivo receptor transcript expression levels, and then examined their response to treatment with triiodothyronine (T3) or cortisol. We find that sothraa and sothrb-L are autoregulated in response to exogenous T3 only during early larval development. T3 did not affect sogra-L expression levels, nor did cortisol alter levels of sothraa or sothrb-L at any stage. While differential expression of the receptors in response to non-canonical ligand hormone was not observed under the conditions in this study, the correlation between sothraa and sogra-L transcript abundance during development suggests a coordinated function of the TH and CS systems. By comparing the findings in the present study to earlier investigations, we suggest that the up-regulation of thraa may be a specific feature of metamorphosis in acanthomorph teleosts.

The novel finding of four distinct prepro-IGF-I E domains in a perciform fish, Sciaenops ocellatus, during ontogeny

In fishes, insulin-like growth factor-I (IGF-I) stimulates growth and differentiation but also plays a role in a number of other processes including osmoregulation, metabolism, immune response and reproduction. This study presents the cDNA encoding multiple prepro-IGF-I transcripts obtained from red drum, Sciaenopsocellatus, and examines differential expression in select adult tissues and during ontogeny. Four distinct transcripts were sequenced which were identical in the coding region for the signal (132 bp) and mature (204 bp) peptides but differed in the coding region of the E peptide by the exclusion of 117 (Ea-1), 81 (Ea-2) or 36 (Ea-3) bp compared to the 222 bp present in Ea-4. Analysis of the pertinent portion of the genomic sequence of this gene suggests that the transcripts are a result of alternative splicing. This is the first report of the expression of all four known prepro-IGF-I transcripts in a teleost other than a salmonid. The deduced amino acid sequences exhibited 70-95% identity with teleosts and somewhat lower identity to other vertebrates (60-75%). Three of the 4 transcripts (Ea-2, Ea-3, Ea-4) were expressed in the liver, ovary, spleen, gall bladder, brain, red muscle, pancreas and spinal cord of adults. Only the Ea-4 transcript was expressed in adult stomach tissue while no signal was detected in pituitary, retina, intestine, adipose or white muscle. In contrast, all 4 transcripts were expressed throughout ontogeny. The apparent expression of the Ea-1 transcript only during the larval stage may indicate a developmental role for this E peptide in red drum.