Behavioral and Neuroendocrine Correlates of Sex Change in the Gilthead Seabream (Sparus aurata)

Sequential hermaphroditism is the most radical form of environmental sex determination observed in fish: functional adult males or females retain their ability to change sex even as adults. Among the factors that affect sex change in these species, the least understood is the social environment. Here, I studied the influences of social context on sex change in the Gilthead Seabream, Sparus aurata, by using the individual‟s dominance rank as an indicator of social status. To understand the role that the brain might play in sex change, I also studied the two main neuroendocrine factors that serve as the sexually differentiated axes of neural plasticity in most teleost species: AVT and GnRH. To do this, I first developed a set of tools designed to address the challenges associated with observing the behavior of aquacultured species. Using these tools, I provide the first in-depth study of seabream captive behavior, including the results of size-matched and sex-matched paired encounters. I found that females are more aggressive than males, but this difference is influenced by gonadal developmental status. I also showed that small but young males are more aggressive than bigger but older females. I cloned the AVT mRNA in seabream, and validated a quantitative assay to measure total brain AVT levels together with GnRH-1, GnRH-2, and GnRH-3 levels. I found that AVT and GnRH-3 levels rise during the onset of the hypothesized sex-change window, and drop to pre-quiescent levels until spawning, when all of these factors seem to increase their expression levels again. I also show for the first time, that GnRH-2 and dominance rank are strongly correlated in seabream during the spawning season but not during quiescence. GnRH-1 was strongly correlated to rank during quiescence but not during spawning. Finally, neither dominance rank nor size were a good predictor of the outcome of sex change, which seems to contradict what has been documented in sequential hermaphrodite reef fishes. I provide a model that accounts for this apparent contradiction and conclude that the Gilthead seabream remains true to the size-advantage hypothesis of sex allocation theory, if size and dominance are seen as proximate, rather than ultimate, factors

Early Marine Migration Patterns of Wild Coastal Cutthroat Trout (Oncorhynchus clarki clarki), Steelhead Trout (Oncorhynchus mykiss), and Their Hybrids

Hybridization between coastal cutthroat trout (Oncorhynchus clarki clarki) and steelhead or rainbow trout (Oncorhynchus mykiss) has been documented in several streams along the North American west coast. The two species occupy similar freshwater habitats but the anadromous forms differ greatly in the duration of marine residence and migration patterns at sea. Intermediate morphological, physiological, and performance traits have been reported for hybrids but little information has been published comparing the behavior of hybrids to the pure species.This study used acoustic telemetry to record the movements of 52 cutthroat, 42 steelhead x cutthroat hybrids, and 89 steelhead smolts, all wild, that migrated from Big Beef Creek into Hood Canal (Puget Sound, Washington). Various spatial and temporal metrics were used to compare the behavior of the pure species to their hybrids. Median hybrid residence time, estuary time, and tortuosity values were intermediate compared to the pure species. The median total track distance was greater for hybrids than for either cutthroat or steelhead. At the end of each track, most steelhead (80%) were located near or north of the Hood Canal, as expected for this seaward migrating species, whereas most cutthroat (89%) were within 8 kilometers of the estuary. Most hybrids (70%) were detected leaving Hood Canal, though a substantial percentage (20%) remained near the Big Beef Creek estuary. More hybrids (7.5%) than pure cutthroat (4.5%) or steelhead (0.0%) were last detected in the southern reaches of Hood Canal.Given the similarity in freshwater ecology between the species, differences in marine ecology may play an important role in maintaining species integrity in areas of sympatry

Hybrid Index Histogram.

<p>Numbers of phenotypic cutthroat (white bars) and phenotypic steelhead (black bars) classified along a continuum of hydrid indices ranging from 0.0 (pure cutthroat) to 1.0 (pure steelhead).</p

Tag type and length summary by year.

<p>Number of each smolt type tagged with either V7 or V9 Vemco acoustic transmitters and each group's average length (± SE) for each year of the study. Only smolts greater than 155 mm were selected for tagging.</p

Dispersal time plotted against distance from Big Beef Creek to each smolt's farthest detection location.

<p>Dispersal time is the time between the last estuary detection and the farthest detection. Negative distances represent movement to southern locations, and positive distances represent movement to northern locations. Locations of cutthroat smolts are represented by dark circles, phenotypic cutthroat hybrids are represented by shaded triangles, phenotypic steelhead hybrids are represented by shaded squares, and steelhead are represented by white circles. Locations of some fish were changed slightly to accommodate viewing of all symbols.</p

Numbers of cutthroat, steelhead, and hybrid smolts last detected at locations within the study area.

<p>South Hood Canal  = >20 km south of the Big Beef Creek estuary, Estuary  =  within 20 km of the Big Beef Creek estuary, HC Bridge  =  within 10 kilometers of the Hood Canal Bridge, Admiralty inlet  = 50 km from estuary, Strait of Juan de Fuca  = 150 km from estuary). Cutthroat (−Ι_H = 0.0) are represented by black bars, phenotypic cutthroat hybrids (−Ι _H = 0.270) by cross-hatched bars, phenotypic steelhead hybrids (−Ι _H = 0.543) by gray bars, and steelhead (−Ι _H = 1.0) by white bars.</p

Box and whisker plots (median, interquartile range, data range, outliers) of migratory track parameters.

<p>(A) residence time (days), the time between ocean entry and last detection, (B) estuary time (days), the time between first and last estuary detection, (C) total track distance (kilometers), the sum of all track segments, and (D) the tortuosity index, which measures the extent to which a track meanders, and is the sum of all track segments divided by the linear distance between the two farthest receivers included in the track.</p