S. Gregory Tolley

THE ROLE OF OYSTERS IN HABITAT USE OF OYSTER REEFS BY RESIDENT FISHES AND DECAPOD CRUSTACEANS

Abstract To assess the role of live oysters in providing habitat, community metrics of resident fishes and decapod crustaceans were compared among 3 habitat treatments: live oyster clusters; cleaned, articulated shell and sand bottom. Sampling was conducted during three seasonally wet and three seasonally dry months using 1-m2 lift nets deployed on an intertidal oyster reef in the Caloosahatchee estuary, Florida. Metrics used to assess relative habitat value included organism density, biomass and species richness. Species-specific comparisons were also made. Results indicate that organism density, biomass and richness were all greater for treatments with shell (live oyster clusters or cleaned, articulated shell) compared with the sand-bottom (no-shell) treatment. Two patterns emerged from species-specific comparisons: (1) species found in live and articulated shell (e.g., flatback mud crab, green porcelain crab) might require shelter; and (2) species found in association with articulated, cleaned shell (i.e., frillfin goby) might use empty oyster boxes for spawning substrate. There was little evidence to suggest that any of the decapods or fishes present were specifically selecting habitat with living oysters present.

INFLUENCE OF SALINITY ON THE HABITAT USE OF OYSTER REEFS IN THREE SOUTHWEST FLORIDA ESTUARIES

Abstract To gauge the influence of salinity on the habitat value of oyster reefs, spatial and seasonal patterns of the presence of reef-resident fishes and decapods were assessed in the Caloosahatchee, Estero, and Faka-Union estuaries of Southwest Florida. Lift nets (1 m2) containing 5 L of oyster clusters were deployed on intertidal reefs at three sites along the salinity gradient of each estuary. Nets were deployed during three seasonally dry and three seasonally wet months for a period of 30 d. Oyster densities were estimated at each site and a number of community metrics were calculated as a measure of habitat use (e.g., organism density, biomass, diversity, dominance, richness). Several metrics increased downstream in one or more systems (e.g., organism density, biomass, diversity) and in general appeared to be more related to salinity than to the density of living oysters present. Although organism density was higher during the wet season for all three systems, biomass was higher during the dry season in the Caloosahatchee. In the Caloosahatchee and the Estero, measures of biodiversity tended to be higher during the dry season. These results suggest that the salinity requirements of the organisms that inhabit oyster reefs should be considered in the planning of oyster-reef restoration or enhancement projects or in the management or alteration of freshwater inflow into estuaries.

Molecular cloning and expression of a 2-Cys peroxiredoxin gene in the crustacean Eurypanopeus depressus induced by acute hypo-osmotic stress.

Peroxiredoxins (Prxs) are a family of ubiquitous proteins that help minimize the harmful effects of oxidative stress by catalyzing the reduction of hydrogen peroxide (H2O2) and organic hydroperoxides to less harmful forms. A full-length cDNA corresponding to a 2-Cys Prx gene was isolated from the flatback mud crab Eurypanopeus depressus and designated as EdPrx-1 (GenBank accession no. EU684547). EdPrx-1 has a major open-reading frame of 594 bp and is capable of encoding a polypeptide of 198 amino acid residues. Like other 2-Cys Prxs, EdPrx-1 protein possesses two conserved cysteine residues that play an essential role for the antioxidant activity of the proteins. The EdPrx-1 protein, as deduced from the cDNA sequence, shows a high level (74-93%) of sequence similarity to the 2-Cys Prxs from other crustaceans as well as those from many arthropod species (73-76% similarity). It shares about 70% sequence similarity with homologs from mammalian species. EdPrx-1 gene is expressed at low level in the gill, hypodermis, and hepatopancreas tissues of the crab under non-stressed condition; however, its expression is elevated about three-fold in the gills under hypo-osmotic stress. This suggests a possible role in protecting against oxidative stress caused by the increased metabolic activities associated with hyperosmoregulation.

Energetics of Swimming in Juvenile Common Snook, Centropomus undecimalis

Little research has been devoted to the investigation of swimming energetics and performance in lie-in-wait predators. This study evaluated activity metabolism and swimming performance in juvenile snook, Centropomus undecimalis, a lie-in-wait predator, by determining oxygen uptake in a tunnel respirometer. Compared to previously reported results for more actively foraging species (i.e., white crappie, rainbow trout), snook exhibited lower maintenance metabolism (58.6 ml O2 kg−1 WM h−1) and greater metabolic expansibility (6.5). Relative critical swimming speeds decreased isometrically with length, implying that small snook are capable of prolonged swimming in velocity regimes similar to those experienced by larger individuals. Swimming was most efficient at the transition between pectoral (low-speed) and caudal (high-speed) propulsion. The results of this study support previous speculation that maintenance costs may be reduced in lie-in-wait predators compared to other fishes.

Acute Response of the Estuarine Crab Eurypanopeus depressus to Salinity and Desiccation Stress

Abstract Hemolymph osmolality changes following exposure to abrupt salinity change in the range of 5–40 ppt (T  =  26°C, full air saturation) and upon exposure to air (T  =  23°C, r.h.  =  30%) were investigated in the flatback mud crab Eurypanopeus depressus (Smith, 1869), a dominant species on oyster reefs in Southwest Florida. During salinity trials, hemolymph osmolality of E. depressus ranged from 751 ± 123 mOsmol kg−1 at 5 ppt (214 ± 32 mOsmol kg−1) to 1188 ± 81 mOsmol kg−1 at 40 ppt (1188 ± 29 mOsmol kg−1). In the salinity range of 5–15 ppt E. depressus exhibited a hyperosmotic pattern of osmoregulation while at 30 and 40 ppt it conformed. In all cases stable hemolymph osmotic concentration was reached in less than 24 h. During desiccation trials, hemolymph osmolality of E. depressus ranged from 971 ± 121 mOsmol kg−1 for unexposed crabs to 1132 ± 169 mOsmol kg−1 after 90 min of exposure. The information obtained from this study adds to knowledge of crustacean stress physiology and may give a clearer picture of the important factors involved in population distribution and the consequences of multiple stressors that may affect the crabs or their oyster-reef habitat.

Freshwater Inflow Effects on Larval Fish and Crab Settlement onto Oyster Reefs

ABSTRACT Planktonic larvae of resident, oyster reef-associated decapods and fishes are subject to variable transport and retention whenever estuarine circulation is altered by freshwater inflow. Because freshwater inflow has the potential to advect larvae either toward or away from oyster reef settlement habitats, we compared the monthly distributions and abundances of larvae and postsettlement stages in Estero Bay, FL, under variable inflows during a 2-y period. Positive correlations between inflow and larval abundances of 2 species (Rhithropanopeus harrisii and Gobiesox strumosus) appeared to be caused by advection of upstream larvae downstream into the study area, whereas similar correlations with postsettlement juveniles of other species (Eurypanopeus depressus and Petrolisthes armatus) suggested bottom-up improvements to postsettlement survival. In contrast, the larvae of many species were advected seaward and away from oyster reef habitats during periods of elevated inflow, creating a spatial gap between the larvae and their landward settlement habitat. The size of this gap was larger for reefs that had greater exposure to freshwater inflows. Larvae displaced too far seaward would have a reduced window of opportunity to find oyster substrate for settlement, thus risking increased aberrant drift and predation loss. Because of the stationary nature of the settlement habitat, advection associated with elevated freshwater inflows was beneficial to some species and detrimental to others, producing winners and losers. The study also suggested that live oyster density was a good indicator of the density, biomass, and richness of oyster reef fishes and decapods. Evidence of temporal resource partitioning (successive peaks in larval densities) was present for those fishes that use empty oyster shell as sites for egg laying and nesting, although this pattern was present only during the first year of the study.

INFLUENCE OF FRESHWATER INFLOW ON REPRODUCTIVE CAPACITY OF THE MUD CRAB EURYPANOPEUS DEPRESSUS INHABITING OYSTER REEFS

Oyster reefs provide structural habitat for resident crabs and fishes, the planktonic larvae of which rely on estuarine transport/retention mechanisms to ensure settlement and subsequent recruitment to reefs. During periods of high freshwater inflow, larvae can be advected downstream and flushed out onto the continental shelf, resulting in an apparent reduction of larval supply within the estuary. However, reduced larval densities in estuaries might also result from decreased reproductive capacity, as dilute salinities on reefs upstream stress adult crabs. This study examined the reproductive capacity of the mud crab Eurypanopeus depressus (Smith, 1869) in response to freshwater inflow using the percentage of ovigerous females present as an indicator. Reproductive capacity on reefs was compared between two sites located near the mouths of two tidal tributaries, each experiencing differing rates of freshwater inflow, and between wet and dry seasons. Abundances of juvenile and adult crabs were significantly reduced during the wet season and at the site experiencing greater freshwater inflow. Length-frequency data indicate that newly recruited juvenile crabs were much less abundant at the site experiencing higher rates of inflow. The percentage of ovigerous females present was also reduced during the wet season but only at the reef experiencing greater inflow. Furthermore, the percentage of ovigerous females present at this site was inversely related to freshwater inflow and positively related to salinity. We suggest that high rates of freshwater inflow and the concomitant reduction in salinity can limit reproductive capacity in the flatback mud crab E. depressus on oyster reefs. Nonetheless, even as reproduction may be inhibited on reefs experiencing high inflow, other reefs subjected to lesser inflow may continue to serve as a source of larvae.