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-[m.sup.2] 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.

KEY WORDS: oyster reefs, habitat use, decapods, fishes

INTRODUCTION

One of the important ecologic functions that oysters provide is habitat for commensal macrofauna. Although 70% of the organic matter filtered by oysters is assimilated (Newell 1988), the remainder provides food for benthic organisms. Furthermore, oysters are ecosystem engineers (sensu Jones et al. 1994) capable of creating extensive biogenic reefs. The secondary production of oysters and the 3-dimensional reef structure attract numerous invertebrates and fishes. Breitburg (1999) classified the fishes found on oyster reefs as residents, facultative residents or transient species and argued that oyster reefs were essential fish habitat for the first group.

Harding and Mann (2001a) compared habitat use by transient species and concluded the term essential fish habitat was inappropriate. Harding and Mann (2001a) suggested that organisms are drawn to oyster reefs because of greater availability and diversity of food, or higher quality of food on reefs compared with other habitats. The xanthid crabs Menippe mercenaria (Menzel & Hopkins 1956) and Panopeus herbstii (Bisker & Castagna 1987, Meyer 1994) feed extensively on oysters, and reef-resident fishes such as naked goby (Gobiosoma bosc), skilletfish (Gobiesox strumosus) and striped blenny (Chasmodes bosquianus) feed on commensal invertebrates (Breitburg 1999).

Oyster reefs have been compared with coral reefs in terms of their structural heterogeneity and vertical relief (Harding & Mann 1999). These complex, 3-dimensional structures create various microhabitats for use by resident macrofauna. Oyster shell serves as spawning substrate for skilletfish (Runyan 1961), Florida blenny Chasmodes saburrae (Peters 1981), feather blenny Hypsoblennius hentz (Breitburg 1999) and frillfin goby Bathygobius soporator (Peters 1983); such fishes select for length and gape of empty oyster shells relative to their own size (Crabtree & Middaugh 1982). Lehnert and Allen (2002) proposed that suitable spawning substrate limits reproduction in reef-resident fishes. Reef structure also provides refuge from predation for many species including the mud crabs Eurypanopeus depressus and P. herbstii (McDonald 1982). Day and Lawton (1988) reported that adult mud crabs prefer substrates with complex topography, and Dittel et al. (1996) suggested that shell rubble provided refuge from predation for mud crab megalopae. Furthermore, red drum Sciaenops ocellatus larvae had lower mortality in mesocosms containing simulated oyster reefs compared with other habitat types (Stuntz & Minello 2001). In the presence of predators, grass shrimp Palaemonetes pugio select oyster-shell pyramids over seagrass- and shallow-water habitats (Posey et al. 1999).

This study assesses the contributions of oyster clusters (live and dead) to habitat use of oyster reefs. That is, are residents selecting reef habitat for its food value, shelter, spawning substrate or all three? In addition to looking at habitat use in individual species, the study utilizes community metrics associated with resident fishes and decapods to assess overall habitat use. Oyster clusters were examined because their production has been proposed as a measure of successful habitat restoration on created reefs (Meyer & Townshend 2000). These clusters are also the dominant feature on Southwest Florida reefs, which are intertidal and exhibit low vertical relief. Sampling methodologies specifically targeted residents (reef and facultative)--the organisms that exhibit the greatest degree of dependence on oyster-reef habitat.

METHODS

Lift nets (Crabtree & Dean 1982) were deployed on an oyster reef in Tarpon Bay, in the Caloosahatchee estuary, to assess the relative use of live oyster clusters by resident fishes and decapod crustaceans. Gorzelany (1986) found greater similarity among oyster-reef associates found at comparable sites (inshore, middle, offshore) in different tidal rivers along the west central coast of Florida than among different sites within the same tidal river. Tolley et al. (2005) reported similar patterns for decapod and fish assemblages on oyster reefs in three Southwest Florida estuaries. The results of the current study conducted at a single site downstream of the mouth of the Caloosahatchee estuary might therefore be predictive for similar locations in estuaries of the region, but should not be extended to reefs farther inshore.

Rainfall and freshwater inflow into Southwest Florida estuaries are seasonal, therefore monthly sampling was conducted during 3 seasonally wet (July-September 2001) and 3 seasonally dry months (March-May 2002) to gauge any influence of changes in salinity on habitat use. Salinity and water temperature were recorded at the beginning and end of each sampling effort. Lift nets (1 [m.sup.2]) were deployed intertidally, just above mean low water, on living oyster reefs for a duration of approximately 30 days (previous collections using Hester-Dendy samplers at the site suggested that this period was sufficient for macroinvertebrate recruitment). An area of bottom approximating 1 [m.sup.2] was cleared of any oyster shell prior to the deployment of each lift net. For details on lift-net construction and deployment see Tolley et al. (2005).

Three treatments were used to assess the role of live oysters in providing habitat for resident fishes and decapods: (1) lift nets containing 5 L (volume displacement) of live oyster clusters; (2) lift nets containing 5 L of cleaned (dead), but still articulated oyster clusters; and (3) lift nets containing no shell (sand bottom). The volume of live oyster clusters used reflected ambient densities on the reef; cleaned clusters were created by immersing live clusters in a series of bleach and water baths with minimal additional mechanical cleaning. New clusters (live and dead) were used for each sampling effort. Three replicates per sampling effort were taken for the treatment using live clusters (6 mo x 1 treatment x 3 replicates), and two replicates were taken for each of the other two treatments (6 mo x 2 treatments x 2 replicates) for a total of 42 samples.

On retrieval of the nets, oyster clusters were removed and associated fishes and decapods were extricated using forceps. Any remaining decapods and fishes were then either removed from the net by hand or by using dip nets to sweep the interior of the lift net. These organisms were then transported on ice back to the laboratory for identification. In the laboratory, fishes and decapods were identified to the lowest taxon practical, measured to the nearest 0.1 mm (shrimp: carapace length, crabs: carapace width, fishes: standard length), and the first 10 individuals of each species were weighed to the nearest 0.01 g wet mass (WM). For species represented by more than 10 individuals in a sample, length-weight regressions calculated previously for each species were used to estimate biomass based upon the mean size of each species collected in each sample.

Organism density (no. [m.sup.-2]), biomass and richness were examined for each sample. Densities of particular species were also compared. Response variables (i.e., density, biomass, richness) were examined using a 2-way factorial ANOVA (2 seasons x 3 shell treatments). Homogeneity of variance was tested using the Levene statistic. Significant differences (P [less than or equal to] 0.05) were resolved using multiple comparison tests (Day & Quinn 1989): Hochberg's GT2 method in cases of equal variance; and the Games-Howell test in cases of unequal variance. Unless otherwise specified, data are presented as mean [+ or -] standard error.

RESULTS

Environmental Factors

Salinity averaged 36.77 [+ or -] 1.57 psu in the dry season compared with 28.20 [+ or -] 7.41 psu during wet months. Because the wet season in Southwest Florida normally extends from mid-June through mid-October, water temperatures were greater in the wet season (28.31 [+ or -] 3.48[degrees]C) compared with dry months (25.97 [+ or -] 6.85[degrees]C). However, no significant seasonal differences in either salinity or temperature were detected.

Density and Biomass

Decapod crustaceans dominated the samples regardless of habitat treatment (Table 1). A total of 10 species of decapods were identified, with the mud crabs Eurypanopeus depressus and Panopeus sp. along with the porcelain crab Petrolisthes armatus and the snapping shrimp Alpheus heterochaelis comprising 84% to 94% of the samples by number. Along with these species, the stone crab Menippe mercenaria and the shrimp Penaeus sp. also occurred frequently in one or more treatments (Table 1). Although fishes were more diverse, with 16 species being represented, only the code goby Gobiosoma robustum was collected in numbers approaching those of the dominant decapods. Other fishes occurring frequently in the samples included the feather blenny Hypsoblennius hentz, frillfin goby Bathygobius soporator, skilletfish Gobiesox strumosus, and gulf toadfish Opsanus beta (Table 1).

Significant variation in organism density was detected among habitat treatments (F[2,36] = 43.0, P < 0.001). Treatments containing live oyster clusters and cleaned-shell exhibited significantly higher densities (no. [m.sup.-2]) than the no-shell treatment (Fig. 1). No significant seasonal or interaction effects were present. The effect of habitat on biomass was similar (Fig. 2), with biomass being significantly higher for treatments with either dead or live oyster clusters compared with sand bottom (F[2,36] = 20.2, P = 0.001). Unlike organism density, biomass did vary significantly among seasons (F[1, 36] = 24.3, P < 0.001) with higher values present during the dry season (Fig. 2). No interaction between habitat and season was detected.

[FIGURES 1-2 OMITTED]

Biodiversity and Composition

Species richness was greater for live and dead oyster clusters compared with sand bottom (F[2,36] = 8.6, P = 0.001) (Fig. 3). No seasonal or interaction effects were detected.

[FIGURE 3 OMITTED]

Comparisons among habitat treatments were also made for species occurring frequently in one or more treatments and yielded two main patterns of habitat use. Densities of Eurypanopeus depressus (F[2,36] = 30.8, P < 0.001) (Fig. 4), Petrolisthes armatus (F[2,36] = 21.8, P < 0.001) (Fig. 5), Alpheus heterochaelis (live clusters: 2.8 [+ or -] 0.5; cleaned shell: 2.8 [+ or -] 0.6; sand bottom: 0.3 [+ or -] 0.6) (F[2,36] = 5.4, P = 0.009) and Opsanus beta (live clusters: 1.3 [+ or -] 0.2; cleaned shell: 1.0 [+ or -] 0.2; sand bottom: 0.1 [+ or -] 0.2) (F[2,36] = 8.8, P = 0.001) were all significantly higher in live oyster clusters or cleaned articulated shell compared with sand bottom. Similarly, stone crab Menippe mercenaria density was greater in live oyster clusters (1.9 [+ or -] 0.3) compared with sand bottom (0.2 [+ or -] 0.3) (F[2,36] = 7.8, P = 0.001).

[FIGURES 4-5 OMITTED]

Seasonal effects were detected for E. depressus, with greater densities occurring during wet months (F[1,36] = 4.5, P = 0.041) (Fig. 4) and for O. beta, with greater densities occurring during dry months (dry: 1.4 [+ or -] 0.2; wet: 0.2 [+ or -] 0.2) (F[1,36] = 21.1, P < 0.001). Such seasonal effects were not detected for the other species mentioned above, and no interaction effects were detected for any of the species examined.

A second pattern of habitat use was exhibited by the frillfin goby Bathygobius soporator (Fig. 6). In this case, density was higher for the treatment with cleaned articulated shell compared with live oyster clusters or sand bottom (F[2,36] = 8.1, P = 0.001). No seasonal effects were detected.

[FIGURE 6 OMITTED]

Two species did not exhibit any habitat effects but did vary seasonally, with higher densities occurring during dry months: Panopeus sp. (dry: 5.3 [+ or -] 0.5; wet: 1.8 [+ or -] 0.5) (F[1,36] = 28.6, P < 0.001) and Gobiesox strumosus (dry: 1.3 [+ or -] 0.3; wet: 0.2 [+ or -] 0.3) (F[1,36] = 7.1, P = 0.012).

DISCUSSION

The assemblage of fishes and decapod crustaceans found in association with oyster clusters during this study is characterized primarily by species commonly found on more temperate oyster reefs (Meyer 1994, Breithurg 1999, Coen et al. 1999a, Coen et al. 1999b, Posey et al. 1999, Glancy et al. 2003) or their tropical-subtropical congeners (Tolley et al. 2005). Many of these species use oyster-reef habitat for various purposes: feeding directly on the live oysters, utilizing empty oyster boxes or clean shell surface as spawning substrate or seeking shelter from predation. This study suggests that the habitat value of oyster clusters to the resident community of fishes and decapod crustaceans is less dependent on the presence of live oysters per se and more dependent on the three-dimensional structure created: articulated, cleaned oyster shell exhibited levels of organism density, biomass and species richness comparable to that of clusters containing living oysters. These metrics were also all significantly higher in treatments with oyster shell compared with sand bottom. Nonetheless, it is important to point out that live oysters are required not only for the creation of oyster clusters in the first place but also in maintaining them through time.

The only seasonal effect detected for these metrics was the occurrence of a higher biomass of fishes and decapods during dry months compared with wet. Because the location sampled was downstream of the mouth of the Caloosahatchee River and was therefore not subjected to the large reductions in salinity that occurred farther upstream, it is unlikely that salinity would explain this pattern. The increased biomass observed during dry months (March through May) compared with wet months (July through September) might be related to somatic and gonadal growth prior to reproduction in some species. For example, Peters (1981) reported that ovary weight of Florida blenny Chasmodes saburrae was greater from March through May and in September compared with all other months in Tampa Bay, Florida. The difference in biomass might also be accounted for by the recruitment of transient species using the reef as juvenile habitat. Fishes made up a greater proportion of the catch during the dry season and juvenile sheepshead Archosargus probatocephalus, silver perch Bairdiella chrysoura, and pinfish Lagodon rhomboides were collected only during dry months.

Although there were no differences in community metrics between live oyster clusters and articulated, cleaned oyster shell, species-specific differences in habitat use were detected. When mean density was compared among treatments for individual species, two patterns emerged--though somewhat predictable based on the life habits of the organisms. The first pattern is represented by species that occurred in greater abundance within treatments using oyster shell. This pattern would be expected for organisms that are utilizing oyster reefs primarily for the structure and refuge they provide. For example, the mud crab E. depressus is an omnivore (McDonald 1982). Though it has not been considered a threat to oyster populations (Meyer 1994), it does prey on juvenile oysters (McDermott 1960, Meyer & Townshend 2000) and on epifaunal bivalves (Milke & Kennedy 2001) and barnacles (Meyer & Townshend 2000). Grant and McDonald (1979) suggested that E. depressus might also avoid desiccation by hiding in oyster shells during low tide. However, Meyer and Townshend (2000) noted that despite the availability of oyster shell substrate and suitable prey, this species was not found until new oyster clusters had developed on created reefs. These results suggest that E. depressus prioritizes structure over food when colonizing new habitat. The toadfish Opsanus beta exhibited the same pattern of habitat use as E. depressus in the current study. Toadfish are known to feed on xanthid crabs (Bisker et al. 1989, Grabowski 2004) and a large individual collected during this study was found with a single Eurypanopeus depressus inside its stomach.

Other species found in greater abundance in oyster clusters and among dead articulated shell included the porcelain crab Petrolisthes armatus and the bigclaw snapping shrimp Alpheus heterochaelis. Although Caine (1975) reported that P. armatus is at times a deposit feeder, feeding directly on detritus, this species is primarily a filter feeder (Caine 1975, Kropp 1981) that uses the oyster cluster for reaching up into the water column. Alpheus heterochaelis, another filter feeder, is commonly found among shells (Williams 1984) and is reported to live symbiotically in the burrows of the black-clawed mud crab Panopeus herbstii in salt marshes (Silliman et al. 2003). Panopeus herbstii does not occur in Southwest Florida; however, unidentified crabs from the P. herbstii complex (Williams 1983) occurred frequently in samples collected during this study.

A second pattern of habitat use was exemplified by the frillfin goby Bathygobius soporator. This species was found in significantly greater numbers in articulated, cleaned shell than in either living oyster clusters or sand bottom. The frillfin goby has been reported to lay its eggs inside oyster shell (Peters 1983); the eggs are subsequently guarded by the male of the species (Breder 1943). This fish also represents the largest species of goby found in association with oyster reefs in the Caloosahatchee estuary. Bathygobius would therefore likely place a premium on habitat with unfouled, articulated oyster shell such as is found in oyster boxes, the empty still-joined valves of dead oysters. This type of microhabitat was created from the articulated, cleaned-shell treatment used in this study. Other fishes such as the naked goby, striped blenny and skilletfish are also known to lay their eggs on the unfouled, inner surface of articulated oyster shell (Runyan 1961, Breitburg 1999).

It is important to note that although attempts were made to standardize (via volume displacement) the amount of oyster shell present in the lift nets, cleaned articulated shell would be expected to have a higher available surface area for use by some species than live oyster clusters. Nonetheless, in most species examined, density did not vary significantly between the two shell treatments.

By capturing organic carbon from the water column and repackaging it in the form of body mass, pseudofeces, or feces, oysters make food available to various benthic organisms living on the reef (Newell 2004). Although some of these residents feed directly on the oysters (e.g., Menippe mercenaria), most feed either on detritus or on other organisms that colonize the reef. Many of the species that reside here in turn serve as forage for important fisheries species (e.g., spotted seatrout Cynoscion nebulosus, McMichael & Peters 1989; red drum Sciaenops ocellatus, Peters & McMichael 1987; bluefish Pomatomus saltatrix, Harding & Mann 2001b) and for birds (e.g., yellow-crowned night-heron, Nycticorax violaceus, Watts 1988).

Equally important, oysters provide complex, 3-dimensional structures that result in various microhabitats. Even after the living oyster is gone, these structures remain for some time, providing refuge from predation and desiccation, as well as high quality substrate for egg laying and nesting. May (1974) reported that xanthid crab abundance on Alabama reefs was related to the availability of oyster boxes--the habitat remaining after a living oyster has died and been removed from its valves. Breitburg (1999) suggested that" 'healthy' populations of oysters would be expected to have a continual low level of mortality of a variety of sizes of individuals and thereby provide a continual supply of nest sites" for resident fishes. This cycle of oyster settlement, growth, death and ultimate degradation of the remaining shell results in a succession of microhabitats available for colonization by other reef residents.

ACKNOWLEDGMENTS

The authors thank Peter Doering of the South Florida Water Management District (SFWMD) for his support and input on the project. Sharon Thurston coordinated sampling and analysis and was aided by a small army of undergraduate interns: Sherith Bankston, Mike Chichester, Julie Farineau, Rashel Grindberg, Matt Hooper, Erin Rasnake, and Lacey Smith. Many of these students were supported by Congressional Grant P116Z010066 awarded through the United States Department of Education. Lesli Haynes and Arielle Poulos conducted preliminary sampling using Hester-Dendy samplers. The authors also thank Rebecca Totaro and Win Everham for their careful reviews of the manuscript. This work was supported by SFWMD grant C-12412-A1.

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S. GREGORY TOLLEY AND ASWANI K. VOLETY

Florida Gulf Coast University, Coastal Watershed Institute, 10501 FGCU Boulevard South, Fort Myers, Florida 33965

TABLE 1.
Decapod crustaceans and fishes collected on an oyster reef in
Tarpon Bay (Caloosahatchee estuary).

                                                            Number
                                                          Collected

Species                     Common Name                 SB    CS    LC

Decapods
  Alpheus heterochaelis     bigclaw snapping shrimp      4    34    51
  Callinectes sapidus       blue crab                    0     5     0
  Eurypanopeus depressus    flatback mud crab           50   270   540
  Libinia dubia             longnose spider crab         0     1     0
  Menippe mercenaria        Florida stone crab           2    10    34
  Palaemonetes vulgaris     marsh grass shrimp          10     1    20
  Panopeus sp.              mud crab                    26    58    66
  Penaeus sp.               penaeid shrimp              10    20    23
  Petrolisthes armatus      green porcelain crab        44   238   549
  Portunas gibbesii         iridescent swimming crab     1     0     0
Fishes
  Anarchopterus criniger    fringed pipefish             0     1     0
  Archosargus
    probatocephalus         sheepshead                   0     1     0
  Bairdiella chrysoura      silver perch                 0     0    13
  Bathygobius soporator     frillfin goby                1    11     2
  Chasmodes saburrae        Florida blenny               0     5     6
  Cyprinodon variegatus     sheepshead minnow            0     0     1
  Eucinostomus sp.          mojarra                      5     0     4
  Gobiesox strumosus        skilletfish                  5    14    13
  Gobiosorna robustum       code goby                   36    85    74
  Haemulon plumieri         white grunt                  0    l      0
  Hypsoblennius hentz       feather blenny               6     8    13
  Lagodon rhomboides        pinfish                      0     7     5
  Lupinoblennius nicholsi   highfin blenny               0     0     1
  Lutjanus griseus          gray snapper                 1     0     0
  Opsanus beta              gulf toadfish                1    13    24
  Symphurus plagiusa        blackcheek tonguefish        1     0     0

Habitat treatments are sand bottom (SB), articulated cleaned shell
(CS), and live oyster clusters (LC).