Mar Biol (2007) 152:149–156 DOI 10.1007/s00227-007-0670-8 RESEARCH ARTICLE Spatial structure of communities on dead pen shells (Atrina rigida) in sea grass beds Pablo Munguia Received: 31 October 2006 / Accepted: 5 March 2007 / Published online: 3 April 2007 © Springer-Verlag 2007 Abstract Delimiting communities in marine habitats is varied among shells within sites and among the eight sites, diYcult because co-occurring species often have diVerent could be related to sea grass characteristics such as blade life histories and the life stages experience the environment density and length either directly (e.g., inhabitants of pen at diVerent spatial scales. The habitat of a particular com- shells directly beneWt from the surrounding sea grass) or munity is embedded within a larger habitat or ecosystem indirectly (e.g., pen shells and sea grass both beneWt from with many species shared between the focal community similar factors such as current and nutrients). Pen shells and the larger system. Pen shells (Atrina rigida) are large were randomly distributed at several spatial scales within bivalves that, once the mollusk dies, provide shelter for the 15 £ 15 m sites as were many motile species. Two motile species and hard substrate for settling larval inverte- exceptions were the shrimp, Palaemon Xoridanus and the brates and egg-laying Wshes. In St. Joseph’s Bay, Florida amphipod, Dulichella appendiculata, whose distributions (29°45ЈN, 85°15ЈW), pen shells are the most abundant were clumped. Most of the sessile species had clumped dis- source of hard substrate, especially inside sea grass (Thal- tributions, tending to be very abundant when they were assia testudinum) beds, where they reach densities of 0.1– present. These pen shell communities provide an opportu- 4.0 m¡2. This study, which was conducted from May to nity for experimental studies of factors aVecting species August 2005, measured the overlap in species densities diversity on small, discrete, naturally occurring habitats. between dead pen shells and the surrounding sea grass communities at eight sites to determine the discreteness of the pen shell communities. Of the 70-epibenthic taxa Introduction recorded, 66% were found on the pen shells but not in the surrounding sea grass habitat. Community structure, which In marine ecosystems, deWning the boundaries for popula- tions and communities has been problematic. Many marine species have at least two distinct life history stages that per- Communicated by J.P. Grassle. sist at very diVerent spatial scales. The dispersal stage (a larva or juvenile) is cast into the water column where it is Electronic supplementary material The online version of this transported to distant settlement habitats (e.g., Roughgar- article (doi:10.1007/s00227-007-0670-8) contains supplementary material, which is available to authorized users. den et al. 1985; Palmer et al. 1996). The more sedentary adult stage may be territorial or sessile (e.g., Olson 1985) or P. Munguia (&) even capable of moving short distances (e.g., Mora and Department of Biological Science, Florida State University, Sale 2002). These two life history stages have the potential Tallahassee, FL 32306-1100, USA V e-mail: [email protected] of acting at di erent spatial scales: an among-habitat (i.e., regional) scale experienced by the disperser stage and a Present Address: local scale experienced by the sedentary stage. Therefore, P. Munguia the spatial arrangement of local habitats can be crucial for Smithsonian Environmental Research Center, 647 Contees Wharf Road, P.O. Box 28, population dynamics and diversity patterns, where commu- Edgewater, MD 21037-0028, USA nities are part of a mosaic of diVerent habitats. 123 150 Mar Biol (2007) 152:149–156 Over the last 20 years, spatial ecology has received a questions: (1) Do the species occupying pen shells consti- large amount of attention (Ricklefs and Schluter 1993). tute a discrete community, or are they just part of a larger Current theory suggests diversity patterns are driven by the community within sea grass beds? More speciWcally, what synergistic contribution of mechanisms at the local scale proportion of pen shell inhabitants is also found in the habi- such as competition and disturbance coupled with mecha- tat surrounding pen shells? If pen shells share a large num- nisms at broader, regional scales such as dispersal and hab- ber of species with the surrounding sea grass habitat, this itat heterogeneity (Cornell and Lawton 1992). In recent would suggests that a pen shell “reef” is nested within the years, theory has addressed not only the relationship larger sea grass habitat. (2) Does pen shell community between spatial scales aVecting diversity, but the mecha- structure vary with spatial scale, speciWcally on a scale of nisms that give rise to spatially-structured diversity patterns 0.1–10 km across St. Joseph Bay? Spatial variation in com- (Chase et al. 2005). Field research has focused on identify- munity structure could occur as a function of environmental ing the mechanisms that regulate diversity in natural sys- conditions including the density of communities in diVerent tems (Holyoak et al. 2005). However, few ecological areas of St. Joseph Bay. I also focus on a subset of pen shell communities have been found to have the appropriate attri- inhabitants to compare the distribution of diVerent species. butes for testing spatial theory (Srivastava et al. 2004). In any given community, populations may grow and dis- There have been two related obstacles in the study of perse at diVerent spatial scales (Huston 1999), which can be spatially structured communities. First, the physical bound- reXected in the distribution of individuals. The diVerence aries of communities are often hard to deWne. Some exam- between the distribution of habitat and species should reX- ples of how ecologists have addressed this problem include ect the way species track habitats: if both habitat and spe- devising methods for estimating diversity at the local scale cies have the same distribution, then species would tend to (e.g., Gotelli and Colwell 2001), and the partitioning of follow such habitat, suggesting that habitat may be limiting. communities across environmental gradients (e.g., Shmida Alternatively, if species have a clumped distribution rela- and Wilson 1985). Second, an often-overlooked aspect in tive to the habitat, this means that a biotic (e.g., habitat spatial ecology is the fact that the diVerent species that selection, competition, dispersal ability) mechanism limits comprise the community may be regulated by processes the distribution, reXected in the aggregated individuals. If that are manifested on very diVerent spatial scales (Huston species have a uniform distribution relative to the habitat, 1999). Species can diVer in their mating strategies, compet- then the above mechanisms are not constraining the distri- itive and dispersal abilities and whether the dispersal stage bution. (e.g., adults or larvae) can select and discriminate potential habitats for settlement (e.g., Keough 1984; Huston 1999; Jenkins 2005; Munguia 2006). Recognizing the spatial Methods scale at which individual species are regulated may help understand the role of the habitat where populations occur Description of natural system as well as the habitat that connects these populations. In this fashion, the spatial area that would be considered a This study was conducted in St. Joseph Bay, Florida (also “community” for one species could in fact be only a part of referred to as St. Joe Bay); a shallow, well-protected bay a larger habitat for another species. The spatial scale at in the northern Gulf of Mexico (Fig. 1). Substrate in the which species function combined with the physical struc- bay is composed of a bare sandy bottom intermixed with ture of communities needs to be considered in order to patches of sea grass (Thalassia testudinum and the less understand diversity patterns. common Halodoule wrightii). Sea grass beds occupy The main objective of this study was to determine approximately 2,560 ha of the bay, mostly on the shallow whether dead pen shells (from Atrina rigida bivalves) were areas (less than 3 m; Wolfe et al. 1988). Live pen shells occupied by a diVerent community than that found in the (Atrina rigida) are found within the sea grass beds, surrounding sea grass habitat in St. Joseph Bay, Florida. anchored in the sand. These large bivalves can also be Pen shells (i.e., Atrina spp. and Pinna spp.) have been found in open sandy areas, however, at much lower densi- found to aVect macrofauna diversity in surrounding habitats ties (personal observation; Kulhmann 1996). When alive, (e.g., Warwick et al. 1997; Cummings et al. 1998), and pen shells have up to 30% of their surface area exposed to even studied as settling habitat for sessile species (e.g., the water column, and are not fully available for coloniza- Keough 1984). Pen shell communities have the potential to tion by other organisms. When the mollusk inside the shell address questions regarding spatially structured communi- dies, the shell becomes increasingly exposed (until the ties (Srivastava et al. 2004), but Wrst their spatial structure whole shell is lying on the sand) and is occupied by a and role within the larger sea grass bed community must be diverse array of species (Munguia 2004), which use the described. Therefore, in this study I asked the following shell as either refuge, egg laying substrate, or settling 123 Mar Biol (2007) 152:149–156 151 Fig. 1 Map of St. Joe Bay with the location of the Weld sites in circles. Shaded area represents sea grass beds. Black areas represent sandy bottom within the sea grass beds. ModiWed from Kuhlmann (1996) habitat. This community persists until the shell breaks sampling, Ziploc bags were carefully placed over the shells down or gets buried in the sand. in situ, and then the bags were sealed and brought to the The discrete habitat boundaries oVered by individual pen surface.