Spatial structure of communities on dead pen shells (Atrina rigida) in sea grass beds

Abstract

Delimiting communities in marine habitats is diYcult because co-occurring species often have diVerent life histories and the life stages experience the environment at diVerent spatial scales. The habitat of a particular community is embedded within a larger habitat or ecosystem with many species shared between the focal community and the larger system. Pen shells (Atrina rigida) are large bivalves that, once the mollusk dies, provide shelter for motile species and hard substrate for settling larval invertebrates and egg-laying Wshes. In St. Joseph's Bay, Florida (29°45ЈN, 85°15ЈW), pen shells are the most abundant source of hard substrate, especially inside sea grass (Thalassia testudinum) beds, where they reach densities of 0.1-4.0 m ¡2 . This study, which was conducted from May to August 2005, measured the overlap in species densities 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 recorded, 66% were found on the pen shells but not in the surrounding sea grass habitat. Community structure, which varied among shells within sites and among the eight sites, could be related to sea grass characteristics such as blade density and length either directly (e.g., inhabitants of pen shells directly beneWt from the surrounding sea grass) or indirectly (e.g., pen shells and sea grass both beneWt from similar factors such as current and nutrients). Pen shells were randomly distributed at several spatial scales within the 15 £ 15 m sites as were many motile species. Two exceptions were the shrimp, Palaemon Xoridanus and the amphipod, Dulichella appendiculata, whose distributions were clumped. Most of the sessile species had clumped distributions, tending to be very abundant when they were present. These pen shell communities provide an opportunity for experimental studies of factors aVecting species diversity on small, discrete, naturally occurring habitats.

Positive and negative interactions control a facilitation cascade

“…Although the density or status of clams did not affect seagrass biomass or productivity, this may not always be the case. Moreover, communities on razor clams, recruitment of epibionts to razor clams, and predation on those epibionts can vary spatially and temporally (Keough 1984, Munguia 2007) and may vary with the successional stage investigated (Munguia 2004). Thus, at the landscape level, community structure will likely be a function of the environmental heterogeneity , variation in the density and traits of the habitat-formers, as well as external recruitment processes.…”

The influence of stage-dependent dispersal on the population dynamics of three amphipod species

“…These shells oVer settling substrate for many invertebrates when the mollusk dies. Pen shells are the most abundant source of hard substrate, in essence becoming "islands" of habitat within the grass beds and sandy substratum for many species found in St. Joe Bay (Munguia 2004(Munguia , 2007.…”

“…Consequently, it is relatively easy to distinguish between the sexes in adult individuals. In St. Joe Bay, these three amphipod species are most abundant on pen shells and very rare in sea grass beds where pen shells are not present, probably because they require or prefer the combination of hard substrate and protection these bivalves provide (Munguia 2007). M. nitida is a relatively common amphipod that occurs subtidally along the northwestern Atlantic coast (BousWeld 1973) and B. unicornis has been reported in the Gulf of Mexico (Thomas 1993).…”

“…The mode by which species disperse may become crucial for determining growth rates and, therefore, potential sources and sinks in metapopulations. The three amphipod species, Neomegamphopus hiatus, Melita nitida, and Bemlos unicornis are not endemic to pen shells; however they colonize this substrate in high densities relative to the surrounding habitat, therefore behaving as metapopulations (Munguia 2007). …”

Exploring coexistence mechanisms in a three-species assemblage

“…Pen shells (including Atrina spp and Pinna spp) are known secondary habitat species, enhancing structure and increasing macrofaunal diversity in their vicinity (Munguia, 2007;Rabaoui et al, 2015). In this study, the positive relationship between pen shells and sea urchins, suggests a facilitation by providing habitat for young and adult sea urchins that are virtually absent in the seagrass meadow (except around shells).…”