Saccate thallus of the red alga Halosaccion glandiforme harbor diverse invertebrate fauna

Abstract Rocky intertidal zones are biologically diverse environments with numerous physical stressors; as such, close associations between organisms often develop to overcome such stressors and enhance survival of associates. As major components of rocky shores, macroalgae support numerous invertebrate species. In this study, we evaluated the habitat-providing role of the red alga, Halosaccion glandiforme (S.G. Gmelin) Ruprecht. This alga, also called sea sacs, is commonly found on rocky shores along the West Coast of North America. During emersion, this seaweed holds water within its saccate thallus, which can potentially serve as microhabitat for various organisms. Little is known about the composition of microfauna associated with this seaweed; as such, we documented richness and abundance of species found inside its thallus. Algal specimens were collected from Charleston, Oregon and transported to the laboratory for further analyses. Of the 119 thalli examined, 12 taxa were documented. Nematodes and copepods were the dominant taxa. Other groups represented included ostracods, turbellarians, halacarid mites, bivalves, rotifers, and three larval types (barnacle cyprids, copepod nauplii, mollusc veliger). Diatoms, crustacean molt remnants, detritus, and sand particles were also observed inside thalli. Species richness and abundance were positively correlated with thalli volume, as well as intra-thalli fluid volume. Several feeding and other behaviors of colonizers were noted; they included herbivory, predator–prey interactions, detritivory, and molting. Our findings demonstrate that H. glandiforme provide refuge for organisms from harsh environmental conditions during emersion periods, and also serve as feeding and nursery grounds for its diverse invertebrate fauna.