MacDonald et al. (1990) has described four general community types. These are communities dominated by Vestimentiferan tube worms ("Lamellibrachia" c.f. "barhami" and "Escarpia" spp.), mytilid mussels (Seep Mytilid Ia, Ib, and III, and others), vesicomyid clams ("Vesicomya cordata" and "Calyptogena ponderosa"), and infaunal lucinid or thyasirid clams ("Lucinoma" sp. or "Thyasira" sp.). Bacterial mats are present at all sites visited to date. These faunal groups tend to display distinctive characteristics in terms of how they aggregate, the size of aggregations, the geological and chemical properties of the habitats in which they occur and, to some degree, the heterotrophic fauna that occur with them. Many of the species found at these cold seep communities in the Gulf of Mexico are new to science and remain undescribed.