Colonies massive, multilayered as a result of extensive frontal budding of zooids, and always encrusting gastropod shells that were formerly occupied by hermit crabs (the hermit crabs themselves have never been found preserved in fossil examples). The original gastropod shell substrate is totally enveloped by bryozoan growth, resulting in a crudely gastropod-shaped colony with a large aperture. Lining this aperture, and forming the living chamber of the symbiotic hermit crab, is bryozoan interior wall calcification, rugose, porous and with lateral zooidal boundaries marked by salient thread-like walls. The colony surface is hummocky owing to a covering of monticules containing cortical zooids. Colonies typically have a chalky appearance and easily crumble, reflecting the aragonitic composition of the bryozoan skeleton.
Autozooids are small, less than 0.5 mm long, blister-like in vertical section, have a variable, rounded polygonal outline shape and are chaotically arranged. The frontal shield is granular, convex and has marginal areolar pores and scattered pores in more central locations. An umbo is usually developed immediately proximally of the orifice within a non-porous area of frontal shield. The small orifice measures 0.10-0.11 mm long by 0.07-0.08 mm wide, and is keyhole shaped with strong condyles dividing the slightly crescent-shaped poster from the larger, well-rounded anter. Overgrown autozooids often have orifices occluded by a closure plate with a groove representing the mould of the marginal opercular scerite. Oral spines are lacking or sparse. Ovicells (usually broken in fossils) are globular with a mostly uncalcified ectooecium and a flat entooecium containing an oval distal window.
Cortical zooids (presumed male zooids) resemble autozooids but are larger in overall size and have a slightly smaller orifice, 0.09-0.10 mm long by 0.07 mm wide.
Avicularia of two types - adventitious and interzooidal – are scattered among the autozooids between monticules. The adventitious avicularia are small, have a subangular rostrum and a calcified pivotal bar. Interzooidal avicularia are large, broad and rounded proximally, with a narrow, acute rostrum and a doubly constricted, ligulate pivotal bar.
The distinctive, gastropod-like shape of Hippoporidra edax colonies, which result from their growth as symbionts of hermit crabs (see Taylor 1994 for a review of such symbioses), immediately distinguishes the species from all other bryozoans occurring in the Coralline Crag. It is more likely to be mistaken with the hydractiniid hydrozoan Hydrocorella circumvestiens (Wood, 1844) (see Caretto 1966, pl. 2, figs 1-7) that also encrusts gastropod shells formerly occupied by hermit crabs. Careful examination of the colony surface, however, reveals the presence of blister-like zooids with keyhole shaped orifices which are lacking in the more solidly calcified hydrozoan.
Formerly identified as Hippoporidra edax, the living British species of Hippoporidra was distinguished by Taylor & Cook (1981) as a new species, H. lusitania. This species is very similar to H. edax but has interzooidal avicularia that are lens-shaped without the acute rostrum developed in H. edax. Other species of Hippoporidra, which ranges from Eocene-Recent, may also be distinguished using the morphology of the interzooidal avicularia and to a lesser extent autozooidal orifice shape (Taylor & Schindler 2004).
Pliocene, Late Zanclean–Early Piacenzian, Coralline Crag Formation, Suffolk, UK.
As currently understood, H. edax is widespread both geographically and stratigraphically, occurring also in the Miocene and Pliocene of France and the southeastern USA, the Pliocene of Belgium, Holland and Italy, and the Recent of the Atlantic coast of the southeastern USA and the Gulf of Mexico (Taylor & Cook 1981). However, it is quite likely that these records represent several different species of similar morphology.