Spirula spirula is a small, muscular species (45 mm ML) found in mesopelagic waters of the tropical open ocean. In the title photographs, the skin of the mantle is missing. The intact mantle is covered with regularly aligned collagen fibers that produce a silvery sheen, as seen on the head in the title photographs (Herring et al., 1981). Spirula carries an unusual internal shell that is calcareous and has the shape of a horn coiled in a single plane without the coils touching one another (open planispiral).

The shell, which retains the phragmocone and siphuncle of its distant ancestors, is used as a buoyancy device. The posterior position of the shell within the body causes the animal to generally orient vertically with the head downward. The unusual general appearance of Spirula with a narrow arm crown, bulging eyes, the peculiar structure of the mantle, the transverse orientation of the fins and the presence of the coiled shell makes this species very different in appearance from all other cephalopods.

The large posterior guard-like sheath of fossil relatives of Spirula seems to be designed to function as a counterweight to maintain the animal in a horizontal position. Such an orientation is particularly important for a bottom-associated animal that swims just above the ocean floor (Naef 1921-23). Presumably the ancestors of Spirula were bottom associated and some remnants of this behavior apparently remains in their life history and distribution (Young, et al., 1998). A small remnant of the sheath exists on the Spirula shell and a remnant of the ancestral habitat remains in Spirula's apparent benthic spawning (Young, et al., 1999).

Brief diagnosis:

A decapodiform ...

• with a calcareous shell that is a phragmocone and has the shape of a coiled horn.