Ram's horn squid

Spirula spirula

ORDER

Sepioidea

FAMILY

Spirulidae

TAXONOMY

Spirula spirula Linnaeus, 1758. Some experts assign this species to a separate order, Spirulida.

OTHER COMMON NAMES

French: Spirule; German: Posthorn.

PHYSICAL CHARACTERISTICS

Partially internal shell is curved ventrally in an open coil; each coil is round in cross section and has transverse septa (dividers) with a siphuncle. There are four series of suckers on the arms. Both ventral arms are hectocotylized in males. There are tentacular clubs with suckers in 16 series; not divided into manus and dactylus. Eyes lack a cornea. Fins are separate, terminal, and lie in a plane nearly transverse to body axis. Large photophore at rear end of body.

DISTRIBUTION

Mesopelagic waters of open tropical oceans.

HABITAT

Found at depths between 1,800–3,280 ft (550–1,000 m), mostly over the slopes of continents or islands where the ocean bottom lies between 3,280–6,560 ft (1,000–2,000 m).

BEHAVIOR

Captive Spirula hang head-downward in the water. This species is able to withdraw its head and arms completely within the mantle; the mantle opening can then be closed by folding over the large dorsal and ventrolateral extensions of the mantle margin. The photophore at the posterior end of the body is known to glow for hours at a time. When the animal is swimming slowly downward, head first, its terminal fins are pointed upward (i.e. posteriorly) and move with a rapid "waving or fluttering motion" that propels it downward (Bruun, 1943).

FEEDING ECOLOGY AND DIET

Nothing is known.

REPRODUCTIVE BIOLOGY

It has been suggested that Spirula lays its eggs on the ocean floor; the capture of very small animals in deep water supports this hypothesis.

CONSERVATION STATUS

Not listed by the IUCN.

SIGNIFICANCE TO HUMANS

The shells, which commonly wash ashore in some areas, are prized by collectors.