Spirula spirulaRichard E. Young
Figure. Left - Side view of the shell of Spirula. Photograph by R. Young of a shell found on a Florida beach. Right - Cut-away view of the shell provided by ray tomography showing the phragmocone with septa and siphuncle. Photograph taken by the atomic and nuclear physics group from the University of Fribourg in Switzerland, and provided by Hans Ueli Johner.
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.
A decapodiform ...
- with a calcareous shell that is a phragmocone and has the shape of a coiled horn.
- Arms with suckers in four series.
- Both arms IV hectocotylized.
- Tentacular clubs without proximal locking apparatus.
- Tentacular clubs with suckers in 16 series; not divided into manus and dactylus.
- Club suckers without circularis muscles.
- Head with tentacle pockets.
- Eyes without corneas.
- Funnel without lateral adductor muscles.
- Mantle locking-apparatus does not reach anterior mantle margin.
- Fins separate, terminal and lie in a plane nearly transverse to body axis.
- Fins with posterior lobes.
- Large photophore at posterior end of body.
- Calcareous shell a phragmocone, curved ventrally in open planispiral; round in cross section and possessing a ventral siphuncle and transverse septa.
- Gills without brachial canal.
- Right oviduct absent.
- Females with accessory nidamental glands.
- Radula absent.
- Ink sac reduced.
- Anal flaps absent.
- Digestive gland paired; esophagus passes between members of the pair.
Virtually all cephalopods have mobile irises but for an oceanic, pelagic decapodiform the extreme contraction seen below in Spirula is unusual.
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Figure. Views of the iris of Spirula Spirula, 23 mm ML, western North Atlantic. Left - Dorsal view. Right - Dorsal-oblique view. Note the the wide-open eyelid and the strongly contracted iris leaving a tiny pupil. Photogaphs by M. Vecchione.
Figure. Views of paralarvae Spirula spirula. Left - Ventral view. Drawing from Clarke (1970). Right - Drawing from Bruun (1943).
The shift in size frequency distribution over time in the region of the Canary Islands, suggests to Clarke (1970) that this cephalopod has a life-span of 18-20 months.
The ventral arms of the male are hectocotylized: They are much longer and thicker than the other arms (see title photograph) and possess a variety of unusual flaps and papillae but lack suckers.
Vertical distributionClarke (1969) found that Spirula occurs at depths between 550 and 1000 m during the day (peak distribution is at 600-700 m) and at night migrates into the upper 100-300 m (peak at 200-300 m).
Geographical distributionMost captures come from the tropical Atlantic and tropical IndoWest Pacific Oceans. Bruun (1943) found that Spirula is distributed mostly over the slopes of continents or islands where the bottom depth is between 1000 and 2000 m. The most numerous captures came from the vicinity of oceanic islands. Indeed, this animal was the most abundant midwater cephalopod collected in the region of the Canary Islands by Clark (1969).
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).
Bruun, A. F. 1943. The biology of Spirula spirula (L.). Dana-Report No. 24. 49pp.
Clarke, M. R. 1969. Cephalopoda collected on the SOND cruise. J.Mar. Biol. Ass. U.K., 49: 961-976.
Clarke, M. R. 1970. Growth and development of Spirula spirula. Jour. Mar. Biol. Ass. U. K. 50: 53-64.
Herring, P. J., M. R. Clarke, S. von Boletzky and K. P. Ryan. 1981. The light organs of Sepiola atlantica and Spirula spirula (Mollusca: Cephalpooda): bacterial and intrinsic systems in the order Sepioidea. J. Mar. Biol. Ass. U.K., 61: 901-916.
Naef, A. 1922 -- Die Fossilen Tintenfische. Gustav Fischer, Jena. 322 pp.
Schmidt, J. 1922. Live specimens of Spirula. Nature, 110, No. 2271.
Young, R. E., M. Vecchione and D. Donovan. 1998. The evolution of coleoid cephalopods and their present biodiversity and ecology. South African Jour. Mar. Sci., 20: 393-420.