Preface
Bobtails in the subfamily Sepiolinae are a controversial group in which species are often difficult to identify morphologically ( Groenenberg et al.). Extensive bottom trawl surveys of the North Sea over several years caught five species: Sepietta oweniana, Sepietta neglecta, Sepiola pfefferi, Sepiola atlantica and a new species of Sepiola. The last two can only be distinguished by DNA sequencing, though a sharp divide was found between their distributions, with S. atlantica south of a line from Petersborough, England to Blåvands Huk, Denmark, and the new species to its north. It is not known whether the Irish and Welsh images in this account are Sepiola atlantica or its morphologically indistinguishable sister species.
Body
Body and tentacles usually about 30mm long, rarely over 45mm. Plump short ovoid mantle usually 12-15 mm long, occasionally to 25mm. Mantle cavity open ventrally and dorsally, except for mid-section of dorsal mantle edge fused to head [image1]. Bases of large thin translucent lateral fins do not reach anterior or posterior of mantle [2]. Translucent whitish exhalent funnel extends from mantle cavity on underside [3]. Ventral surface coloured as dorsum [4] or paler [7], no demarcation line between dorsum and venter. Translucent mantle, head, arms and base of fins covered in chromatophores able to change colour in less than a second from translucent white with glistening silver viscera to various shades and combinations of black, gold and brown mottling. Bacteria in viscera can produce luminescence.
Head
A little wider than mantle. Large rounded eye mounds with large, highly developed lateral eyes with horizontal slit apertures [1] that can see forwards, sideways and backwards.
Arms & tentacles
Eight arms, and two captorial tentacles kept coiled by mouth [6] except when shot out to seize prey. Each arm has two rows of transparent suckers [7], increasing to 4-8 rows distally on dorsal arms. 8 longitudinal rows of suckers on captorial tentacle clubs.
Internal hard parts
Reduced horny pen (chitinous gladius) within mantle. Chitinous “parrot’s-beak” at base of arms (TOL).
Sepiola atlantica
- Short plump ovoid mantle, total length body and tentacles to 45mm.
- Eight arms, and two captorial tentacles kept coiled by mouth [6].
- Two rows of suckers on each arm, 4-8 rows distally on dorsal arms.
- Bases of large, thin, translucent, lateral fins do not reach anterior or posterior of mantle [2].
- Mantle cavity open ventrally and dorsally except for mid section of dorsal mantle edge fused to head [1].
Rossia macrosoma (della Chiaje, 1829) (MSIP)
- Short plump ovoid mantle, total length body and tentacles to 180mm.
- Eight arms and two captorial tentacles kept coiled by mouth.
- Two rows of suckers on each arm, 4 or more rows distally.
- About ten rows of small suckers on captorial tentacle club.
- Bases of short, fleshy, rounded, lateral fins do not reach anterior or posterior of mantle.
- Mantle cavity open round entire perimeter with pronounced lip, without any fusion to head.
Sepietta oweniana d’Orbigny 1839 (MSIP)
- Stout ovoid mantle, total length body and tentacles to 110mm.
- Eight arms and two captorial tentacles.
- Dorsal pair of arms joined by membrane at base, intervening cleft obviously shorter than between other arms.
- Left dorsal arm, hectocotylus, greatly widened on male.
- About thirty rows of very small suckers on captorial tentacle club.
- Bases of large thin lateral fins do not reach anterior or posterior of mantle.
- Mantle cavity open ventrally and dorsally except for mid section of dorsal mantle edge fused to head.
Sublittoral inshore shallows, occasionally stranded in shore pools, to 150 metres. Clean sand or fine shell gravel substrate. Sometimes nektonic; fins move like wings to propel forwards or backwards [2]. Exhalent funnel expels water steadily downwards to hover motionless [3], and strongly forwards to escape backwards (mantle-first) from danger [5]. Often benthic during daytime, lying buried in sand [8] awaiting small crustacea. Concealment takes 20 seconds; exhalent funnel blows depression in sand, blown sand settles on top of sepiole; arms move any stones and brush sand to complete cover.
By expanding or contracting different chromatophores, colour can change dramatically in less than a second to provide camouflage [9] or startle and distract attackers, often with dark sienna [4]. Sometimes discharges viscous sepia to distract predator as it jets away. Small crustacea [6] caught by rapidly uncurled pair of long captorial tentacles. Predators include fish. During mating on seabed, male holds female from below and behind while specialised left dorsal arm, hectocotylus, transfers spermatophores into her mantle cavity (TOL image). Between 30 and 150 eggs deposited on firm surfaces in each of several nocturnal spawning bouts spread over about three weeks. Female dies shortly after spawning. Young about 1.5mm long at hatching. Resemble miniature adults with relatively shorter arms and tentacles.
Current taxonomy: World Register of Marine Species (WoRMS)
Gosse, P.H. 1856. The aquarium. 2nd ed. London. pp.58-66(Archive)
Groenenberg, R.S.J., Goud, J., Heij A.de & Gittenberger,E. 2009 Molecular phylogeny of North Sea Sepiolinae (Cephalopoda: Sepiolidae) reveals an overlooked Sepiola species. J. Mollus. Stud. 75(4): 361-369
Hardy, A., 1970. The open sea: Its natural history. Part one: The world of plankton.
Marine life information network (MARLIN)
Marine species identification portal (MSIP)
Morton, J.E., 1967. Molluscs. London.
Tree of Life (TOL)
Yau, C. & Boyle, P.R., 1996. Ecology of Sepiola atlantica (Mollusca : Cephalopoda) in the shallow sublittoral zone. J. mar. biol. Ass. U.K. 76, 733-748. (pdf).