Author: Ian Smith October 2012
Body
Wide, flattened, oval mantle (body excluding head and arms), up to about 400mm long . Front dorsal edge of mantle protrudes as rounded lobe reaching almost to between eyes [image1]. Lateral fin extends full length of body on each side; not united at posterior [2]. Exhalent respiratory funnel extends from mantle cavity on underside [3]. Ventral surface whitish [4]. Dorsal surfaces of mantle, head and arms covered in tiny elastic pigment spots (chromatophores) in three layers: bright yellow (outer layer), orange-red, and black-brown. A lower fourth layer of non-elastic cells (iridophores), exposed when other chromatophores contracted, reflects brilliant white light like snow crystals [5], and, when mixed with other colours, can cause glow of superficial iridescence e.g. green [4], blue [13] or violet [14].
Head
Large, highly developed eyes usually part-covered by W-shaped eyelids. Pupils are horizontal slits, so slit aperture permits good vision [6]. Eyes surrounded by substantial ridges [5].
Arms & tentacles
Eight visible arms, shorter than head, including two lateral arms like sickle blades [7]. Each arm has four rows of short-stalked suckers [8]. Two extendible long tentacles hidden in pits either side of mouth, except when shot out to seize prey.
Shell & beak
Internal shell, “cuttlebone”, up to about L. 270mm x W.105mm; largest mollusc shell likely to be found beached on British and Irish shores. Elongate oval, almost parallel sides and evenly rounded posterior, width about 40% of length (Flickr) . (Following measurements relate to 210mm long cuttlebone.) Composed of sheet (gauge 0.6mm) of translucent, horny-yellow, conchiolin [16], similar to pen of squids, but only visible at the edges until erosion of overlying white, hard, thin, rugose, calcareous, dorsal layer (gauge 0.7mm) with many growth lines; below conchiolin a layer (20mm thick) [17] of over 100 soft chalky laminae [18] held apart by innumerable pillars [19] enclosing gas filled cavities. Earliest, narrowest, cavities at dorsal posterior, latest, widest cavities (0.6mm or more between laminae) ventral, extending to anterior. Gas causes cuttlebone to float when separated from body. Cuttlebone positioned dorsally over visceral mass; acts as supporting endo-skeleton to body. Rate of laminae creation varies 18.7/month in summer - 1.6/month in winter in Brittany (le Goff et al.).
Chitinous “parrot’s-beak” at base of arms (Flickr).
Sepia officinalis
- Wide flattened oval mantle up to about 400mm long.
- 8 visible short arms; 2, usually hidden, extendible long tentacles. [7]
- 4 rows of short-stalked suckers on each arm. [8]
- Lateral fin extends full length of body on each side; do not unite at posterior. [2]
- Front dorsal edge of mantle protrudes as rounded lobe reaching almost to between eyes. [1]
- White chalky cuttlebone up to about L. 270mm x W.105. Sides almost parallel. Width about 40% of length. Yellow periostracum on side flanges (Flickr).
Sepia elegans Blainville, 1827 (ICTIO)
- Mantle (body excluding head and arms), up to 100mm long.
- 8 visible arms, 2, usually hidden, extendible tentacles.
- 2 rows of short-stalked suckers on each arm.
- Somewhat flattened globose body, brownish dorsum lacking any pattern, whitish ventrally.
- Lateral fins extend full length of mantle on each side, do not unite at posterior.
- Front dorsal edge of mantle protrudes as acutely angled lobe between eyes.
- Rose flushed dorsal surface on soft, calcareous, lanceolate cuttlebone; edges taper to both ends. Width about 30% of length.
- Posterior spine on cuttlebone does not protrude from living body.
- Tentacular club short and blunt. 6-8 suckers in each row.
- Lives over sandy and muddy seabeds offshore, to south and west of Britain.
Sepia orbignyana (Ferrusac, 1826) (ICTIO)
- Mantle, (body excluding head arms and tentacles), up to 120mm long.
- 8 visible arms, 2, usually hidden, extendible tentacles.
- 4 rows of short-stalked suckers on each arm.
- Somewhat flattened globose body, reddish brown lacking any pattern, ventrally paler.
- Lateral fins extends entire length of mantle, do not unite at posterior.
- Front dorsal edge of mantle protrudes between eyes as large rounded lobe.
- Rose, orange or pale golden yellow (Flickr) flushed dorsal surface on soft, calcareous, narrowly oval, cuttlebone; width about 30% of length.
- Long posterior spine on cuttlebone protrudes from living body.
- Tentacular club has over 100 suckers arranged in rows of 5; median row has 2 large and 3 very large suckers.
- Lives over muddy seabeds offshore, mainly to south of Britain.
Sublittoral, inshore shallows in summer, offshore to 250 metres. Nektonic; fins ripple to propel forwards or backwards [4], and buoyancy controlled by varying gas / water content of cavities in cuttlebone. Exhalent funnel turns in any direction to squirt strong jet of water to rapidly escape danger [3]. Often benthic, lying on / in sand, during daytime.
By expanding or contracting different chromatophores, colour can change dramatically in 0.6 of a second to a wide range of hues and patterns that reflect the Sepia’s activity or emotion. Waves or ripples of colour can move rapidly across the suface. When swimming, frequently dark brown dorsally with dark ‘zebra’ stripes on either side; effective camouflage when among long leaves and shadows of Zostera [2], or unbanded brown when over brown seaweed [15]. Resting on sand, yellow-brown mottling assumed and sand blown onto back [9], and mantle surface may be raised into granules resembling sand [10]. Over seabed with light pebbles, body may go dark with blotches of white [11].
When waiting for prey, middle pair of arms often held upright [10] and lateral arms spread sideways. Buried shrimps exposed by water blown from exhalent funnel onto sand and seized by two rapidly extended long tentacles. Crabs seized from behind, bitten with strong parrot’s-beak jaws and subdued with poison.
Predators include large fish, dolphins and man (British catch mainly exported). Eaten by sea-birds, at least when already dead; beak marks frequent on beached cuttlebones [17]. If threatened, whole animal may go white except for two black spots resembling large eyes, and, as it jets away, whole animal instantly changes to blackish to confuse predator. Sometimes discharges ink (sepium) to distract predator as it flees.
Breeds at age 18-20 months in English Channel, 14-16 months further south. Male assumes zebra pattern as approaches female. Fertilization internal, via male’s left arm, adapted for transfer of spermatophore packages, into female’s mantle cavity. Eggs, enclosed in tough, black, onion-shape case [12], expelled individually over several days through respiratory funnel and attached by stalk to seaweed or other objects to form bunch of up to 300 black “grapes”. Young hatch as miniature adults with a yolk reserve. Female dies shortly after breeding.
N.E. Atlantic from Shetland to Canary Islands, and in Mediterranean (GBIF map). Around all coasts of Britain and Ireland; main range English Channel to Anglesey and Kent. Rapidly spreads north in warm summers, with numerous mortalities in cold winters. Cuttlebone floats, so may be beached far from inhabited areas. (UK interactive distribution map, NBN ).
Current taxonomy: World Register of Marine Species (WoRMS)
Hardy, A., 1970. The open sea: Its natural history. Part one: The World of
Plankton.
le Goff, R. ,Gauvrit, E., Pinczon du sel, G. & Daguzan, J. , 1998. Age group determination by analysis of the cuttlebone of the cuttlefish Sepia officinalis L. in reproduction in the Bay of Biscay. J. Moll.Stud. 64(2): 183-193. London.
Morton, J.E., 1967. Molluscs.London.
Turk, S.M., Light, J.M, and Tompsett, P.E., 2000. A large Cornish and Scillonian wreck of cuttlebones Sepia officinalis and # S. orbigniana. The Conchologists’ Newsletter 155: 414-419.
Yonge, C.M, & Thompson, T.E., 1976. Living marine molluscs.London
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