Family THYSANOTEUTHIDAE

Museums

Museum list

Regional Maps

Family THYSANOTEUTHIDAE

Compiler and date details

C.C. Lu, National Chung Hsing University, Taichung, Taiwan

Introduction

The family Thysanoteuthidae Keferstein, 1866 comprises two genera, Thysanoteuthis Troschel and Cirrobrachium Hoyle. Only the monotypic genus Thysanoteuthis is well known. Cirrobrachium is known from larval forms and fragments of adults from the Atlantic only and its taxonomic status is unclear.

Thysanoteuthis rhombus was recorded from among the stomach contents of lancetfish from the south-western Pacific and north-eastern Indian Ocean by Rancurel (1970) and Fujita & Hattori (1976). Nesis (1979) recorded net-caught specimens from the Australasian region and the species was first recorded from Australian nearshore waters by Dunning (1982).

Thysanoteuthis rhombus may exceed 800 mm mantle length (ML); females spawn gelatinous sausage-shaped egg masses 150 to 200 mm in diameter and up to 1 m long. A spiral double row of eggs of up to 2 mm diameter is contained in the external layers and these masses have been found near the surface. Around Japan, spawning occurs during the summer months; the smallest hatchlings are 1.1 mm ML (Misaki & Okutani 1976; Suzuki et al. 1979). Lancetfish (Alepisaurus), yellowfin tuna, spotted dolphin and blue marlin all prey on T. rhombus (Clarke 1966; Rancurel 1970, 1976; Fujita & Hattori 1976; Wolff 1982).

Thysanoteuthis rhombus is an oceanic species, generally caught in the upper 50 m of the water column. Juveniles apparently are capable of leaping out of the water but not of 'flying’ as observed in ommastrephids and onychoteuthids. Adults are slow swimmers (Nishimura 1966). In Australian waters, thysanoteuthids have been observed singly or in pairs, although groups of up to 20 have been observed elsewhere (Clarke 1966).

This species occurs circumglobally in tropical surface waters and in the East Australian Current system as far south as north-eastern Tasmania. Juveniles have also been recorded from tropical waters off the North West Shelf.

Diagnosis

Thysanoteuthis rhombus is characterised by a funnel locking cartilage with a long narrow longitudinal groove and a short broad transverse groove, a muscular mantle, and rhombic fins extending the length of the mantle in adults. Arm suckers are biserial and the tentacular suckers are tetraserial. The lateral arms are strongly keeled in large specimens. Long cirrate trabeculae on the arms support a well-developed web, and buccal connectives attach to the ventral borders of the ventral arms. The left ventral arm in males is hectocotylised. No photophores are present.