Family TREMOCTOPODIDAE Tryon, 1879

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Family TREMOCTOPODIDAE Tryon, 1879

Compiler and date details

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

TREMOCTOPODIDAE Tryon, G.W. Jr 1879. Cephalopoda. Manual of Conchology. 1: 1–316. 1-316 pp.
Type genus:
Tremoctopus delle Chiaje, 1841.

Introduction

This pelagic octopod exhibits tremendous size and sexual dimorphism. The females reach a large size (up to 2 m in total length), the males are dwarf. The body is muscular and firm. The two dorsal pairs of arms are longer than the two ventral pairs. The web connecting the dorsal arms pairs is deepest, with the web being very shallow to almost absent ventrally. Two pairs of cephalic water pores are located at the dorsal and ventral aspects of the head. The third right arm of mature males is hectocotylised; modification of the hectocotylisation is extensive; the hectocotylus develops in an enclosed sac.

The family Tremoctopodidae was established by Tryon (1879) based on the genus Tremoctopus Delle Chiaje, 1830. Many other reports on this animal have been published subsequently on specimens captured in the North Atlantic, the Mediterranean Sea and the Pacific Ocean (Jatta 1896; Berry 1912; Naef 1923; Akimashkin 1963). Robson (1932) first revised the genus and, more recently, Thomas (1977) reviewed its systematics, distribution and biology.

Tremoctopus is a cosmopolitan, pelagic octopod genus, its members inhabiting tropical, subtropical and temperate waters. According to Thomas (1977), the family Tremoctopodidae consists of three species level taxa. Tremoctopus gelatus Thomas, 1977 is found off the eastern coast of Florida, Hawaii Islands and the Indian Ocean. Tremoctopus violaceus violaceus Delle Chiaje is found in the Atlantic Ocean, the Mediterranean Sea, Gulf of Mexico and the Caribbean Sea; and T. violaceus gracilis is found in the Pacific and Indian Oceans. Tremoctopus is epipelagic, living in the surface waters (Clarke & Lu 1975; Lu & Clarke 1975; Roper & Young 1975).

The first record of the family from Australian waters was that of Lu & Phillips (1985) who recorded Tremoctopus violaceus from the coast of New South Wales. Zeidler (1989) expanded the distribution, including records for Queensland, South Australia and Western Australia.

Thomas (1977) described the reproductive system of Tremoctopus violaceus in detail. Tremoctopus lacks a penis. The Needham’s sac is blind and the entire sac with the enclosed spermatophore is transferred to the spermatophore reservoir at the tip of the hectocotylised arm. Only a single spermatophore is produced in the lifetime of the male. The spermatophore reservoir connects with the penial filament by means of a duct that passes into the filament.

Tremoctopus autotomises its hectocotylised arm during mating. Sexually mature females are frequently found with these detached arms lying in the mantle cavity, with the ‘penis’ missing and spermatophore reservoir burst.

Females produce large egg masses comprising more than 100 000 small eggs. The egg stalks are cemented together to form root-like structures, which are held by the suckers of the female’s dorsal arms where the eggs are brooded (Naef 1923; Hamabe 1973).

The young Tremoctopus usually carry pieces of Portugese-Man-of-War, Physalia, tentacles in the dorsal and dorso-lateral arms and its suckers are filled with batteries of Physalia nematocysts (Jones 1963). This functions as a defence mechanism.

The food of Tremoctopus is varied. d’Orbigny (1840, in Férussac & d'Orbigny 1835–1848) reported finding abundant shells of pteropod molluscs in young animals, and Thomas (1977) found the adult females feed chiefly on small fish. Zeidler (1989) reported finding fish scales, cephalopod flesh, green and brown algae and polychaete jaws in Australian specimens. Predators of Tremoctopus include Alepisaurus ferox (Rancurel 1970).

Diagnosis

The body is muscular and firm. The two dorsal pairs of arms are longer than the two ventral pairs. The web connecting the dorsal arms pairs is deepest dorsally and very shallow to almost absent ventrally. Two pairs of cephalic water pores are located at the dorsal and ventral aspects of the head. The arm suckers are in two rows. The family exhibits pronounced sexual dimorphism in size. Females are large, reaching to 2 m in total length. Males are dwarf. The third right arm of mature males is hectocotylised; the hectocotylised arm develops in a pouch below the eye and is detachable at maturity.