Family IDIOSEPIIDAE

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Family IDIOSEPIIDAE

Compiler and date details

July 2001 - C.C. Lu, National Chung Hsing University, Taichung, Taiwan

Introduction

Members of the monogeneric family Idiosepiidae Appellöf, 1898 are the smallest of all cephalopods in the world (known maximum size of mature males 17 mm, females 22 mm). The family is distributed in the Indo-west Pacific from Japan to Australia, and South Africa. It has not been reported from Atlantic waters. Seven nominal species are known. In Australian waters, Idiosepius pygmaeus and I. paradoxus are known from Queensland waters, their southern limit of distribution has not been established. Idiosepius notoides, an Australian endemic species, is known in southern Australian waters from southern New South Wales southward and westward to at least the Albany area (Lu & Phillips 1985).

The type species of the family, Idiosepius pygmaeus was first described by Steenstrup in 1881 from specimens collected in a surface net at 4°20'N 107°20'E, as well as a specimen from Zamboango. Berry (1921) reported the first Australian record of the family from South Australia, naming it Idiosepius notoides. Allan (1945) reported the presence of Idiosepius pygmaeus in Australia. Cotton and Godfrey (1940) also recorded I. paradoxus as from Australia. Lu & Phillips (1985) questioned the validity of the records for I. pygmaeus and I. paradoxus as no specimens of either were available for examination. Subsequent collections have confirmed the occurrence of both species, as well as I. notoides, in Australian waters.

This family is characterised by lack of an external shell and presence of a vestigial chitinous internal shell. A unique oval adhesive organ present on the dorsal surface of the mantle is used by the animal to attach itself to seaweed or seagrass blades. This glandular organ is located chiefly in the integument layer; its external surface is uneven with irregularly arranged furrows and pits. Adhesion of the animal is achieved by a sticky substance secreted by the organ (Sasaki 1923).

Idiosepids are commonly found in seagrass beds where they attach to the undersides of leaves. In Japan, I. paradoxus was found to feed chiefly on gammalid amphipods as large as its own body (Sasaki 1929). The predator bites the back of the prey just at the point where the heart is located. The animal is able to extend its buccal mass to as long as its first arm, and move it freely in all directions. By doing so, it can eat out the soft flesh of the prey without the external chitinous skin being broken.

Natsukari (1970) reported on the egg-laying behaviour and embryonic development of Idiosepius paradoxus in western Japan. A female laid eggs once or twice at three day intervals and can lay 25 to 64 eggs in total. One or two days after the final egg-laying, the female died. In captivity, in glass jars, the female adhered upside-down to the bottom of the jar. She ejected eggs one at a time from the funnel, receiving the egg in her arms. She then positioned herself by grasping the bottom with the dorsal three pairs of arms and stuck the egg to the bottom, using her tentacles. After sticking an egg, she slid backwards by hard flapping of her fins and repeated the egg laying process, taking about 30 seconds each time. The egg is elliptical in shape, measuring 0.87–0.91 mm in length and 0.67–0.72 mm in width. The incubation period of the eggs is 15–17 days at an ambient water temperature of 18.5–22.6°C.

English (1981) reported that an aquarium-kept Idiosepius species from the Sydney area, laid eggs on the side wall of the tank beneath pieces of aquarium tubing, or on the undersides of seagrass leaves. She suggested that under natural conditions, this species of Idiosepius lays its eggs on hard substrata such as bivalve shells or the undersides of blades of seagrass. The main breeding period is reported to be through the spring and summer; the life span is reported to be approximately 12–15 months.

Jackson (1986) studied Idiosepius from northern Queensland and confirmed the occurrence of both I. pygmaeus and I. paradoxus in the Townsville area. Adults of Idiosepius paradoxus are planktonic while those of I. pygmaeus have a more nektonic life. The two species show some degree of niche separation. Idiosepius pygmaeus has adapted to an estuarine existence while I. paradoxus has successfully established in the continental shelf plankton community. Studies on Idiosepius pygmaeus in the tropical water of Australia show that it has faster growth rates and shorter life spans than I. paradoxus: males mature as young as 42 days old and females as young as 60 days. The greatest ages recorded for individuals were 67 days for males and 79 days for females (Jackson 1988; Jackson & Choat 1992).

Ecological Descriptors

Marine.

Diagnosis

The mantle is elongate, slightly pointed at the posterior end. An oval attachment organ is located on the dorsal surface of the mantle. The fins are small and kidney-shaped, and are attached laterally at the posterior end of the mantle, at a slightly oblique angle in relation to the longitudinal body axis. The anterior edge of the mantle is not fused with the head. The head is prominent with large, bulbous eyes that are covered by a cornea. The nuchal cartilage is lacking. The funnel-mantle locking cartilage is peg-like in all except the southern Australian species, Idiosepius notoides, which has a simple, straight cartilage. The arms are short with two rows of suckers. The tentacles are slender, short and retractile. The tentacular clubs are not expanded, with two to four rows of suckers. Both ventral arms of mature males are hectocotylised. The modification involves loss of suckers on most of the arms and the tip of left ventral arms become bilobed.