Family ARGONAUTIDAE

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Family ARGONAUTIDAE

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

Introduction

Members of the family Argonautidae Naef, 1912 are pelagic octopods that exhibit extreme sexual dimorphism in size. Females attain a large size, over 100 mm mantle length (ML), while the males are minute, up to 15 mm ML depending on the species.

Female Argonauta secrete a thin, laterally compressed calcareous structure. It is a single-chambered egg case with a flat keel fringed by two rows of tubercles. The lateral sides of the shell are ribbed with the centre pressed in, or bent outwards into a prominent ‘ear’ or ‘horn’. The shell provides protection and flotation and allows attachment of the eggs (Young 1960).

Records and studies of Argonauta date back several hundred years. Many names, records and descriptions exist in an extensive bibliography ranging from sale catalogues of curios to serious zoological literature. Robson (1932) gave extensive listings of names, literature and the available biological information. Nesis (1982) recognised only four valid species worldwide: Argonauta argo, A. nodosa, A. boettgeri and A. hians.

McCoy (1882) described the first argonaut from Australia, Argonauta oryzata (now considered to be a junior synonym of Argonauta nodosa). Brazier (1892) in his catalogue of the Australian Museum collection listed A. argo, A. nodosa, A. hians and A. boettgeri from Australian waters. Lu & Phillips (1985) updated the distributional records and confirmed the presence of A. argo, A. nodosa and A. hians in Australian waters. The presence of A. boettgeri remains to be confirmed.

Argonauts feed primarily during the day (Nesis 1977), apparently on pelagic molluscs. Okutani (1960) reported that, off Japan, female Argonauta boettgeri prey on the pteropod, Carolina tridentata. Nesis (1977) also found that pelagic molluscs e.g. heteropods and pteropods, are the dominant prey of A. boettgeri from the western Pacific, with other octopods less frequently among the food items. Remains of heteropods have also been found in the stomachs of A. hians (Nesis 1977).

Argonauts themselves are preyed on by many predators. Okutani & Suzuki (1975) reported Argonauta boettgeri in the stomach contents of a yellowfin tuna from the Indian Ocean. Rancurel (1970) reported finding A. argo, A. hians and A. nodosa(?) in the stomach contents of Alepisaurus ferox from the south-western Pacific. In Australian waters, A. nodosa was found in the stomach contents of the Australian fur seals, Arctocephalus pusillus doriferus, in the Bass Strait and southern Tasmania (Gales et al. 1994).

Males of Argonauta boettgeri and A. hians mature at about 7 mm ML, presumably the maximum size attained (Nesis 1977). Argonauta argo males mature at 8 mm ML (Naef 1923). No data are available on males of A. nodosa. The hectocotylus is lost during mating, and all males die soon after their first mating (Nesis 1977).

In females the maximum shell (egg case) length exceeds the mantle length and the animal can be completely withdrawn into shell. In Argonauta boettgeri and A. hians the shell is formed at a mantle length of 6.5–7 mm (Nesis 1977), and at the same size in A. argo (Naef 1923).

Mating occurs shortly before the first eggs are laid. Female Argonauta boettgeri, taken at length 11–13 mm ML, already had hectocotyli in the mantle cavity. Egg laying begins when females reach a size of 14–15 mm; female A. hians, 18–20 mm ML, have laid eggs. The size at which egg-laying commences differs in different regions of the world's oceans (Nesis 1977). Female A. argo mature at about double the size of A. boettgeri and A. hians (Naef 1923).

The eggs are laid in clusters consisting of many small eggs connected by a stalk. They are attached to the apex of the shell, occupying its posterior part. Clusters of eggs at different developmental stages are found in the shell. Nesis (1977) found that in Argonauta boettgeri the egg clusters can be clearly divided into three parts; each has eggs at a similar developmental stage. The first portion lies closest to the aperture of the shell and contains eggs at an early stage of development. The second portion is found in the middle of the mass and contains eggs at stages ranging from the appearance of red eye pigment to the beginning of chromatophore formation. The third portion is located farthest from the aperture of the shell and contains eggs with embryos that are fully formed and ready to hatch; the chromatophores, ink sac and dark coloured eyes are fully formed. Reid (1989) found similar development in the egg mass of A. nodosa from southern Australia.

Hatching takes place at night. Newly hatched larvae are released and can be caught in plankton samples at night. Egg laying probably also occurs at night. Nesis (1977) suggested that the three stages of development in the egg clusters represented the products of three successive nights. Egg incubation lasts three days at temperatures of 26–29°C.

Female argonauts begin to reproduce very early and continue to grow and reproduce for a long time. Shells of a large size range have been collected. Shells of recently mature females contain only a small cluster of eggs at the first stage of development. Large females generally have clusters containing eggs at all three stages.

Both Argonauta hians and A. boettgeri are known to cling to any object floating on the surface of the sea, including other argonauts (Nesis 1977). Williamson (in Voss & Williamson 1971) reported finding a chain of argonauts clinging to each other. Nesis (1977) reported that up to 20–30 argonauts of similar size can be involved in such a chain, with the first female usually holding onto some inanimate object while other females in the chain hold onto the proceeding one, on the ventral part of the shell.

Argonauts are epipelagic living predominantly in the upper 100 m (Lu & Clarke 1975; Roper & Young 1975). Nesis (1977) believed that mature animals are not restricted to the surface layer but live at all epipelagic depths.

Argonauta argo and A. hians are cosmopolitan, occurring in tropical to subtropical oceans while A. boettgeri occurs in the tropical and subtropical waters of the Indo-west Pacific. Argonauta nodosa is known only from the Southern Hemisphere, in the Indian Ocean and Pacific Ocean.

In Australia, A. nodosa is known from southern Australia from waters off New South Wales, Victoria, Tasmania and South Australia. Argonauta argo is found in the warmer waters off Western Australia and north of Gabo Island, Victoria. Argonauta hians is known from the North West Shelf of Western Australia.

Diagnosis

The body is firm, with the mantle thin but muscular. The mantle locking apparatus is well developed and complex. The arms bear two rows of suckers, and are connected by shallow web. Sexual dimorphism is pronounced. Males are dwarf with the entire left arm III hectocotylised, developed in a pouch beneath the eye; at maturity, the hectocotylised arm is autotomised. In females, the distal tips of dorsal arms bear broad, expanded, membranous, glandular flaps that secrete and hold the thin, fragile shell (egg case).