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Hydromyles globulosa (Rang, 1825)

Overview

This naked pelagic snail is about 1 cm long. It is ball-shaped, slightly transparent and intensely orange or yellow. When swimming the wings and the two tentacles are the same size as the wings and prominent. In retracted condition the animal is just an orange ball. It lives in the Indo-Pacific Ocean in the upper water layers, where it occurs in massive blooms (Hydromyles globulosa detail, Hydromyles globulosa drawing). Also see the Hydromyles globulosa.mov movie.

Taxonomic Description

According to Rang (1827) the animal is blue, somewhat transparent with oval wings and a very thin and flexible "shell" of a yellow colour. The wings resemble those of Thliptodon and Laginiopsis. They are spur-like, being somewhat broader at the distal end near the attachment to the body (Hydromyles globulosa). The posterior footlobe is long, the lateral footlobes are rounded and directed anteriorly. Tesch states that the median and lateral footlobes are clearly separated, but at the same time suggested that the median footlobe may be not homologous with the typical gymnosomatous median lobe. The visceral mass, fixed to the head, is attached by two pairs of muscles to the outer integument which has no direct contact with the visceral mass (Hydromyles globulosa line). The animals are able to contract completely so that the contracted specimens look like balls. All anterior parts: the head, the footlobes, the tentacles and the wings are then covered by the mantle-like body fold which closes on top of the retracted organs. The muscles in between the body and this "mantle" suggest that the outer cover of the specimen is not homologous to the molluscan mantle. The hook sacs are absent, the radula and jaw are reduced. There are some rows (up to 5) of chitinous plates with some rows of points or small denticles directed towards the buccal aperture. The rows of chitinophore cells are somewhat irregular and there is a resemblance between the chitinous plates and the jaw of Cliopsis krohni. The radula is composed of about ten rows of teeth with the formula 1-1-1 (Hydromyles globulosa rad, Hydromyles globulosa radul, Hydromyles globulosa radula). The median plate is moon-shaped and without clear cusps or dents. The animals are so contractile that measurements are not accurate.
The body length is about 10 mm max.

Juveniles

A special description is not available. The juvenile stage develops in the mother animal and the young specimens become free when they are the shape of small adults.

Reproduction

The species is a protandric, ovoviviparous hermaphrodite. The young specimens develop in the accessory sexual gland, and when growing they break through its wall to complete development in the body lumen of the female. A large number of adults always contain embryos. A regression of the visceral mass in the adult females with embryos is always seen, and a complete decomposition of the visceral mass in the fully pregnant stage is supposed to occur. The juveniles are delivered by bursting of the body. The contractility of the pregnant specimens diminishes and their skin configuration becomes less glandular in character.

Ecology

It is a carnivore. Ink production in this species is described by Lalli and Gilmer (1989). (See also Cephalobrachia macrochaeta).

Distribution

H. globulosa is a subtropical species of the Indian and Pacific Oceans, see the Hydromyles globulosa map, and is frequently very abundant. It is probably less abundant in real equatorial waters in the Pacific. Currents strongly influence the range of this species as demonstrated by transport of populations to the northern part of the NE-Pacific and to the southern corner of the Tasman sea. The greatest concentration of specimens is found in the Indo-Malayan area and Tasman Sea.

Types

Psyche globulosa Rang, 1825: 284, pl. 6, fig. 1.
The types could not be traced.

Hydromyles globulosa