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Phylliroe bucephala Péron and Lesueur, 1810

Overview

Leaf-like laterally flattened pelagic mollusc, without papillae and completely transparent. The 'X'-shaped digestive gland is characteristic. It lives in the upper water layers of warm waters in all oceans (Phylliroe bucephala2, Phylliroe bucephala 4). The juveniles are parasitic on the medusa, Zanclea costata.

Taxonomic Description

The body is laterally compressed, elongate and fish- or leaf-like. It is perfectly transparent (Phylliroe bucephala). The tail is long, more than 16% of the body length. There are no papillae but one pair of well developed tentacles (the rhinophores) are present. The foot is reduced to a pedal gland not disturbing the body outline. The head, before the rhinophores, is downward directed with a terminal mouth. The anus is situated on the right lateral side in the centre of the body. The digestive gland, and other organs are seen through the skin. The digestive gland is 'X'-shaped. There are 3 to 5 gonads. The radula formula is 3-1-3 to 5-1-5. The most central or two most central lateral teeth are distinct from the outer ones in having dentation on both sides, the outer laterals have only dentation on the inner border. The median teeth are unicuspoid, with small denticulation on both sides of the median cusp.
Body length up to 55 m.

Juveniles

A special description is not available.

Reproduction

This species is hermaphroditic. The veligers, without shell, are found in the bell of Zanclea costata (Mnestra parasites Krohn is its senior synonym but Zanclea costata was proposed in favour for stability of nomenclature). The parasiting juveniles grow larger in the bell and reach the stage where they can execute swimming movements. At that time the Zanclea costata meduse becomes degenerated. The mollusc is still connected to the inside of the umbrella with its rudimentary foot. The meduse now becomes smaller after being eaten by Phylliroe bucephala and only a small remnant is seen in a two week old specimens. After this stage, when the mollusc has grown to about 12.0 mm long, the rest of the meduse is thrown off. These data on this brood parasitism are given by Martin and Brinckmann (1963). The development is illustrated in the following series of pictures showing the Phylliroe bucephala 1°stage in the bell of Zanclea costata, Phylliroe bucephala 2°stage, Phylliroe bucephala 3°stage and finally the Phylliroe bucephala 4°stage where Zanclea costata is nearly absorbed, except for a small portion near the foot of the mollusc. The full grown specimens sometimes have remains of the medusa sticking to the pedal gland.

Ecology

This epipelagic species is highly bioluminescent and emits flashes of light when disturbed. It feeds on Zanclea costata., Oikopleura albicans and Siphonophora. The temperature range of this species is at least 18°-25°C.

Distribution

This species lives in all oceans, including Mediterranean, see the Phylliroe bucephala map.

Phylliroe bucephala