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Taxonomische classification
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Fol, 1872

A small appendicularian. The trunk is ovoid. The tail is narrow, being 2-4 mm; the length of the tail is about four times the body length; the tail has a narrow tail muscle and two distinct, characteristic sub-chordal cells at about 1/2 to 2/3 down the length of the tail (well distinguishable even in unstained material).
In this species the sexes are separated (which is not the case in all other known species of Appendicularia). Ovary or testis posteriorly.
Mouth with a small lower lip. The endostyle is rather voluminous, posteriorly prolongated to just near the anus. The buccal glands are small and spherica. The right stomach lobe forms a cloacal sac behind the entrance to the intestine.

The length of the body (i.e. without the tail) is usually between 0.5 and 1.0 mm but may reach 1.3 mm.

Depth range

Distribution in the North Sea
Occurs in the whole North Sea. Common in coastal and brackish waters and estuaries. Also in Wadden Sea, Kattegat and W Baltic (occasionally E Baltic also).

World distribution
A semi-cosmopolitan, neritic species from warm and temperate waters. Atlantic, Indian and Pacific Oceans. Also present in Mediterranean Sea and Red Sea. From all Appendicularia, O. dioica is the most eurytherm and euryhaline species.

Oikopleura dioica