Eudistoma Caullery, 1909

Monniot, Françoise, 2009, Some ascidians from Indonesian marine lakes (Raja Ampat Islands, West Papua), Zootaxa 2106, pp. 13-40 : 26

Genus Eudistoma Caullery, 1909

The identification of lobed colonies with pedunculate erect lobes is complicated by the uniform structure of the zooids, and their general strong contraction when fixed. The intraspecific variability of pigmentation is common. The shape and size of the larvae are more reliable characters to differentiate species but they are not often described in the literature. This difficulty arises with the Raja Ampat material.

Successive descriptions of specimens collected in the Pacific and identified as Eudistoma laysani ( Sluiter, 1900) or Eudistoma viride Tokioka 1955 are not satisfying. Sluiter’s type specimen (ZMA TU 795) is in 4 pieces of 1 to 4 lobes, the largest 7mm. There is no trace of pigment spots close to the oral siphon and the peduncle of the lobes contains fecal pellets. These colonies are too small and too contracted to observe the anatomical details. No larvae are present in Sluiter’s material.

The type specimen of E. viride has been searched in vain.

Among the characters described in different papers, no true differences can be found in the thorax or abdomen, except the size of the larvae when it is given, their shape being the same. The dark pigment spots at the base of the oral siphon helps the identification but in some zooids of large colonies these spots are missing (personal observation). After a long time of fixation the pigment sometimes fades or disappears. Nevertheless it must be noted that in Raja Ampat the collectors have successfully separated the species in the field, which, after dissection, have effectively shown different larvae.

The identifications here are essentially based on the larvae: large in E. laysani 1 to 1.1mm in length, and 0.5mm for E. viride .