Flabellina sp. 9 from the Andaman Sea

Dear Lawrence,

Thanks for these photos, which give us such a good view of the colour pattern and the external anatomy. As to identifying species to genera from external features, I am afraid most rules of thumb are broken in most genera, and definitely all families, of the aeolids.

The arrangement of the cerata in vertical rows or in arches, whether the rows are single or double, the position of the anus and the renal opening, and the shape of the rhinophores, are all useful external indicators. Unfortunately when you try and make a key or set of rules, there are so many exceptions it is not worthwhile. Internal anatomy, such as the shape of the radular teeth etc is vital.

To illustrate external anatomy I have chosen two species with a similar pattern of orange lines to Flabellina sp. 9. Cratena simba is a typical example of that genus with the first group of cerata on each side being arranged in a single arch, while the rest of the cerata are arranged in single spaced rows.

The second example is Phidiana anulifera, which some would call Facelina anulifera. The cerata in Phidiana are usually arranged in vertical rows, the most anterior cluster on each side consisting of 6 or so rows, and the number of rows in the other clusters diminishing as we move posteriorly.

In Flabellina sp. 9 the cerata are arranged on raised ridges or 'brackets', in an arch-like pattern. This is quite unlike any Facelina, but is reminiscent of some species of Flabellina. Another possibility is that it is a new species of Pteraeolidia, as it has many external similarities to P. ianthina. However until we know something of its anatomy we can't be sure, though I would be very surprised if it turned out to be a Facelina or Phidiana.

 Best wishes,
Bill Rudman