Aeolidiella alderi: new species in Dutch waters

Aeolidiella alderi (the first 3 photo's)

While we (KNNV Strandwerkgroep Waterweg-Noord) were beachcombing and turning stones at low tide at Burghsluis (Oosterschelde, the Netherlands), Jurriën van Deijk found a 18 mm small sea-slug. Attached under a stone with hardly any undergrowth. We made some photo's to be able to identify the species. At first I thougt it was a Facelina or Coryphella species. 

It is a pity I just made some photo's for identification and not in its natural surroundings. On the other hand: they were fortunately good enough for identification.

After studying my photo's and looking at the species found in Dutch waters, I thought: let's put it up for question on the Facebook 'study group NE Atlantic Nudibranchs'. 8 minutes later Joao Pedro Silva from Lisbon, Portugal (it is a small world!), commented: 'I think the aeolid is Aeolidiella alderi. Look at the elongated cnidosacs of the first cluster of cerata'. So, I had been looking in the wrong direction: this would be a new species for the Netherlands. Of course there is always some doubt, because a 100% identification is only possible by extracting the radula or comparing DNA. But after other positive identification comments of several people and studying the characteristics I am sure it is Aeolidiella alderi (Cocks, 1852).


One of the identification features: the eyes are visible just behind the rhinophores.

It has no English or Dutch name. As a Dutchmen it is a bit presumptuous to give it a common English name, but I will give it a go: collared sea-slug. Likewise in Dutch: gekraagde vlokslak. This way I emphasize one of the characteristics of this small nudibranch: 'the numerous cerata have white cnidosacs which are particularly large in the anterior cluster, giving a white collar or ruff to the animal' (www.habitas.org.uk). In layman's words: the outgrowths at the front of the animal (not the large 'horns') have large white containers (for storing the stinging cells of sea-anemones) forming a white collar.

For the Dutch readers: ruff betekent molensteenkraag (de bekende witte kraag die figureert op schilderijen uit de Gouden Eeuw). 

For further reference look here and especially for key identification features and a comparison with related species Aeolidia papillosa and Aeolidiella glauca here. 

Its big nephew: the common grey sea-slug, Aeolidia papillosa (Linnaeus, 1761) (NL: grote vlokslak).

No white collar and the rhinophores and upper side are densely pigmented. It is not difficult to distinguish the two species.

Aeolidiella glauca (Alder & Hancock, 1845) (NL: kleine vlokslak). Again no common name. Glauca means bluish-grey or green. The authors Alder and Hancock must have been colour blind or more plausible, described a preserved and so often discoloured specimen as a then new species. Who will give it a proper common name? Both photo's of A. glauca were staged/not in a natural surrounding.

The body of A. glauca is broader than that of A. alderi and again: it has no white collar.

It has been a wonderful year for 'nudilovers' in the Netherlands, as was 2011. The Oosterschelde was riddled with Geitodoris planata (NL: millennium wratslak) and harlequin, Polycera quadrilineata (NL: harlekijnslak). I have read messages like: 'this dive I have seen more than 200 harlequins'. Rare species like Janolus hyalinus (NL: wrattig tipje) were seen. In 2011 divers discovered 4 new species for the Netherlands at the Doggersbank.

Geitodoris planata (millennium wratslak)

So, has something changed in Dutch waters or is it increased interest for nudibranchs amongst divers? The latter is certainly true. This year a 'Zoekkaart Nederlandse Zeenaaktslakken' (an identification card for Dutch Nudibranchs) is published and I think they will sell a lot of them to the diving community.

The harlequin, Polycera quadrilineata (NL: harlekijnslak)


I want to thank Jurriën van Deijk for finding A. alderi. And Joao Pedro Silva, Marco Faasse, Brendan Oonk, Bernard Picton and Rob Dekker for their comment. 

Janolus hyalinus (NL: wrattig tipje)