Nudibranchs are a supply of limitless fascination to many underwater photographers, with whole ‘festivals’ held in nudi hotspots comparable to Anilao within the Philippines; there’s even an occasion within the chilly waters of Scotland. Nudibranchs can generally be mistaken for flatworms, and vice versa, and while each are fascinating and sometimes equally colourful, it’s helpful to have the ability to decide which is a flatty and which a nudi. Right here’s a easy information to a number of the most blatant bodily variations. Nudibranchs are a part of the mollusc grouping. Thee are many subdivisions and complicated clarifications that excite taxonomists and confuse the remainder of us, however for our puposes I’ll persist with this straightforward classification. The Spanish Dancer (Hexabranchus sanguinea) above is a big nudi, which exhibits all the important thing nudibranch options: sensory organs often called rhinophores on the entrance finish, and bare gills (actually nudi and branchus) on the dorsal floor. This fellow, Pseudobiceros fulgor, is a typical flatworm. It lacks gills or any apparent protrusions from the physique and is, because the identify suggests, an annelid, a sort of worm. The problem can begin when the 2 biologically distinct sorts of animal begin to resemble one another. Take the Noticed Doris pictured under, for instance. You have to look actually near see the animal’s rhinophores. Its vestigial gills are out of view. You might simply confuse this nudi for a flatworm. Rhinophores usually have advanced whorls and textures to extend their floor space, thus aiding with their chemosensory operate. This critter (Chromodoris sp.) is an simply recognised animal and advertises its toxicity (which it derives from its sponge primarily based weight-reduction plan) with brilliant warning colours. Generally flatworms will seem to have rhinophores, however they’re truly simply extensions of the physique margin. P. bedfordii is a basic instance. Right here’s one other flatworm species, this time from the Maldives, which exhibits typical ‘false horns’. The extra troublesome animals are the sacoglossans, which seem like each or neither relying in your viewpoint. They’re in reality molluscs which feed on marine algae. This one is Thuridella hopei from the Mediterranean. Sea Hares comparable to this Dolabella auricularia at a whopping 30cm have fairly distinct protrusions that give them their frequent identify. They’re molluscs, regardless of wanting similar to some flatworms.