Felimare agassizii

Felimare agassizii is a dorid nudibranch in the family Chromodorididae. Bergh described it in 1894 as Chromodoris agassizii, and it has also been reported as Hypselodoris agassizii; both combinations are now treated as synonyms ^[1].

It is set apart by a double yellow and pale blue band around the mantle edge, together with white gills tipped in blue. It is a sea slug of the tropical eastern Pacific ^[2]. It eats sponges, and in Peru it is known only from 2 records on the northern coast ^[3],[4].

The most useful feature is the mantle edge: a solid double band, yellow on the inside and blue on the outside, broken at two points, between the rhinophores and halfway along the back. The gills are white with blue tips ^[2].

At Huatulco, on the southern Pacific coast of Mexico, the specimens measured were between 9 and 55 mm long ^[5].

Similar species

• Felimare californiensis: It is similar in size and general colour and the two overlap in the southern Gulf of California, but its gills are the colour of the back and it lacks the double yellow and blue band on the mantle edge ^[2].
• Felimare sechurana: A species of northern Peru that lacks the yellow and pale blue mantle band; according to its describers, the two ranges do not overlap ^[2].

The review of Peruvian nudibranchs gives it a range from the Gulf of California to the northern coast of Peru, and also mentions records from the Gulf of California (Mexico), the Galápagos Islands (Ecuador) and Malpelo Island (Colombia) ^[4]. Another taxonomic study, by contrast, states that to the south it reaches only Panama, the Galápagos Islands and Colombia, leaving Peru out ^[2].

It is a benthic species ^[4]. At Huatulco it turned up at 3 sites whose bottoms combine corals with rocks or large boulders; one of them also had algae and plenty of sponges. Those sites were surveyed between 2017 and 2018 by snorkelling and scuba down to 18 m ^[5].

It belongs to the genus Felimare, erected by Marcus and Marcus in 1967. A molecular analysis of the chromodorids gathered into that genus every eastern Pacific, Atlantic and Mediterranean species previously placed in Hypselodoris, and that work already lists the species as Felimare agassizii ^[6]. In a later genetic study of the genus, the specimens identified as Felimare agassizii formed a group of their own, with strong statistical support ^[2].

Its diet was studied in Mazatlán Bay, in the Gulf of California, from stomach contents and faeces. There it proved polyphagous: it ate 9 species of demosponges, 8 of them with spicules and 4 in the order Hadromerida; the remaining one is a horny sponge, Dysidea uriae ^[3].

In the same work 23 opisthobranch species were examined and only 5 had sponge remains in the gut; this species was one of them. Together, fishes and opisthobranchs fed on about 46 % of the sponge species known from the bay, although the authors conclude that this predation seems to have little bearing on where sponges grow or how abundant they are ^[3].

The same authors recall that an earlier study from Nayarit had proposed that this species fed on a sponge of the genus Euryspongia, which they consider probably a Dysidea, and they note that Hypselodoris species show a high specificity for sponges related to Dysidea ^[3].

For tropical species that in Peru have only been recorded at the far north, among which the review of Peruvian nudibranchs includes this one, two explanations are put forward: their presence may be temporary, tied to the displacement of warm water, or they may have adapted permanently after several El Niño events ^[4].

In Peru it has been reported from Cancas, Tumbes, where Nakamura recorded it in 2006 at 7 to 8 m depth, and from Lobos de Tierra Island, Lambayeque, according to a 2013 paper. Photographs of Peruvian specimens were published in 2007 and 2013 ^[4].

These are only 2 records, both from the north. Divers at Los Órganos should compare it with Felimare sechurana, the other Felimare species reported for Peru, recorded at Punta Veleros and described in 2017 from a specimen collected at Foca Island ^[4],[2].