Chromolaichma sedna

Chromolaichma sedna is a dorid nudibranch in the family Chromodorididae ^[1] that feeds on sponges in Mazatlán Bay, Mexico ^[2]. Since 2024 the name has been kept for animals from the Gulf of California and the eastern Pacific, because the western Atlantic populations turned out to be a separate sister species, Chromolaichma hemera ^[3].

Its tissues hold terpenoids that showed antibacterial activity and toxicity to fish in laboratory tests ^[4],[5]. The 2023 review of the nudibranchs recorded in Peru does not list it among the confirmed species nor among those that may potentially occur ^[6].

It was described in 1967 as Casella sedna and later moved through the genera Glossodoris, Chromodoris and Doriprismatica; Chromodoris fayeae is one of its synonyms ^[1]. It is the type species of Chromolaichma, a genus set up in 1977 that the 2012 molecular analysis folded into Doriprismatica ^[7] and that was reinstated in 2018 for this species and 2 others ^[3].

The Atlantic sister species has a translucent white body, a mantle edged by a yellow marginal band with a thinner red band inside it, the top third of the rhinophore lamellae red and red tips on the gills; according to the same study, Chromolaichma sedna differs from it by a whiter ground colour, a narrower yellow band and a mantle edge with fewer undulations ^[3]. The specimen collected at Huatulco, Mexico, measured 31 mm ^[8].

Similar species

• Chromolaichma hemera: It lives in the western Atlantic and the Caribbean, not in the Pacific; its ground colour is less white, its yellow band is wider and its mantle edge has more undulations than that of Chromolaichma sedna ^[3].

The type locality is in the Gulf of California, and the Pacific specimens studied in 2024 came from the Gulf of California, Cabo San Lucas and San José del Cabo in Mexico, Costa Rica and the Gulf of Chiriquí in Panama ^[3]. In 2025 it was recorded for the first time in Oaxaca, Mexico, from one specimen at Isla Montosa, an island with patches of coral, sand and large boulders; that study gives its known range as the Gulf of California to Ecuador ^[8].

In Mazatlán Bay, Mexico, it was collected on rocky subtidal bottoms by scuba diving and snorkelling ^[2]. In the 2024 study, 20 specimens of this species and its Atlantic sister were collected by hand on scuba at depths between 5 and 15 m ^[3].

For the genus, the 2024 study summarises a lifespan of about 1 year, simultaneous hermaphroditism and planktotrophic larvae with high dispersal potential ^[3]. The authors caution that these are literature data, gathered for the genus where possible and that may reflect patterns of broader groups ^[3].

Eastern Pacific populations are genetically very uniform: 3 eastern Pacific specimens differed by 0 to 0.7 % in the COI gene ^[7], and in 2024 the mean divergence was 0.45 %, with every haplotype close to a central one from Mexico and the Gulf of California ^[3]. Against the Atlantic populations the minimum COI divergence was 13.6 %, a split attributed to the closure of the Isthmus of Panama 2.8 million years ago ^[3].

In Mazatlán Bay, analysis of gut contents and faeces showed that this species had the most varied diet among the sponge-eating opisthobranchs: 16 prey species, all spicule-bearing demosponges, 8 of them in the order Hadromerida ^[2]. The Huatulco study cites that result and describes it as polyphagous ^[8].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

Specimens collected in Baja California, Mexico, yielded 2 sesterterpenoids, sednolide and sednolide 22-acetate; the first inhibited growth of the marine bacterium Vibrio anguillarum at 100 µg per disk ^[5].

Specimens from Costa Rica contained several scalarane-type sesterterpenes ^[4]. One of them was toxic to the fish Gambusia affinis at 0.1 ppm and moderately inhibited mammalian phospholipase A2, with an IC50 of 18 µM ^[9],[4].

In 2024 this species and its Atlantic sister were used to calculate the first molecular divergence rates for chromodorid nudibranchs, with the closure of the Isthmus of Panama as the calibration: 4.9 % per million years in COI and 2.0 % in 16S ^[3]. Its metabolites are part of the study of bioactive compounds from marine molluscs ^[4].

The 2023 literature review of Peru's nudibranchs, with 31 confirmed species and 9 that may potentially occur, does not include Chromolaichma sedna, so there is no published record of the species in Peruvian waters up to that date ^[6].