Author: (A. Adams & Reeve, 1850)

Order: Nudibranchia  Family: Chromodorididae

Maximum Size: 15 mm

Sightings: Sunshine Coast, Whitsundays

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Mexichromis trilineata (A. Adams & Reeve, 1850)

Mexichromis trilineata is a small species that rarely grows larger than 12 mm. The main background colour of both mantle and foot is a light purple but some specimens may be more vivid in colour. There is a thin white margin to the mantle that is repeated on the foot and tail. Rarely, there may be some yellow/orange within the white margin. As it’s name trilineata suggests, it bears three parallel longitudinal lines on the dorsum, a medial line from between the rhinophores running posteriorly to and surrounding the gill pocket, and a shorter lateral one each side. The lines are white but may carry markings within the white of orange or yellow that present as stripes, dashes or spots. Rudman suggests that, at times, as the animal grows larger the yellow/orange may expand to cover the white completely. The white lines themselves may present as mere dashes or spots rather than distinct lines or there may only be a single medial line with the lateral two absent. That presentation can sometimes lead to confusion with some species of Verconia. These variations are not overly common in some regions so it is to be expected that when the species was first described the authors who gave it that name of trilineata would not have been aware of all the possible variations. Large defensive mantle glands opening ventrally are present submarginally usually around the entire periphery and are regularly spaced, at times being quite conspicuous. Often, the posterior-most of these glands are significantly larger than the rest. Some specimens may exhibit between one to three of these glands on the posterior of the mantle only,SP the rest of the mantle periphery being devoid. The rhinophore stalks are purple with white clubs that have varying amounts of orange, the most common presentation being orange to the base and the tip with white in-between. The gills are white with a purple base and having varying amounts of orange up the rachis and on the tip.

It feeds upon a blue sponge of the Dysidea genus and it is not uncommon to see a group of several feeding together and even hollowing out cavities in the sponge as they consume it.

Spawn is laid as a spiral ribbon on edge with an undulating appearance. It is purple/pink in colour. The development type is unknown.

Distribution is the Western Pacific Ocean including Okinawa, Philippines, Indonesia, Melanesia and Queensland, Australia.

Previously known as: Pectenodoris trilineata. The genus Pectenodoris was raised by Rudman in 1984 with this species as the type (and only) species but the genus did not survive the Chromodorididae review, by molecular sequencing, and the subsequent nomenclature changes of Johnson & Gosliner in 2012.

Nearly every taxonomist who has put pen to paper about this species has waxed lyrically about its beautiful appearance.

Originally described as: Goniodoris trilineata

This species was also described by Bergh in 1905 as Chromodoris virgata and is listed under that name in Willan & Coleman, 1984.

References:
– Adams A. & Reeve L. (1848-1850). Mollusca. In A. Adams (ed.), The zoology of the voyage of H.M.S. Samarang, under the command of Captain Sir Edward Belcher, C.B., F.R.A.S., F.G.S., during the years 1843-1846. Reeve & Benham, London, x + 87 pp., 24 pls. [i-x, 1-24, pls 1-9 (1848); 25-87, pls 10-24 (1850)].

– Bergh, L. S. R. (1905). Die Opisthobranchiata der Siboga-expedition. Siboga-Expeditie. 50: 1-248, pls 1-20.

– Willan, R.C. & Coleman, N. (1984). Nudibranchs of Australia, Neville Coleman, AMPI: 26-27

– Rudman, W.B. (1984) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: a review of the genera. Zoological Journal of the Linnean Society 81: 115-273.

– Rudman, W.B. (1986) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: Noumea purpurea and Chromodoris decora colour groups. Zoological Journal of the Linnean Society 86(4): 309-353.

– Johnson, R.F. & Gosliner, T.M. (1998). The genus Pectenodoris (Nudibranchia: Chromodorididae) from the Indo-Pacific, with the description of a new species. Proceedings of the California Academy of Sciences, 50(12): 295-306.

– Wilson, N.G. (2002) Egg Masses of Chromodorid Nudibranchs (Mollusca: Gastropoda: Opisthobranchia). Malacologia, 2002, 44(2): 289-305.

– Johnson, R.F., Gosliner, T.M. (2012). Traditional taxonomic groupings mask evolutionary history: A molecular phylogeny and new classification of the chromodorid nudibranchs. PLoS One 7 (4): e33479.

– This Species Profile has been modified from a previously published article in Dive Log Magazine’s – Critter ID with NudiNotes Column, Issue: #349 (August 2017): 12 by David A. Mullins

David A. Mullins, January 2020