Mexichromis trilineata usually ranges from 7 mm to 15 mm in length, with an elongate to broadly oval body shape. The tail extends slightly beyond the posterior margin.[7]Mexichromis trilineata have three yellow longitudinal bands outlined in white, with the entire mantle edged with a fine white line. They have a few patches of yellow edged with white around the mantle edge, and the defensive mantle glands, arranged around the edge in roughly two series, are visible as pale pink patches.[8] The gills and rhinophores are typically violet at their bases and then translucent white with red markings.[9]
Although individuals of Mexichromis trilineata usually have three longitudinal lines as its name suggests, sometimes these lines are broken into a series of shorter lines, while occasionally the outer two lines are absent. Some specimens have yellow or orange markings on the white lines.[6]
^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.
^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: Nouméa purpurea and Chromodoris decora colour groups. Zoological Journal of the Linnean Society 86(4): 309–353.