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Mexichromis festiva (Angas 1864)

Michael Thompson 2016


Mexichromis festiva is a dorid nudibranch part of the Mexichromis genus in the Chromodorididae Super Family, a taxonomic family of shell-less marine gastropod molluscs, known for their wide array of colours. M. festiva are unique due to their radular morphology, foregut anatomy, and reproductive system differences helps identify the species. The species is found along the eastern coastline of Australia. M.festiva perform numerous defensive strategies to avoid predations. Little knowledge is known on the population of the species however their main threat is reef walking.

Physical Description

  • External Morphology
    • Colour
      • M. festiva mantle is white, as well as a sequence of brightly coloured orange or golden yellow markings forming either a broken or continuous line along the edge ofthe mantle. Along the dorsal side of the external body plan are scattered patterns of purple to red spots; in the specimen collected these spots were purple. In other specimens it’s seen that the spots can be slightly raised from the mantle. The base of the rhinophore stalk of the species are translucent white with the club is similar to the purple-red colour of the spots; in the sample collected the rhinophores were purple. The branchial plumes (gills) are translucent white along the base whilst the rachis upper half of the gill is the purple-red colouration. Both the body and muscular foot of the nudibranch are white, in some cases there’s a purple submarginal border along the foot.However the band is often very faint, and the colour diffuses along the edges,this band can run from both sides of the head towards the posterior tip on the foot(Rudman, 1983).
    • Shape
      • M. festiva has a bilaterally symmetrical external body plan; this excludes the spot patterns along the dorsal side of the body. The mantle is an elongated oval with the anterior being slightly wider and the mantle overlapping down along the sides. There are up to nine individual gill branches that situated in a circle around the anus and when extended fully the fills form a cup-shaped arrangement. During locomotion the foot along the posterior section becomes largely visible behind the mantle equalling to approximately a third of the mantle length. Mantle Gland swelling can be notable along the edge of the mantle, however the swellings in this specimen aren’t greatly noticeable (Rudman,1983). The external features and colouration of M. festiva are labelled and more noticeable in Figure 1.
    • Size
      • Chromodorididae nudibranchs are known to range in size between <1mm to >30cm, although most species tend to be approximately 2-3cm in size. The M. festiva specimen was approximately 4mm in length when crawling (Figure 2.). It’s assumed that the specimen can grow to up to 10mm in length, however it’s maximum length is unknown.
  • Similarity to Other Species
    • M. festiva was first described by Angas (1864) along the Sydney Harbour, his description of the external colouration of the species matches the specimen collected.Other observations of the species described variations with the colour of the spotted pattern being more reddish, the golden yellow coloration of the border of the mantle being more broken elongated marks on the edge, and the presence of the submarginal purple band along the posterior point of the foot. Thus Angas’s description of the species further supported the identification of the sample collected despite the variations in colouration (Rudman, 1983, Angas,1864). Using coloration, the closest matching species to M. festiva is Mexichromis mariei. In M. mariei, the colour of the spots are a deep violet colour and the submarginal border on the foot is always a heavy purple unlike the faintness of the colouration in M. festiva (See Figure 3). A more discrete difference between the two species is seen in the radular morphology; M. mariei has more cusp-like denticles on the teeth of the radula than M.festiva (Rudman, 1983). Between the two species, the alimentary canal where the buccal bulb and oral tube relative size is similar. Another similarity is the jaw rodlets; in both species they are bicuspid, whilst other species of Mexichromis are either tricuspid or multicuspid. Other studies have also stated that M. festiva to be similar to Hypselodoris nudibranchs due to their relative high body and narrow mantle edge. M. festiva reproductive system is also similar to Hypselodoris as there’s a large branching vestibular gland that situated along the beside the female gland mass. What differs is seen in the large exogenous sperm sac (like Chromodoris nudibranchs), however Mexichromis sperm sac is situated below the vagina (Rudman, 1984).
Figure 1
Figure 2


Life History and Behaviour

Anatomy and Physiology

Biogeographic Distribution

Evolution and Systematics

Conservation and Threats