Author: Bergh, 1877

Order: Nudibranchia  Family: Discodorididae

Maximum Size: 110 mm

Sightings: Sunshine Coast

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Discodoris boholiensis Bergh, 1877

Discodoris boholiensis is a broad, oval-shaped and large species (to 110 mm) but the mantle is thin and fragile with a highly undulating edge. The body is flat but has a distinctive narrow central longitudinal ridge – the visceral hump, commencing at the rhinophores and terminating at the gill. These features can make it appear more like a flatworm than a nudibranch, at first glance. This gives Discodoris boholiensis a different look to other species of the genus. The surface of the notum is covered in many, tightly packed, small, conical and simple tubercles. These tubercles are stiffened by spicules but are not caryophyllidia. The rhinophore and gill pockets have raised rims that also carry tubercles. The rhinophores have long stalks and when not affected by current or surge the clubs are carried on the stalks angled somewhat posteriorly. The gill usually consists of six tripinnate plumes, surrounding the anal papilla, and they emerge from the gill pocket angled posteriorly. The gill pocket appears to open at the posterior end of the mid-dorsal ridge. The flexibility of the thin mantle allows it to readily contort to suit any refuge and slip into small crevices for safety.

The background colour is a dark cream becoming darker towards the mantle edge however the appearance is greatly affected by the colours, distribution and concentration of the multitude of tubercle tips that may be white, red, brown or black. The mantle margin carries a very thin white line inside of which is a broader band that is dark brown or dark red. This band varies in width and is frequently intruded upon by the colour of the adjacent notum. It also carries white spots and short transverse white lines. The median ridge is dark brown or dark red often with white blotches or lines. The rhinophore stalks are translucent brown with a white line running up their posterior face. The clubs are dark brown with white pigment on the edge of some of the lamellae and bears a white line running up the anterior face to link with the white apical tip. The gill leaflets too, are dark brown but the gill rachises (the supporting branching stems) are white to grey.

Mantle segments are easily autotomized with the thin fragile “flaps” each side being readily cast off when the animal is stressed.

The creamy coloured spawn is laid upon the substrate as a coiled ribbon, attached by one edge, the free edge being longer is undulating. It feeds upon sponges.

Distribution is broadly Indian Ocean and Western Pacific Ocean.

Although the colouration and patterning of the dorsum of Discodoris boholiensis is variable, it usually cannot be confused with other species and this can therefore be diagnostic.

Discodoris boholiensis is the type species of Discodoris. It is named after Bohol Island in the Philippines – the Type Locality.

The author, Bergh, described another species, Discodoris meta, at the same time thinking that it was most probably a variety of Discodoris boholiensis. Valdes 2002, could find no differences and synonymized D. meta with D. boholiensis. Dayrat, 2010 redescribed the species and concurred.

David A. Mullins – March 2021

References:

– Bergh, L. S. R. (1877). Malacologische Untersuchungen. In: Semper, C., ed. Reisen im Archipel der Philippinen. theil 2, heft 12. Wiesbaden: Kreidel

– Bergh, L. S. R. (1905). Die Opisthobranchiata der Siboga-Expedition, Monographie 50. E. J. Brill, Leiden, Netherlands. 248 pp.

– Rudman, W. B., (1999 September 21). Discodoris boholiensis Bergh, 1877. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/discboho and associated messages.

– Behrens, D., spawn image by Miller, M. (2000). Opisthobranch of the Week, Week: 237. Mike Miller’s Slug Site. Available at: http://slugsite.us/bow/nudwk239.htm and link to http://slugsite.us/bow/i131_boh.htm

– Valde ́s, A ́. (2002). A phylogenetic analysis and systematic revision of the cryptobranch dorids (Mollusca, Nudibranchia, Anthobranchia). Zoological Journal of the Linnean Society 136: 535–636.

–  Dayrat, B. (2010). A monographic revision of basal discodorid sea slugs (Gastropoda, Opisthobranchia, Nudibranchia, Doridina). Proceedings of the California Academy of Sciences. Series 4, vol. 61, suppl. I, 1-403