Roboastra tentaculata is a species of sea slug, a polycerid nudibranch, a marine gastropod mollusc in the subfamily Nembrothinae in the family Polyceridae.
Etymology "Tentaculata" is used to describe Roboastra tentaculata due to the presence of "enlarged" oral tentacles. It was known that these elongate outward-turned oral tentacles with dorso-lateral grooves are a trademark of the genus Roboastra (and the not-yet established Tyrannodoris. However, as the species was initially placed in Tambja where no other species had such oral tentacles, the characteristic was deemed very significant.
Taxonomic History At its establishment in 2005, R. tentaculata was placed in genus Tambja. The authors acknowledged even in its naming that the well-developed, dorso-laterally grooved oral tentacles far more resembled Roboastra than any known Tambja, but in their analysis of internal morphology via dissection it was deemed too dissimilar from species placed in Roboastra at that time.
However, after more phylogenetic analysis Nembrothinae as a whole, it was found that the then-named Tambja tentaculata formed a sister-clade with Roboastra gracilis.Pola, M.; Cervera, J.L.; Gosliner, T.M.; 2007. Phylogenetic relationships of Nembrothinae (Mollusca: Doridacea: Polyceridae) inferred from morphology and mitochondrial DNA. Molecular Phylogenetics and Evolution. 43, 726–742. The following year, the authors took their own suggestion and officially reassigned the species as Roboastra tentaculata in order to maintain the monophyly of Roboastra.Pola, M., Cervera, J.L., Gosliner, T.M., 2008. Description of the first Roboastra species (Nudibranchia, Polyceridae, Nembrothinae) from the Western Atlantic. Bulletin of Marine Science. 83: 391-399.
Interestingly, the Roboastra species that R. tentaculata was deemed too dissimilar from at its establishment were removed from Roboastra years later and placed within their own genus, Tyrannodoris.Willan, R.C. & Chang, Y.-W. [Yen-Wei]. (2017). Description of three new species of Tambja (Gastropoda, Nudibranchia, Polyceridae) from the western Pacific Ocean reveals morphological characters with taxonomic and phylogenetic significance for traditional Polyceridae and related 'phaneorobranch' nudibranchs. Basteria. 81 (1-3): 1-23.
Strangely, in this same study, the diagnositics for Roboastra referred to Roboastra gracilis exclusively, and in the acknowledgements section emphasizes "...resurrected the overlooked genus Roboastra in 1967 with only a single included species (Burn, 1967) [still only this one species]". Yet Roboastra tentaculata was not included within the systematics for any genus, in fact mentioned only once in passing. Despite the study's insistence that pre-rhinophoral sensory organs, a trait that R. tentaculata possesses, are indicative of the genus Tambja and do not occur outside of the genus, R. tentaculata was not among the Tambja list of species. Even then, the redescribed diagnositics for Tambja would anyways exclude R. tentaculata, again on account of its well-developed dorso-laterally grooved oral tentacles. So, it seems the species as a whole was unaccounted for within the study.
External Morphology Roboastra tentaculata is a limaciform Nembrothinid, 20mm when living, with a long pointed end of foot. The body is smooth, without pits. The genital pore is located on the right side halfway between the gills and rhinophores. The brim-like anterior margin of the head dips in the center, like if the letter "V" or "U" shrugged. There exist pre-rhinophoral organs, an ovular collection of lateral slots, between the rhinophores and oral tentacles. Their oral tentacles are very well developed, grooved dorsolaterally along the outward face; a requisite characteristic of the genus Roboastra. The rhinophoral sheaths are well developed, and contained fully retractile perfoliate rhinophores with roughly 20 lamellae. 5 nonretractile bipinnate gills form a semicircle around the anal papilla.
The ground colour is green with yellow bands running the length of the body, (though looking upon it, one might think it is yellow with green bands). The genital pore, both rhinophores and rhinophoral sheaths, gills, and the full margin of the notum are green. The oral tentacles are green, with yellow pigment at the base. The arrangement of the yellow bands are described as thus: "Three dorsal bands; two submarginal and one mid-dorsal; two submarginal yellow bands run along inner sides of notal lateral margins and rhinophoral sheaths, joining at posterior end of notum and again at anterior part of head; from this junction another yellow band extends posteriorly to anterior end of rachis of central branch continuing again for short distance behind tubular anal papilla. Two further bands along each side of body; upper bands join between anterior notal margin and oral tentacles; lower yellow bands arise from base of oral tentacles, almost reaching posterior end of foot and uniting or remaining separate (in different specs). Minute brown pigmentation surrounds outer edges of all yellow bands."
Misidentification Roboastra tentaculata has had a history of being misattributed to a currently unnamed species of Tambja. This has occurred frequently in both guidebooks and academic texts, even in the texts of the authors who established the species.
The Sea Slug Forum's pages for Tambja tentaculata (=Roboastra Tentaculata) contain a mix of correctly identifiedPola, M., 2005 (Aug 19). Tambja tentaculata - some more photos. [Message in] Sea Slug Forum. Australian Museum, Sydney. Accessed on: 2026-07-10. R. tentaculata, and also a great deal of the unnamed Tambja species.Teresa Zuberbühler, 2007 (Mar 15). Re: Tambja tentaculata from Borneo. [Message in] Sea Slug Forum. Australian Museum, Sydney. Accessed on: 2026-07-10.
In Pola et al. (2014), in Figure 9H,Pola, M.; Padula, V.; Gosliner, T.M.; Cervera, J.L.; (2014) Going further on an intricate and challenging group of nudibranchs: description of five novel species and a more complete molecular phylogeny of the subfamily Nembrothinae (Polyceridae). Cladistics. Volume 30, Issue 6 pp. 607-634. the individual depicted feeding on bryozoans is not Roboastra tentaculata, but in fact the unnamed Tambja species.
Correctly labelled Tambja tentaculata (=Roboastra tentaculata) can be found in Pola et al. (2005) Figure 1A, and Pola et al. (2006) Figure 19D.Pola, M.; Cervera, J.L.; Gosliner, T.M.; 2006. Taxonomic revision and phylogenetic analysis of the genus Tambja Burn, 1962 (Mollusca, Nudibranchia, Polyceridae). Zoologica Scripta. 35, pp491–530.
There are continued observations of a great deal many other Roboastra animals, and, especially in regards to a similar blue-and-yellow animal,Hengky Dotulong, 2006 (Feb 14). Tambja tentaculata from Bali. [Message in] Sea Slug Forum. Australian Museum, Sydney. Accessed on: 2026-07-10. there is generally disagreement on which are unique unnamed species and which are R. tentaculata. The addition of juvenile Roboastra muddy the waters further, as juvenile Nembrothinids are often less pigmented and have fainter markings, and some mistake less distinctJerry, B.R., 2006 (May 1). Tambja or Roboastra? from Indonesia. [Message in] Sea Slug Forum. Australian Museum, Sydney. Accessed on: 2026-07-10. blue-and-yellow animals as simply juvenile R. tentaculata. Overall, Roboastra with pre-rhinophoral lateral slots is a diverse area that requires further study for confident differentiation.
The Sea Slug forum page entitled "Tambja sp. 6" has unintentionally compiled a good record of the variety of Roboastra that are often confused for R. tentaculata.Tambja sp. 6. Sea Slug Forum. Australian Museum, Sydney. Accessed on: 2026-07-10.
Distribution This species was described from Guam. The existence of some very similar possibly undescribed yellow-and-blue species of Roboastra makes distribution difficult to verify, but there is another confirmed record of R. tentaculata occurring in the Philippines.Koehler, Erwin. 2019. iNaturalist Observation: https://www.inaturalist.org/observations/20902694. Accessed on: 2026-07-08
Diet As of the 2017 study, there have been no observations of Roboastra tentaculata nor its sister-species Roboastra gracilis feeding. All predaceous sea slugs previously classified in Roboastra have been moved to Tyrannodoris, so there is currently no precedent of diet within the genus. There have been "sightings" of R. gracilis preying upon other sea slugs, but upon further examination all instances have in fact been Tyrannodoris luteolineata, whose juvenile form resembles R. gracilis to the extreme.
It is speculated that Roboastra feed on bryozoans, much like Tambja does, as photographs of both members of the genus depict with high frequency individuals crawling upon bryozoans.
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