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Oxynoe
sp. #1
 
Oxynoe sp. #1
Maximum size:  47 mm (including "tail").

Identification:  This species has a thin, transparent shell that's covered by undivided parapodia. The body is papillate and the foot extends well beyond the posterior margin of the shell. The posterior portion of the foot appears to increase in relative length as it grows. (Note 1) It is light olive-green with yellow flecks and white blotches. The body is decorated with gold-ringed, blue ocelli that have dark blue dots in their centers. (Note 2) Blue ocelli are also present on the body under the shell (showing through the shell in young animals).

Natural history:  Numerous Oxynoe sp. #1 were recently found, on Maui, in patches of Caulerpa taxifolia growing in a patchy Halimeda kanaloana bed at a depth of about 9 m (29-30 ft). The lack of records between the 1800s and 2021 suggests that the species may be very sporadic in occurrence. (Note 3) It feeds on Caulerpa taxifolia and lays flattened, cream egg masses. It's diurnally active and uses it's flexible foot margins to grasp the Caulerpa's fronds. (see photos) (Note 4)

Distribution:  Maui and Oahu: also known from the western Pacific and French Polynesia.

Taxonomic notes:  Oxynoe sp. #1 is a close match for "Oxynoe sp. 2" in Gosliner, et. al. (2018). The genus was first reported from Hawaii by Adams in 1855 (described as Lophocercus krohnii). His description, however, was based only on the shell. Morch, in 1863, reported  the genus from the "Sandwich Islands" using the name Lophocercus viridis Pease 1861 (since changed to Oxynoe viridis and originally described as "from the Pacific Islands"). Neither the shell description by Adams nor the Morch record can be clearly assigned to either "Oxynoe sp. 2" or O. viridis as illustrated in Gosliner, et. al. (2018). Therefore, although it's possible that the older records were a different species, we're lumping the new animals with them by "process of elimination." If it becomes possible at some point to separate them, Katherine Shepherd and Jenna Szerlag should be credited with the first record of Oxynoe sp. #1.

Photo:  Rebecca Bicker: Maalaea Bay, Maui; Feb. 14, 2021.

Observations and comments:

Note 1:  Gosliner, et. al. (2018), referring to the genus, mentions that the elongate "tail" can be used as a propeller to swim away if they are disturbed and can be autotomized to distract predators. Secretion of milky, defensive fluid is also mentioned. Autotomization and regeneration might be an alternative explanation for the variation in "tail" length.

Note 2:  The blue spots with gold rings aren't as precisely matched to the blue and gold ocelli found in the sea hares Stylocheilus striatus and Phycophila euchlora as are the markings in some other possible mimics. But, perhaps they are close enough so that Oxynoe sp #1 is still deriving some benefit from the resemblance (since the sea hares are known to concentrate toxins from the cyanobacteria they eat)?

Note 3:  Previous Oahu records from the 1800s lack habitat data.
 
Note 4:  On Feb. 14, 2021 Katherine Shepherd and Jenna Szerlag found 10 animals on a 4 m (15 ft) diameter patch of Caulerpa taxifolia growing in a patchy Halimeda kanaloana bed. They ranged from 17 to 40 mm in length and could "stretch out longer." They were observed feeding actively on the Caulerpa and crawling between clumps. Several were resting on egg masses.

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