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| Bathydevius caudactylus Robison & Haddock, 2024 |
Highlights:
• We present a previously undescribed genus and species of deep-sea nudibranch.
• It has unique morphology and is not closely related to any known families.
• In a molecular phylogeny, it resolves as a distinct sister to the rest of known nudibranchs.
• The species is bioluminescent and represents the third independent origin of bioluminescence among nudibranchs.
Abstract
We describe an exceptional nudibranch, new to science, from bathypelagic depths in the eastern North Pacific Ocean. More than 100 individuals of Bathydevius caudactylus gen. et. sp. nov. have been observed in the water column at depths between 1013 and 3272 m. Twenty spawning individuals were observed on the seafloor at depths between 2269 and 4009 m. Anatomy, diet, behavior, bioluminescence, and habitat distinguish this surprising nudibranch from all previously described species, and genetic evidence supports its placement in a new family.
Keywords: Nudibranch, Bathypelagic, bioluminescence, Deep-sea, New species, New genus, New family
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| Bathydevius caudactylus gen et sp. nov. in situ, from a video frame grab. |
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| Hood configuration modes for Bathydevius caudactylus gen et sp. nov.: (a) fully expanded; (b) the hood is closed around an open sphincter; (c) the hood is bi-lobed, closed along its vertical axis; (d) invagination in the upper margin with a pinched projection in the lower margin; (e) the peripheral lip of the hood is rolled back against the outer surface of the bell; (f) in a propulsive pulse. |
Systematics
Class GASTROPODA.
Subclass HETEROBRANCHIA.
Order NUDIBRANCHIA
Suborder incertae sedis.
Family Bathydeviidae fam. nov.
Bathydevius gen. nov.
Bathydevius caudactylus sp. nov.
Diagnosis: Bathydevius caudactylus gen. et sp. nov. (Fig. 1) is a mostly transparent, gelatinous nudibranch composed of three distinct regions: the head, supporting a voluminous, bell-shaped oral hood; a body enclosing the internal organs and bearing the columnar ventral foot; and a spatulate tail fringed with 9–16 caudal appendages “dactyls.” The dorsal surface has two stubby, unsheathed rhinophores and a gently curved ridge of gills with the anal opening just posterior to the center of the gill ridge. Ventrally, the body merges into a short, cylindrical foot. Visible through the transparent body, the stomach is typically red, the rugose digestive gland is orange or brown, and in mature individuals, gonadal tissue on the right side of the digestive gland is opaque white.
Etymology: The generic name Bathydevius gen. nov. describes a deep-living deviation, to reflect its diversion from the evolutionary path of its relatives in appearance, molecular signature, and natural history. The species name caudactylus sp. nov. refers to the finger-like projections that line the posterior margin of the tail.
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| Bathydevius caudactylus gen et sp. nov. on the seafloor in the posture associated with the release of an egg-bearing ribbon. Distance between the red laser dots is 29 cm. |
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| Bioluminescence of Bathydevius caudactylus gen et sp. nov., recorded in situ. Light emanates from the surface of the oral hood and within the dactyls (at left). |
Bruce H. Robison and Steven H.D. Haddock. 2024. Discovery and Description of A remarkable bathypelagic Nudibranch, Bathydevius caudactylus, gen. et. sp. nov. Deep Sea Research Part I: Oceanographic Research Papers. 214; 104414. DOI: doi.org/10.1016/j.dsr.2024.104414
