''Bathydevius'' (Greek for "deep-living deviation") is a

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of unusual, highly distinctive

nudibranch Nudibranchs () are a group of soft-bodied marine gastropod molluscs, belonging to the order Nudibranchia, that shed their shells after their larval stage. They are noted for their often extraordinary colours and striking forms, and they have b ...

(

sea slug Sea slug is a common name for some Marine biology, marine invertebrates with varying levels of resemblance to terrestrial Slug, slugs. Most creatures known as sea slugs are gastropods, i.e. they are Sea snail, sea snails (marine gastropod moll ...

) native to deep waters of the

North Pacific Ocean North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography. Etymology The word ''north'' ...

. It contains a single species, ''B. caudactylus'' ("finger tail"), and is the only known member of the monotypic family Bathydeviidae.

Taxonomy

''Bathydevius'' is a bizarre nudibranch with a highly unusual appearance, ecology, and taxonomy. It is the first known nudibranch to inhabit the

bathypelagic zone The bathypelagic zone or bathyal zone (from Greek βαθύς (bathýs), deep) is the part of the open ocean that extends from a depth of below the ocean surface. It lies between the mesopelagic above and the abyssopelagic below. The bathypela ...

, and only one of very few pelagic, free-swimming genera within this generally

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

group. It has a large, bell-shaped oral hood that it can use to catch prey, superficially similar to that of a

Venus flytrap The Venus flytrap (''Dionaea muscipula'') is a carnivorous plant native to the temperate and subtropical wetlands of North Carolina and South Carolina, on the East Coast of the United States. Although various modern hybrids have been created ...

and closely resembling that of the nudibranch ''

Melibe ''Melibe'' is a genus of sea slugs, nudibranchs, marine gastropod mollusks in the family Tethydidae. Most nudibranchs are carnivores, but their prey is usually sessile or slow-moving animals such as sponges or bryozoans. In contrast, ''Melibe' ...

''. However, phylogenetic studies indicate that ''Bathydevius'' is not closely related to ''Melibe'', and is rather either (based on

rRNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosomal ...

) the most basal nudibranch genus or (based on

mtDNA Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the DNA contained in ...

) the

sister genus In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...

to the

Bathydorididae Bathydorididae is a taxonomic family of dorid nudibranch, shell-less marine gastropod mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant spe ...

Distribution

A majority of observations of ''Bathydevius'' are off the coast of California from the

Monterey Canyon Monterey Canyon, or Monterey Submarine Canyon, is a submarine canyon in Monterey Bay, California with steep canyon walls measuring a full in height from bottom to top, which height/depth rivals the depth of the Grand Canyon itself. It is the la ...

, but individuals have also been observed near

Davidson Seamount Davidson Seamount is a seamount ( underwater volcano) located off the coast of Central California, southwest of Monterey and west of San Simeon. At long and wide, it is one of the largest known seamounts in the world. From base to crest, ...

, off the coast of

Point Conception Point Conception (Chumashan languages, Chumash: ''Humqaq'') is a headland along the Gaviota Coast in southwestern Santa Barbara County, California, United States. It is the point where the Santa Barbara Channel meets the Pacific Ocean, and as ...

, and off the coast of

Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...

. In addition, two individuals of a similar nudibranch were observed near the

Mariana Trench The Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deep sea, deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maxi ...

, which would potentially extend the range of this genus or its relatives to the western Pacific.

Description

''Bathydevius'' ranges between to in size, and when fully expanded, its hood has a diameter of . The flexible hood can be shaped into a variety of configurations, to the extent that the highly prehensile margin of the hood can form notches. This hood is used to catch planktonic prey such as the

shrimp A shrimp (: shrimp (American English, US) or shrimps (British English, UK)) is a crustacean with an elongated body and a primarily Aquatic locomotion, swimming mode of locomotion – typically Decapods belonging to the Caridea or Dendrobranchi ...

Boreomysis ''Boreomysis'' (from Greek '' Boreas'', the god of the northern winds, and the genus name '' Mysis'') is a mysid crustacean genus, the type of the subfamily Boreomysinae of the family Mysidae. Majority of the species are found in the ocean deep ...

''. Its tail has finger-like protrusions, hence the specific epithet ''caudactylus''. It is

bioluminescent Bioluminescence is the emission of light during a chemiluminescence reaction by living organisms. Bioluminescence occurs in multifarious organisms ranging from marine vertebrates and invertebrates, as well as in some fungi, microorganisms inc ...

, having the ability to scintillate blue across the surface of the oral hood and the protrusions on its tail, and is only the third known nudibranch lineage to display bioluminescence, which appears to have been independently evolved in each. ''Bathydevius'' appears to neither sink nor rise when suspended in water, suggesting that it has a density roughly equivalent to that of seawater.

Life history

Although ''Bathydevius'' generally swims in midwater, it descends and attaches to the seafloor when ready to breed. When attached, it releases a ribbon of eggs. At least 20 individuals have been observed attached to the seafloor. As with most other gastropods, it is

hermaphroditic A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...

Discovery

''Bathydevius'' was first discovered in February 2000 by the

Monterey Bay Aquarium Research Institute The Monterey Bay Aquarium Research Institute (MBARI) is a private, non-profit oceanographic research center in Moss Landing, California. MBARI was founded in 1987 by David Packard, and is primarily funded by the David and Lucile Packard Foun ...

using the ROV ''Tiburon'', at a depth of in

Monterey Bay Monterey Bay is a bay of the Pacific Ocean located on the coast of the U.S. state of California, south of the San Francisco Bay Area. San Francisco itself is further north along the coast, by about 75 miles (120 km), accessible via California S ...

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

. The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

specimen was collected in January 2004, and over 157 individuals were documented in dives spanning from 2000 to 2021; during this period of study, it came to be nicknamed the "mystery mollusk" until its taxonomic identity was determined. Following extensive documentation of its taxonomy, ecology, and life history, it was finally described in November 2024. At least one spawning ''Bathydevius'' individual has survived being captured alive and brought to the surface. It spawned within an aquarium tank and released an egg ribbon that attached to the bottom of the tank, which successfully developed into larvae.

References

{{Taxonbar, from1=Q131138396, from2=Q131138378, from3=Q131138400 Monotypic gastropod genera Nudibranchia Molluscs of the Pacific Ocean Gastropods described in 2024