ウミウシ用語集

The anterior end of the foot. Described as round, tentacular, angular, etc. In English: "(anterior) foot corners."

The region just below the tip of an appendage or organ. For example, the presence and colour of subapical bands on the cerata is often diagnostic when distinguishing closely related species.

When multiple names are considered to refer to the same organism, they are called synonyms. Under the principle of priority in the International Code of Nomenclature, the earliest published name is valid, and later ones become synonyms.

A single hard, needle-like structure inside or at the tip of the penis. Its length, curvature, and whether it is hollow vary among species and are crucial for distinguishing externally similar relatives.

Multiple small spines or projections arranged on the surface of the penis. The arrangement, individual size, and shape of these spines are diagnostic for species identification — distinct from the single penial stylet found in some other taxa.

Species that are morphologically nearly indistinguishable but recognized as genetically or ecologically distinct. Among nudibranchs, many visually similar species groups have been split into multiple species through molecular phylogenetic analysis.

A vernacular Japanese umbrella term for the marine gastropod lineages historically grouped under "Opisthobranchia" (後鰓類). Umiushi is not a formal taxonomic term — it is a cultural and informal grouping.

Opisthobranchia is no longer recognised as a valid clade in modern classification, as molecular phylogenetics has shown it to be paraphyletic. But the lineages that used to sit within it continue to be grouped together colloquially as umiushi. Following Kashio et al. (2025-) and Nishida (2024) A Field Guide to the Ecology of Sea Slugs, this site treats the following twelve groups as umiushi: Rhodopoidea, Acteonida, Ringipleurida, Pleurobranchida, Doridida, Cladobranchia (Nudibranchia sensu stricto), Umbraculida, Cephalaspidea, Anaspidea, Pteropoda, Sacoglossa, and Acochlidiida.

Some Japanese conchologists restrict umiushi to Nudibranchia only; this site does not follow that narrower usage.

The primary food source for Sacoglossa. Algae species are closely linked to nudibranch distribution and feeding ecology.

An organ found mainly in dorid nudibranchs. It produces and secretes chemical defense substances (repellent compounds) to protect against predators.

An organ found in mollusks. Located primarily on the dorsal side, it covers and protects internal organs.

The primary food source for dorids. Many dorids are specialist sponge feeders. Some species sequester sponge-derived chemicals for their own defense.

Single-celled symbiotic dinoflagellates, mainly of the genus Symbiodinium, that live within the tissues of cnidarians such as corals and anemones, supplying their hosts with photosynthetic products. Some aeolid sea slugs in the genus Phyllodesmium ingest zooxanthellae from soft-coral prey and retain them alive within their cerata, indirectly profiting from the algae's photosynthesis.

Sensory papillae densely covering the dorsal surface of dorid nudibranchs, especially Discodorididae. Each consists of a central spicule surrounded by radiating supporting spicules, resembling a clove (caryophyllum) — hence the name. The structure scatters light to produce a velvety appearance and is a key diagnostic character of genera such as Jorunna.

A life history in which an animal spends its entire life — egg, larva and adult — drifting in the open water column without ever settling onto the seafloor. Among gastropods, holopelagic life is restricted to a few lineages such as the pteropods, the genus Glaucus, and some heteropods. Freedom from the bottom allows broad distribution, but holopelagic species are also directly exposed to fluctuations in temperature and prey availability.

A primitive visual organ that can detect light and dark but cannot form images. Usually located near the base of the rhinophores.

A defense strategy of resembling other organisms in appearance or behavior. Includes Batesian mimicry (resembling toxic species) and Müllerian mimicry (mutual resemblance among toxic species).

Bright body coloration that signals toxicity or unpalatability to predators. The vivid colors of many nudibranchs are thought to serve as aposematic signals.

The publication in which a species was first described and named. Serves as the source of the scientific name.

A former subclass that once encompassed sea slugs and some shelled gastropods. Molecular phylogenetics revealed it to be non-monophyletic, and it has been formally abandoned as a taxonomic group. Still used informally as a convenience term.

Projections extending from around the mouth. Oral tentacles have sensory organs for detecting food at close range.

Mating behavior between hermaphroditic nudibranchs. Typically performed by aligning the genital openings on the right side. Usually involves mutual exchange of sperm.

The membrane between the oral tentacles.

In dorids, located centrally within the branchial plume. May protrude as an anal papilla.

Food source for some nudibranchs, particularly seen in Arminida and Polyceridae.

Individual leaf-like or branch-like structures that make up the branchial plume (secondary gills). Their number and arrangement are used for species identification.

Nudibranchs lay egg masses on substrates after internal fertilization. The shape of egg masses (spiral, ribbon-like, etc.) can be used for species identification.

A feeding organ of mollusks consisting of a ribbon-like structure with rows of teeth used to scrape food. The shape and arrangement of teeth is an important morphological character for species identification.

The voluntary shedding of a body part. Known examples include autotomy of cerata in aeolids, and the remarkable head-body autotomy with complete regeneration seen in some sacoglossans.

The pointed tip of a radular tooth. The shape, slenderness, and presence/absence of additional denticles on the cusp of central or lateral teeth are key characters for species identification.

Multiple scientific names applied to the same taxon. The earliest published name is the senior synonym (valid name), and later names are junior synonyms.

An organ possessed by many aeolid nudibranchs. Located at the tips of cerata, it stores nematocysts obtained from hydrozoan prey and can discharge stinging cells when attacked.

Tree-like, branched outgrowths of the skin that cover the dorsal surface of some sea hares such as Bursatella leachii, providing camouflage that breaks up the body outline against algal turf and reef substrate. Their presence and branching density vary among aplysiid lineages and are used as a diagnostic character at the genus and species level.

Fine folds (ridges) on the rhinophores. In English, referred to as "lamellae." The presence and arrangement of lamellae are important identification features.

The types of food consumed by nudibranchs. Highly diverse: spongivores, cnidarian feeders, algae feeders, bryozoan feeders, and even predators of other nudibranchs.

Sensory organs, typically a pair on the head. Their shapes vary: lamellate, smooth, papillate, enrolled, or ear-shaped. In English, they are called "rhinophores" — meaning "nose-bearers."

A tubular structure that houses and protects the rhinophores. Found in species that can retract their rhinophores. The shape of the rim (smooth, crenulate, etc.) is a diagnostic character.

A group belonging to the suborder Dendronotina. Dorsal appendage morphology varies widely, from tree-like branching forms (Dendronotus, etc.) to genera with very different morphologies. Currently classified under the superfamily Dendronotoidea and family Dendronotidae.

The opening of the reproductive organs on the right side of the body. In hermaphroditic nudibranchs, both male and female genital openings are located here.

An imaginary line indicating the center of the body.

Finger-like outgrowths arrayed along the dorsal surface of aeolid sea slugs (Order Nudibranchia sensu stricto). Each ceras houses a branch of the digestive gland; in species with cnidosacs at the tip, undischarged stinging cells from cnidarian prey are stored there and reused for defence. In photosymbiotic species such as Phyllodesmium, the cerata are flattened into broad paddles that hold zooxanthellae and serve as solar panels.

A group including side-gill slugs. Characterized by a gill on the right side of the body. Formerly known as Notaspidea.

The teeth flanking the central tooth in each row of the radula. The number per side and the shape vary greatly among species. Many aeolids lack lateral teeth altogether (uniserial radula).

Wing-like lateral extensions found in sea hares (Anaspidea) and others. Sometimes used for swimming.

The dentate (toothed) edge of the jaw of a sea slug. The shape and arrangement of denticles along this edge are important diagnostic features used in species identification.

The geographical location where the holotype of a species was collected.

A group belonging to the suborder Arminida. The genus Armina has gill lamellae on the ventral side of the foot margin, but the group also includes genera without lamellae. Molecular phylogenetics has questioned its monophyly, and its classification remains fluid.

The single central tooth in each row of the radula. It is typically larger than the lateral teeth and bears a prominent central cusp, often flanked by smaller denticles.

The area permanently submerged below the low tide mark. Where divers search for nudibranchs.

The area between high and low tide marks. A primary location for finding nudibranchs during shore surveys.

Rocks lying loose on the seabed. Turning them over often reveals nudibranchs — a classic habitat for many species.

A group characterized by a branchial plume on the posterior dorsum. Now classified as the separate order Doridina, independent from Nudibranchia. The most species-rich group of sea slugs.

A characteristic flattened head structure found in Cephalaspidea (the "headshield slugs"). Aids in burrowing through sand.

A group including bubble shells and their relatives. Most possess a shell and inhabit sandy or muddy substrates.

The ability to incorporate chloroplasts from consumed algae into one's own cells. Found in some Sacoglossa. Whether incorporated chloroplasts perform photosynthesis long-term varies by species — in many, they are digested relatively quickly.

Respiratory organs for gas exchange — absorbing dissolved oxygen and releasing carbon dioxide. In advanced gastropods like nudibranchs, the original gills were lost and new gills emerged from other parts of the body. These are called secondary gills.

Finger- or cone-shaped small protuberances on the body surface, widely seen on the dorsum and parapodia of nudibranchs. Their shape, size, branching state and distribution pattern are important species-level characters. Branched forms are termed dendritic papillae; minute forms with a central spicule and surrounding radial spicules are termed caryophyllidia. Distinct from internal anatomical structures such as the penial or anal papilla, for which Japanese uses the simpler term 乳頭.

A group known for kleptoplasty — the ability to incorporate chloroplasts from algae they consume. They have a distinctive feeding method of sucking out cell contents from algae.

Specimens designated in the original description other than the holotype. Used to demonstrate the range of variation within the species.

The primary food source for aeolid nudibranchs. Many species steal nematocysts from hydroids and store them in their cnidosacs.

The muscular ventral surface used for locomotion. Its shape, width, and coloration can be used for identification.

Describes rhinophores lacking lamellae (folds) on their surface. Used in describing rhinophore morphology.

The single specimen designated as the name-bearing type of a species in its original description. The scientific name is permanently linked to this specimen.

A group of nudibranchs bearing dorsal appendages called cerata. Belongs to the suborder Aeolidida, spanning multiple superfamilies. Many possess cnidosacs at the tips of their cerata.

A group including sea hares. Typically large-bodied, many possess a reduced internal shell. Some species release purple ink as a defense mechanism.

The scientific name currently considered correct under the International Code of Nomenclature.

In sea butterflies and sea angels (Pteropoda), the foot is laterally expanded into a pair of wing-like structures used for swimming with a flapping motion. Although gastropod parapodia share a name with the similarly positioned outgrowths of polychaete worms, the two are not homologous. Functionally and morphologically these "wings" parallel the parapodia of sea hares, but are further specialised for flight-like swimming in the open ocean.

One of the major taxonomic groups of sea slugs. Adults lack a shell and have lost the original gill (ctenidium). Includes the suborders Aeolidida, Dendronotina, and Arminida, among others. The order Doridina was formerly included but is now treated as a separate order.

A mass of eggs laid by nudibranchs. Species display characteristic shapes including spirals, ribbons, and strings.

An organ having both male and female reproductive functions. Most nudibranchs are simultaneous hermaphrodites, producing both eggs and sperm. Reproduction is typically sexual through copulation, though some species like Hydatina can reproduce asexually through fission.