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Nudibranch vs Sea Slug: Every Nudibranch Is a Sea Slug — But Not the Reverse

Nudibranch vs Sea Slug: Every Nudibranch Is a Sea Slug — But Not the Reverse

May 1, 2026 ·

Every nudibranch is a sea slug — but not every sea slug is a nudibranch.

"Sea slug" is an informal English umbrella term for shell-less or near-shell-less marine snails. "Nudibranch" is a specific scientific group that lives inside that umbrella, alongside several other sea slug lineages that most divers, casual readers, and even a few popular field guides cheerfully lump together.

This is the difference, why it matters, and the six animals you can use to recognise the major sea slug groups on sight.

What "sea slug" actually means

"Sea slug" is not a taxonomic label. It is a piece of English that biologists started using because they needed a friendly word for "marine gastropod that has lost or drastically reduced its shell." Nothing more.

Around six thousand species across many independent gastropod lineages get called sea slugs. They all share one evolutionary trick — abandoning the shell — but they reached that trick from different starting points. Some breathe with feathery dorsal gills, some with paired side-flaps, some with internal cavities. Some graze algae, some hunt sponges, some catch jellyfish in mid-water. The only thing that holds the umbrella together is the absence of an obvious external shell.

That makes "sea slug" useful in conversation and risky in writing. If a nature documentary describes a "sea slug feeding on coral," it has not told you very much. The same sentence could apply to half a dozen unrelated animals with completely different biologies.

What a "nudibranch" is (and why it has just been split into two)

Nudibranchs are one specific lineage within the sea slug umbrella. The name comes from Latin nudus (naked) and branchia (gill): adults have their respiratory organs out in the open, either as a feathery rosette on the back or as rows of finger-like projections called cerata.

Until very recently, every textbook and field guide listed nudibranchs as a single order — Nudibranchia — divided into two suborders informally called dorids and aeolids. In 2025, Martynov and Korshunova published a major reorganisation built on a decade of molecular work in the North Pacific. They proposed that the two halves are different enough to be treated as separate orders rather than suborders:

  • Order Nudibranchia (sensu stricto) — the aeolids and their relatives, with finger-like cerata along the back.
  • Order Doridida — the dorids, with a flower-like gill plume on the back and tubercles or smooth mantles.

This is a consequential change for catalogues and field guides, but for our purposes here it has a useful side effect: it makes the dorid–aeolid split visible at a glance instead of buried inside the same order. The next section uses one representative from each.

(A full explanation of Martynov and Korshunova's argument, and why the North Pacific drove it, lives in a separate article.)

A quick tour of the sea slug family tree

Six animals, six lineages, every one of them a sea slug.

1. Order Doridida — Phyllidia varicosa

Phyllidia varicosa is the type species of both the genus Phyllidia and the family Phyllidiidae, which means it has been the reference point for "what a phyllidiid dorid looks like" since Lamarck named it in 1801. Black ground colour, raised yellow tubercles, no exposed gill plume on the back (Phyllidiidae breathe through secondary gills under the mantle, an unusual arrangement among dorids).

The yellow-on-black pattern is a warning. Phyllidia sequester isocyanide-class terpenoids from their sponge prey — a defensive chemistry first isolated from this exact species by Burreson, Scheuer, Finer and Clardy in 1975 (the famous 9-isocyanopupukeanane paper, named after the type-locality beach at Pūpūkea, Oahu). A 1963 anecdote already had a Hawaiian fisherman watching every fish in his lobster bucket die when a single Phyllidia varicosa was added; the chemistry caught up with the observation a decade later.

If you see a stocky black slug studded with yellow bumps and no gill rosette, it is probably a phyllidiid, and it is almost certainly chemically defended.

2. Order Nudibranchia (sensu stricto) — Phyllodesmium longicirrum

If Phyllidia shows you a typical dorid, Phyllodesmium longicirrum shows you a typical-but-extreme aeolid. It is the largest known aeolid in the world, sometimes exceeding 150 mm — most of its relatives are an order of magnitude smaller — and its body is fringed by long, flattened cerata that look more like leaves than fingers.

The cerata are not just decorative. Phyllodesmium feeds on soft corals (Alcyoniidae and Xeniidae), and several species, including longicirrum, retain the corals' photosynthetic algae — zooxanthellae of the genus Symbiodinium — alive inside their digestive branches. The flattened cerata work as solar panels, exposing the borrowed algae to sunlight. Rudman's 1981 study showed that this photosymbiosis is reproducible across multiple Phyllodesmium species, and a 2009 revision by Moore and Gosliner placed longicirrum at the extreme end of the morphological gradient.

Most aeolids store stinging cells from their cnidarian prey for defence. P. longicirrum does not have those. Instead, it relies on terpenoid compounds borrowed from its soft-coral diet — a different chemistry, same idea.

3. Sacoglossa — Elysia marginata

Sacoglossans are the algae-grazing branch of the sea slug umbrella. Most of them are small, green, leaf-shaped animals, and many practise kleptoplasty — they suck the cytoplasm out of algal cells, keep the chloroplasts working inside their own tissues, and run partial photosynthesis for days or weeks. A sea slug that also functions, briefly, as a plant.

Elysia marginata took kleptoplasty into stranger territory in 2021. Mitoh and Yusa, working in Japan, photographed wild and laboratory individuals doing something nobody had documented in any animal of comparable complexity: cutting their own heads off and growing a new body. The slug's neck has a pre-formed weakness; the head detaches, walks away, and over about seven days regenerates a heart, gut, and full body from the remaining tissue. The discarded body keeps moving for hours and decays over weeks. The same study showed the same trick in Elysia atroviridis, suggesting the ability is not unique to a single species.

The leading explanation is parasite removal — the discarded body in atroviridis often carried internal copepods. Whatever the trigger, it makes E. marginata the only sea slug currently known to literally throw its body away.

4. Aplysiida — Bursatella leachii

The sea hares are the heavy infantry of the sea slug umbrella: large-bodied, soft, often with paired parapodia they can use as swimming flaps. The genus Aplysia is the household example, and we covered Aplysia kurodai in an earlier article in this series.

Bursatella leachii looks almost nothing like Aplysia, and it is the same lineage. Its parapodia are fused along the back, so it cannot swim. Its body is covered in branched, tree-like skin outgrowths called dendritic papillae, which break up its outline against algal turf. It tends to occur in dense aggregations on shallow muddy or seagrass bottoms — sometimes hundreds of individuals creeping over the same patch of substrate, occasionally even mating in chains.

A note on identification: a 2020 molecular study by Bazzicalupo and colleagues showed that what was historically called Bursatella leachii across the Indo-Pacific is in fact a complex of cryptic species. The name Bursatella ocelligera has been resurrected for the Indo-Pacific population, including most Japanese records. Calling a Pacific specimen "B. leachii" is not exactly wrong — that has been the convention for two centuries — but it is no longer current.

The point for this article is the body plan: papillate, fused-parapodia, gregarious. Sea hares are not all Aplysia-shaped.

5. Cephalaspidea — Chelidonura varians

The Cephalaspidea — the headshield slugs — are named after a flat, shovel-like extension of the head used for ploughing through sand. Many cephalaspideans still keep a small shell, often hidden internally; a few, like Chelidonura, have lost it entirely.

Chelidonura varians, described by Eliot in 1903, is the most photogenic ambassador of the group: a flat, cobalt-blue body striped or stippled in white and yellow, with a forked posterior tail that gives the genus its name (Greek khelidōn, "swallow"). The headshield is what tells you this is not a nudibranch. Run a finger forward from the rhinophores: in a nudibranch you find a notum and tentacles, in Chelidonura you find that pancake-flat shield.

Chelidonura hunts acoel flatworms — not the most glamorous prey, but tracking acoel chemistry across reef sand makes the headshield genuinely useful.

Aglajidae is a small family — under 90 valid species — but morphologically more diverse than its size suggests. The 2018 phylogenetic revision by Zamora-Silva and Malaquias divides it into 15 genera, seven of them brand new: Biuve, Camachoaglaja, Mannesia, Mariaglaja, Niparaya, Spinophallus and Tubulophilinopsis, alongside the older names Aglaja, Chelidonura, Melanochlamys, Nakamigawaia, Navanax, Noalda, Odontoglaja, Philinopsis and Spinoaglaja. Every one of them is still recognisably a "headshield slug" — they all carry that same pancake-flat shovel out front — but the body plans run wider than the family name implies. Chelidonura itself is the long-tailed reef-flat species you can spot from a metre away. Melanochlamys species are stubby, almost cylindrical, and burrow through sandy bottoms in temperate waters. Navanax inermis, the California aglaja, routinely tops 200 mm and hunts other slugs by following slime trails. Tubulophilinopsis species look almost like little cuttlefish from above, with a raised, helmeted head. Nakamigawaia spiralis, described from Japan, has a small spirally coiled internal shell that is a clear outlier in the family. And Noalda is so anomalous — the only aglajid with a partially external bulloid shell — that the same paper proposed pulling it out of Aglajidae altogether and treating it as incertae sedis within Cephalaspidea.

Chelidonura varians itself, neatly, has been spared the taxonomic shuffle. The 2018 tree placed it inside the strict Chelidonura clade alongside the type species Chelidonura hirundinina, with samples from Okinawa, Lord Howe, Lizard Island and Madagascar all clustering together. Many of the species that used to share a genus page with it have moved out — C. fulvipunctata is now Biuve fulvipunctata, C. inornata, C. mandroroa, C. sandrana and C. alexisi are now in Mariaglaja, C. pilsbryi, C. gardineri, C. lineolata and C. reticulata are now in Tubulophilinopsis — but C. varians stays put. So when a guidebook from 1995 calls it Chelidonura varians and one from 2024 calls it Chelidonura varians, that is the rare case of a name surviving a full molecular revision unchanged.

6. Pteropoda — Clione limacina

Stop calling pteropods "pteropods" and you would never guess they were sea slugs. They are pelagic — most spend their entire life cycle drifting in open water — and they swim with their parapodia spread out as a pair of muscular wings. The shelled half (Thecosomata) are "sea butterflies"; the shell-less half (Gymnosomata) are "sea angels."

Clione limacina, described by Phipps in 1774 from the Arctic, is the canonical sea angel: a translucent, finger-sized body, rust-coloured digestive tract visible through the skin, two pairs of wings beating slowly through cold water. It hunts Limacina helicina — the sea butterfly — almost exclusively. When it catches one, a set of muscular hooks called buccal cones erupts from its head and pulls the prey out of its shell. There is no nicer way to put this: the angel eats the butterfly, and only the butterfly.

That predator–prey pairing makes the pair a flagship for ocean acidification monitoring. Limacina builds an aragonite shell that dissolves in acidifying water; if the butterfly fails, the angel starves. North Pacific populations long called C. limacina were split off as Clione okhotensis by Yamazaki and Kuwahara in 2017, but the North Atlantic / Arctic animal remains C. limacina sensu stricto.

Six animals, six body plans, six different ways to be a sea slug. Only number 1 and number 2 are nudibranchs.

A footnote on Opisthobranchia

A lot of older field guides — and a surprising amount of Wikipedia — still file these animals under "Opisthobranchia." That word used to refer to a class containing all the lineages above, plus a few more.

Molecular phylogenies in the 2000s and 2010s showed that Opisthobranchia is not a real evolutionary group: some of its members are more closely related to land slugs and pond snails than to each other. By the late 2010s most working malacologists had stopped using the term except as a historical convenience. The lineages are now nested under the broader subclass Heterobranchia, and the sea slug umbrella is informal again — exactly as it always should have been.

If you see "Opisthobranchia" used in a 2010s-or-later writing as if it were a current group, treat it the way you would treat "the planet Pluto": once a useful label, no longer the organising idea.

Common confusions

Three sources of mix-up. The first is not a sea slug at all; the second and third are sea slugs.

  • Polyclad flatworms (genera like Pseudoceros, Pseudobiceros) — not sea slugs at all. They are not even molluscs; they belong to the phylum Platyhelminthes. No foot, no rhinophores, no internal anatomy of any sea slug type. Online, their flat, vivid bodies get them routinely mislabelled as "sea slug."
  • Pleurobranchids (Pleurobranchida) — sea slugs, but not nudibranchs. Pleurobranchida is a separate order within Nudipleura, sister to Nudibranchia. Often equated with nudibranchs in casual writing; they belong under the sea slug umbrella, but they are a different lineage at the order level.
  • Shelled Acteonimorpha (Hydatina, Bullina, Micromelo, etc.) — sea slugs, but a different lineage from headshield slugs. Historically grouped within Cephalaspidea, but since Bouchet & Rocroi (2005) they are placed in their own subterclass Acteonimorpha (containing the superfamily Acteonoidea; Hydatina, Bullina, Micromelo belong to family Aplustridae within it). They still carry a thin external shell, which gets them "looks like a snail" and quietly skipped in most popular guides.

A note on ウミウシ (the Japanese counterpart)

English-speaking readers sometimes assume "sea slug" is a tidy translation of the Japanese word umiushi. It is not. Japanese has its own version of the same elastic-umbrella problem.

When the term umiushi (海牛, literally "sea cow") was first applied scientifically in 1892, by Fujita Tsuneoki, it covered only one small group of dorids. By the time Yoshiaki Hirano's 2000 reference Umiushi-gaku came out, the word was in popular use as a near-synonym of Opisthobranchia — exactly the broad sweep that English uses for "sea slug." After Opisthobranchia fell apart, Japanese workers had to put the term back together from scratch, listing twelve component clades; this work is still ongoing.

The historical arc takes about 130 years. We tell the long version in a separate Japanese-language article (with English translation). Mention it here only as a reminder that the looseness is not an English-language quirk.

Why does the difference matter?

Three reasons.

For divers. "I saw a nudibranch" tells your dive buddy nothing useful if you actually saw a sea hare or a sea angel. The lineages have different behaviours, different distributions, and different photographic etiquette (you do not poke a phyllidiid; you absolutely do not lift a Spanish dancer). Naming the group correctly is part of seeing it correctly.

For citizen scientists. iNaturalist and WoRMS index records by clade. If you upload a Bursatella photo as "nudibranch" or a Clione as "snail," your data does not flow into the right place.

For media. Every viral "blue sea slug" article on the internet calls Glaucus atlanticus a nudibranch. That happens to be correct — Glaucus is in Order Nudibranchia (sensu stricto). The same articles often use the term interchangeably with Pteraeolidia, with Hexabranchus, even with Bursatella. Those four animals are in three different orders. The umbrella collapses if you pretend it is a single thing.

Browse by group

SEASLUG.WORLD catalogues species across every group above — over four thousand entries from contributors around the world. If something here caught your attention, the lineage browse pages are the fastest way in:

Or start at the front page and pick a colour: en.seaslug.world.

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