# Sea Slug — Facts, Threats & Conservation

*Clade Nudibranchia — over 3,000 described species in order Nudibranchia*

> Sea slugs, especially nudibranchs (order Nudibranchia), are shell-less marine molluscs with over 3,000 colourful species; some steal chloroplasts from algae for photosynthesis and others store stinging cells from jellyfish prey, and most are not individually assessed on the IUCN Red List.

**IUCN status:** Not Evaluated as a group — shell-less molluscs; some steal chloroplasts or stinging cells from prey  ·  **WARN range:** World oceans, Shallow reefs and temperate coasts worldwide

## Quick facts
| Fact | Value |
| --- | --- |
| Phylum | Mollusca — gastropod molluscs |
| Shell | None in adult nudibranchs |
| Size | Mostly mm to about 10 cm |
| Kleptoplasty | Chloroplast theft in sacoglossans such as Elysia |
| Defence | Stolen nematocysts or prey toxins in cerata |
| Habitat | Shallow marine reefs and rock pools |
| IUCN status | Not Evaluated for most species |

## Scientific classification
- **Kingdom:** Animalia
- **Phylum:** Mollusca
- **Class:** Gastropoda
- **Order:** Nudibranchia
- **Shell:** Absent in adults — secondary loss
- **Described species:** Over 3,000 nudibranchs
- **Not:** Not fish, not insects — molluscs

## Conservation status
- **Status:** Not Evaluated
- **Population:** Not applicable at clade level
- **Trend:** Unknown; reef loss and warming affect prey-specific species
- **Assessed:** Most species not assessed
- **CITES:** Not listed
- High discovery rate and low assessment priority mean most nudibranch diversity lacks formal status despite reef dependence.

## Key facts: Sea Slug
- Nudibranchs are shell-less marine gastropods — molluscs, not insects or fish — with exposed gills (cerata) on the back.
- Some sacoglossan sea slugs steal chloroplasts from algae and photosynthesise temporarily — kleptoplasty.
- Others sequester nematocysts from cnidarian prey and use them in their own cerata for defence.
- Bright colours often advertise toxicity or distastefulness — aposematism copied from prey chemistry.
- Most species are small — millimetres to a few centimetres — and specialised to specific prey and habitats.
- Climate change, reef degradation and collection affect local populations; most species lack IUCN assessments.

## Molluscs without shells
Snails are gastropod molluscs; nudibranchs lost the external shell secondarily. Larvae may carry a brief shell that disappears at metamorphosis. The body is soft, often vividly coloured, with paired rhinophores (sensory tentacles) on the head and cerata on the dorsum serving respiration and sometimes defence.

They crawl on a muscular foot like any gastropod. 'Sea slug' also covers non-nudibranch groups — sea hares, sap-sucking slugs — but nudibranchs dominate popular recognition because of colour diversity on reefs.

## Stealing chloroplasts
Sacoglossan slugs such as Elysia chlorotica feed on algae and incorporate chloroplasts into their own tissues. The plastids continue photosynthesising for days to months, supplementing the slug's energy budget. The slug must maintain the plastids with stolen algal genes — a partial symbiosis that blurs the line between feeding and farming.

Kleptoplasty is not universal among sea slugs; it is a specialised trick in a subset. It made Elysia a model organism in photosynthesis research and popular science writing.

## Stealing stings
Aeolid nudibranchs feed on hydroids, anemones and corals and pass undischarged nematocysts through the digestive tract to cnidosacs at cerata tips. Predators biting cerata receive a sting the nudibranch did not evolve itself. Chemical defences from sponge prey provide another route to toxicity.

Aposematic colour patterns — orange, blue, purple contrasts — warn fish that the slug tastes bad or stings. Convergence is common: unrelated species mimic each other's warning colours.

## Reef specialists and data gaps
Nudibranchs are often prey-specific — one species eats one sponge, one bryozoan. Reef loss and warming remove both slug and food. Most of 3,000+ species have no IUCN assessment; discovery rates on tropical reefs still add new species annually.

Divers and rock-pool searchers record range shifts as seas warm. Sea slugs are canaries for reef invertebrate diversity that never generates headline extinction listings.

## What WARN does
WARN publishes free marine education for overlooked invertebrate diversity. Nudibranchs are among the most photographed reef animals and among the least assessed — kleptoplasty and cnidocyte theft deserve accurate explanation beyond dive-blog shorthand.

If this guide helps you understand wildlife and the pressures it faces, a gift to WARN supports habitat protection and free public education in our partner countries.

## Frequently asked questions: Sea Slug
### What do sea slugs eat?
Specialised diets — sponges, hydroids, anemones, algae, bryozoans depending on species. Nudibranchs often feed on one prey type; kleptoplastic species eat algae and retain chloroplasts.

### Where do sea slugs live?
Marine environments worldwide — shallow reefs, rock pools and temperate coasts. Nudibranchs crawl on substrate; none live on land or in freshwater.

### Are sea slugs dangerous?
Some concentrate toxins or stolen stinging cells and should not be handled. Most are harmless to humans if observed. Bright colours often warn of distasteful or stinging species to fish.

### How long do sea slugs live?
Typically weeks to about a year depending on species — short-lived compared with fish. Exact lifespan varies with food availability and temperature.

### How big are sea slugs?
Most nudibranchs are a few millimetres to about 10 cm. Some tropical species reach larger sizes; most rock-pool finds in Britain are small and colourful.

### Are sea slugs related to garden slugs?
Both are gastropod molluscs, but sea slugs are marine and nudibranchs are shell-less; garden slugs are pulmonate land gastropods. Shared name reflects shell reduction, not close relation.

### Are sea slugs endangered?
Most species lack IUCN assessments. Reef degradation, warming and prey loss threaten specialists locally; many species remain undescribed or unassessed despite being reef-diverse.

## Sources
- [Natural History Museum — Nudibranchs](https://www.nhm.ac.uk/discover/nudibranchs.html)
- [MarineBio — Nudibranchs](https://marinebio.org/species/nudibranchs/)
- [Encyclopedia of Life — Nudibranchia](https://eol.org/pages/4644)

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Full guide: https://worldanimalrescuenetwork.org/wildlife-guides/sea-slug