---
title: Sap-sucking slugs (Sacoglossa)
scientific_name: Sacoglossa
common_name: Sap-sucking slugs
rank: Order
published: 2024-11-19T18:41:51.573Z
updated: 2026-08-23T21:14:20.722Z
tags: Sap-sucking slugs, Sacoglossa, Red Sea, Marine Life, Diving
url: https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs
---

# Sap-sucking slugs (*Sacoglossa*)

## Quick facts

- **Rank:** Order
- **Scientific name:** Sacoglossa
- **Body shape:** Invertebrate - Not a fish - coral, mollusc, crustacean, echinoderm and other invertebrate groups.
- **Species listed here:** 16

## How to recognise this group

- drains algal cells and keeps the chloroplasts, so most are green
- small, typically 1-5 cm
- body form varies: side flaps, leaf-like cerata, or even a two-valved shell
- almost always sitting on the green alga it feeds from

These slugs run on sunlight using machinery they did not build. A sacoglossan feeds by punching a single tooth through the wall of an algal cell and drinking the contents, and rather than digesting everything it takes, it keeps the chloroplasts. They are moved into the lining of the gut, which branches through the whole body like a tree, and there they go on photosynthesizing, in some species for weeks. A well fed slug can then live a long time without eating anything, powered by an alga that is no longer alive. The group name, Sacoglossa, means sac tongue, for the pouch where the animal stores the teeth it has blunted instead of dropping them.

No other animal manages this. Corals and giant clams keep whole algal cells alive as lodgers, which is a very different thing from lifting out one organelle and running it yourself, because a chloroplast normally depends on a steady supply of proteins from the algal nucleus it has been separated from. How the slugs keep them working is still argued over. It is also why so much of this group is green: the color is the crop rather than the animal.

One of them can be beheaded and walk away from it. In a Japanese laboratory, individuals of Elysia marginata, a species listed on this coast, cut through their own necks, and the head crawled off and regrew a complete new body, heart included, in about three weeks. The old body went on moving for days. A head that can photosynthesize does not need a gut to stay alive while it builds one, which is the leading explanation.

Sixteen species are listed in this catalogue and almost every one is tied to a particular green alga, so finding them means searching the weed, Halimeda and Caulerpa and Bryopsis, from a few centimeters away. Some carry leaf shaped flaps and pass for a piece of the plant they are draining. One still has a real two valved shell and looks like a small clam that decided to crawl.


## Species (16)

- [Blunt Elysia (Elysia obtusa)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/blunt-elysia)
- [Decorated Thuridilla (Thuridilla decorata)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/decorated-thuridilla)
- [Dwarf Elysia (Elysia pusilla)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/dwarf-elysia)
- [Elegant Cyerce (Cyerce elegans)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/elegant-cyerce)
- [Exquisite Bivalved Snail (Julia exquisita)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/exquisite-bivalved-snail)
- [Green Lobiger (Lobiger viridis)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/green-lobiger)
- [Hairy Elysia (Elysia tomentosa)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/hairy-elysia)
- [Hooded Elysia (Elysia marginata)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/hooded-elysia)
- [Jordan's Oxynoe (Oxynoe jordani)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/jordans-oxynoe)
- [Kevin Lee's Placida (Placida kevinleei)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/kevin-lees-placida)
- [Mercier's Elysia (Elysia mercieri)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/merciers-elysia)
- [Ocellate Sap-sucking Slug (Plakobranchus ocellatus)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/ocellate-sap-sucking-slug)
- [Oriental Polybranchia (Polybranchia orientalis)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/oriental-polybranchia)
- [Shaun the Sheep Slug (Costasiella kuroshimae)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/shaun-the-sheep-slug)
- [Thompson's Elysia (Elysia thompsoni)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/thompsons-elysia)
- [Three-lobed Elysia (Elysia trilobata)](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs/three-lobed-elysia)

---

*Source: [Red Sea Creatures](https://redseacreatures.com/taxon/invertebrate/molluscs/sap-sucking-slugs)*
*Updated: August 23, 2026*