---
title: Sea squirts (Ascidiacea)
scientific_name: Ascidiacea
common_name: Sea squirts
rank: Class
published: 2024-11-19T18:41:51.573Z
updated: 2026-08-23T21:14:21.461Z
tags: Sea squirts, Ascidiacea, Red Sea, Marine Life, Diving
url: https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts
---

# Sea squirts (*Ascidiacea*)

## Quick facts

- **Rank:** Class
- **Scientific name:** Ascidiacea
- **Body shape:** Irregular - No standard fish outline - seahorses, frogfishes, ghost pipefishes.
- **Species listed here:** 12

## How to recognise this group

- sessile, firmly attached to rocks or other substrates
- sac-like, blob-shaped, or forming encrusting colonies
- two distinct openings (siphons) on the surface
- often brightly coloured in reds, yellows, blues, or purples
- contract rapidly and squirt jets of water when disturbed

A sea squirt starts life able to swim and think, and then gets rid of both.

The larva is a tadpole with a muscular tail, a nerve cord, a light sensor and a tilt sensor, and it has hours rather than days to find somewhere to live. When it does, it lands head first and cements itself down by the front of its head, and then dismantles the equipment it will never need again. The tail is absorbed. The notochord goes. Most of the larval nervous system is broken down and recycled, and what remains of the brain is a single knot of nerve tissue sitting between the two siphons. It is a permanent trade: movement given up for a fixed address in a good current.

What is left is a bag with two holes. Water in through one siphon, through a perforated basket lined with mucus that strains everything edible out of it, and out through the other. Squeeze one, or walk over a colony at low tide, and both siphons fire at once. The name is entirely literal.

The blood is where it gets strange. Cells in many species collect vanadium, a metal present in seawater only in traces, and concentrate it to levels millions of times higher than the water they are pumping, storing it inside the cell at an acidity close to that of stomach acid. No one has settled what it is for. Defense is the usual guess, and it remains a guess.

Not all of them are single animals. Many are colonies: dozens of individuals a few millimeters across, embedded in one shared tunic and arranged in rings and stars around a shared exit hole, so that what looks like a painted crust on a wreck is a settlement with communal plumbing. On this coast they take the shaded, awkward, current-swept surfaces that corals leave alone.


## Species (12)

- [Black Colonial Ascidian (Botrylloides niger)](https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts/black-colonial-ascidian)
- [Black-mouth Ascidian (Ascidia melanostoma)](https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts/black-mouth-ascidian)
- [Blue Club Tunicate (Rhopalaea idoneta)](https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts/blue-club-tunicate)
- [Brick Ascidian (Eusynstyela latericius)](https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts/brick-ascidian)
- [Eilat Star Ascidian (Botryllus eilatensis)](https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts/eilat-star-ascidian)
- [Manina's Ascidian (Metandrocarpa manina)](https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts/maninas-ascidian)
- [Red-throated Ascidian (Herdmania momus)](https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts/red-throated-ascidian)
- [Ridged Sea Squirt (Pyura gangelion)](https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts/ridged-sea-squirt)
- [Rodrigues' Didemnum (Didemnum rodriguesi)](https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts/rodrigues-didemnum)
- [Sheep's Foot Ascidian (Polycarpa mytiligera)](https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts/sheeps-foot-ascidian)
- [Spiny Sea Squirt (Halocynthia spinosa)](https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts/spiny-sea-squirt)
- [Turkish Ascidian (Rhodosoma turcicum)](https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts/turkish-ascidian)

---

*Source: [Red Sea Creatures](https://redseacreatures.com/taxon/invertebrate/tunicates/sea-squirts)*
*Updated: August 23, 2026*