---
title: Crustaceans (Crustacea)
scientific_name: Crustacea
common_name: Crustaceans
rank: Subphylum
published: 2024-11-19T18:41:51.573Z
updated: 2026-08-23T21:28:58.234Z
tags: Crustaceans, Crustacea, Red Sea, Marine Life, Diving
url: https://redseacreatures.com/taxon/invertebrate/crustaceans
---

# Crustaceans (*Crustacea*)

## Quick facts

- **Rank:** Subphylum
- **Scientific name:** Crustacea
- **Subcategories:** 7

## How to recognise this group

- rigid outer shell
- body often visibly divided into segments
- multiple pairs of jointed legs and antennae

A crab, a spiny lobster, a barnacle on a wreck and the specks in a mouthful of seawater are all the same animal built to different specifications.

The specification is a row of segments, each carrying a pair of limbs made to one shared design, and evolution has spent half a billion years reassigning them. Toward the front one pair became jaws, the pair behind it became more jaws, then claws, then walking legs, then the paddles under a shrimp's tail, and the gills are limb branches too. Nothing gets invented; the same appendage is reissued with a new job. It is why the front of a lobster's mouth is a stack of six pairs of different tools, and why a barnacle feeds with what are anatomically its feet.

Two pairs of antennae is the giveaway. An insect has one pair, a spider none, a crustacean two, and if you can see two sets of feelers on a small animal in the sea you are almost certainly looking at one. The relationship runs the other way as well: the current evidence puts insects inside the crustacean family tree rather than beside it, which makes a fly something close to a crustacean that walked out of the water and stayed out.

The cost of the design is that armor cannot stretch, so growth happens in steps and there is no growing between them. The reach it buys is enormous: copepods too small to see, probably the most numerous animals on the planet, at one end, and crabs measured in meters across the legs at the other. Nearly all of them hatch as the same thing, a one-eyed larva called a nauplius, drifting in the plankton and resembling nothing it will become.

What a diver meets on this coast is a handful of branches off that tree: crabs wedged in coral heads, shrimps in every crevice, hermit crabs working the rubble at night, clawless spiny lobsters under the ledges, mantis shrimps watching from burrows, and barnacles cemented to anything hard that has been in the water long enough.


## Subcategories

- [Barnacles (Cirripedia)](https://redseacreatures.com/taxon/invertebrate/crustaceans/barnacles)
- [Crabs (Brachyura)](https://redseacreatures.com/taxon/invertebrate/crustaceans/crabs)
- [Hermit crabs (Paguroidea)](https://redseacreatures.com/taxon/invertebrate/crustaceans/hermit-crabs)
- [Lobsters (Achelata)](https://redseacreatures.com/taxon/invertebrate/crustaceans/lobsters)
- [Mantis shrimp (Stomatopoda)](https://redseacreatures.com/taxon/invertebrate/crustaceans/mantis-shrimp)
- [Shrimps (Caridea)](https://redseacreatures.com/taxon/invertebrate/crustaceans/shrimps)
- [Squat lobsters (Galatheidae)](https://redseacreatures.com/taxon/invertebrate/crustaceans/squat-lobsters)

---

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