---
title: Prickly Pen Shell (Pinna muricata)
scientific_name: Pinna muricata
common_name: Prickly Pen Shell
taxon: Clams
published: 2026-08-11T07:40:36.446Z
updated: 2026-08-11T09:02:15.454Z
tags: Prickly Pen Shell, Pinna muricata, Clams, Red Sea, Marine Life, Diving
url: https://redseacreatures.com/taxon/invertebrate/molluscs/clams/prickly-pen-shell
---

# Prickly Pen Shell (*Pinna muricata*)

## Quick facts

- **Group:** Clams (*Bivalvia*)
- **Size:** up to 15 cm
- **Depth:** 1-20 m
- **Diet:** Filter feeder
- **Activity:** Diurnal
- **Rarity:** Uncommon
- **Habitat:** Sand and seagrass, buried point-down
- **Reproduction:** Broadcast spawner, Gonochoric

## How to recognise it

- brittle fan-shaped shell standing upright out of the sand
- about a dozen ribs, often carrying short open spines
- only the wide upper edge shows, the point is buried
- shell to 15 cm

A brittle fan standing point-down in the sand with only its wide upper edge showing, up to 15 cm (6 in) long, ribbed with about a dozen lines that often carry short open spines. Divers usually meet it by accident, because a buried pen shell is a hard edge in soft ground and it cuts.

The animal anchors itself with byssus, a bundle of protein threads spun from a gland in the foot and hardened in seawater. In the Mediterranean, the same material from a larger relative was woven into sea silk, a cloth so fine and so laborious that surviving pieces sit in museums. The threads hold the shell upright in shifting sediment while the top edge filters passing water.

A broken pen shell does not heal well, and the animal spends its whole life in one spot, so a seagrass meadow with pen shells in it is a meadow that has been left alone. That makes them a decent field indicator of how much traffic a patch of sand actually gets.

## Also known as

- **English:** prickly pen shell
- **Japanese:** イワカワハゴロモ
- **Russian:** Пинна колючая

## Scientific synonyms

- *Atrina aequilatera* (E. von Martens, 1880)
- *Pinna (Quantulopinna) muricata* Linnaeus, 1758
- *Pinna aequilatera* E. von Martens, 1880
- *Pinna cancellata* Mawe, 1823
- *Pinna hawaiensis* Dall, Bartsch & Rehder, 1938
- *Pinna semicostata* Conrad, 1837
- *Pinna zebuensis* Reeve, 1858
- *Quantulopinna delsa* Iredale, 1939
- *Quantulopinna delsa howensis* Iredale, 1939

## Range

Indian Ocean, Kenya, Madagascar, Mozambique, Red Sea, Republic of Mauritius, Seychelles, South Africa, Tanzania

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*Source: [Red Sea Creatures](https://redseacreatures.com/taxon/invertebrate/molluscs/clams/prickly-pen-shell)*
*Updated: August 11, 2026*