---
title: Bag Pen Shell (Streptopinna saccata)
scientific_name: Streptopinna saccata
common_name: Bag Pen Shell
taxon: Clams
published: 2026-08-11T07:40:36.453Z
updated: 2026-08-11T08:41:49.264Z
tags: Bag Pen Shell, Streptopinna saccata, Clams, Red Sea, Marine Life, Diving
url: https://redseacreatures.com/taxon/invertebrate/molluscs/clams/bag-pen-shell
---

# Bag Pen Shell (*Streptopinna saccata*)

## Quick facts

- **Group:** Clams (*Bivalvia*)
- **Size:** up to 20 cm
- **Depth:** 2-25 m
- **Diet:** Filter feeder
- **Activity:** Diurnal
- **Rarity:** Uncommon
- **Habitat:** Wedged in crevices in coral and rock
- **Reproduction:** Broadcast spawner, Gonochoric

## How to recognise it

- shell growing out of a crevice rather than out of sand
- irregular, bag-like outline that follows the shape of the hole
- thin translucent shell, often with a wavy gaping edge
- shell to 20 cm

A pen shell that gave up on sand. This one grows out of a crevice in coral or rock, and because the shell forms around whatever space is available, no two look alike: the outline is irregular and bag-like, thin, translucent, with a wavy gaping edge, up to 20 cm (8 in) long.

Growing to fit is a real strategy rather than an accident. Wedged in a hole, the animal needs no anchor line and no burrow, and it is far harder for a fish to reach than one standing exposed in a meadow. The cost is that it can never move, and that its shape is dictated by a crack it settled in as a larva.

What shows underwater is the gape, a wavy dark line with the mantle just inside it, usually surrounded by growth. It closes slowly and reopens slowly, and if the crevice is tight the two valves may not meet at all - the shell simply grew to the shape of its cell.

## Also known as

- **English:** baggy pen shell
- **Japanese:** カゲロウガイ
- **Chinese:** 袋狀江珧蛤
- **Russian:** Пинна мешковидная
- **KO:** 굵은주름키조개

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