Dead Man's Fingers

Overview

Dead Man's Fingers (Codium fragile) is a distinctive green seaweed recognized by its dark green, spongy, and finger-like branches. Originally native to the Pacific Ocean near Japan, this resilient alga has spread globally and is now considered an invasive species in many coastal regions, including the North Atlantic.

Appearance

This species forms bush-like clusters of cylindrical, dichotomously branched fronds that arise from a spongy, disc-shaped holdfast. The branches are typically 3–10 mm in diameter and possess a soft, velvety texture due to the closely packed, microscopic structures called utricles that form the outer layer. While often 15–40 cm long, individual fronds can reach up to 1 meter in length. When exposed during low tide, the branches hang limply from rocks, resembling dark, swollen fingers.

Distribution & habitat

Codium fragile is highly adaptable and thrives in a variety of marine environments, including rocky shores, cobble beaches, seagrass beds, and oyster reefs. It frequently colonizes human-made structures such as piers, docks, and breakwaters. It is commonly found in the low intertidal and subtidal zones, where it can form dense mats that may outcompete native algal species.

Biology & behavior

Remarkably, the entire clump of Codium fragile is composed of a single, giant multinucleate cell. It is an "ecosystem engineer" capable of altering its environment; its buoyant branches can sometimes detach and float, potentially displacing shellfish or interfering with local marine life. The species is known for its ability to regenerate if damaged and can exhibit biofluorescence under certain light conditions.