Red-spotted Horseshoe

Overview

The Red-spotted Horseshoe (Protula tubularia) is a sedentary marine polychaete worm belonging to the family Serpulidae. It is easily recognized by its rigid, white, calcareous tube and a large, retractable branchial plume that fans out in a distinctive U-shape or horseshoe pattern.

Distribution & habitat

This species is widely distributed across the Mediterranean Sea, the North Sea, the English Channel, and parts of the Atlantic Ocean, with records also extending to the Indian Ocean and other global regions. It typically inhabits hard substrates such as rocks and reefs, ranging from the lower shore down to depths of 100 meters, and is frequently found in both illuminated areas and shaded environments like caves.

Appearance

Protula tubularia constructs a smooth, white, sinuous tube that lacks an operculum, a feature that distinguishes it from similar serpulids like Serpula vermicularis. The branchial plume, which can reach up to 10 cm in diameter, consists of two spiral lobes. These tentacles are highly variable in color—ranging from white to red or orange—and are often marked with small, paired red spots that function as eyespots.

Biology & behavior

As a sedentary organism, the worm remains fixed within its tube, extending its plume to facilitate both respiration and filter feeding. It is highly sensitive to light and vibrations, rapidly retracting its plume into the tube when disturbed. Reproduction is sexual, involving the release of gametes into the water column; fertilized eggs often develop within a transparent sac attached to the tube before the larvae settle onto a suitable substrate.