Red Sea Squirt

Overview

The red sea squirt (Halocynthia papillosa) is a solitary, sessile tunicate easily recognized by its vibrant red to orange, bladder-like body. It is a common inhabitant of the Mediterranean Sea and parts of the eastern Atlantic, where it thrives in shaded, rocky environments.

Appearance

This species features a firm, ovoid, and cartilaginous tunic with a distinctively rough, granular texture. It possesses two siphons—an oral siphon at the top for water intake and an atrial siphon on the side for expulsion—both of which are surrounded by fine, rigid bristles that act as sensory organs and protective filters. When disturbed, the animal can retract its siphons and contract its body significantly.

Distribution & habitat

While most common in the Mediterranean, it is also found along the Atlantic coast of Portugal. It is a sciaphilic (shade-loving) species, typically found in low-light conditions such as rocky crevices, overhangs, cave walls, and at the base of seagrass beds. It is a benthic organism, usually attached to hard substrates.

Feeding

As a suspension feeder, it draws seawater through its oral siphon. An internal branchial structure filters out microorganisms and oxygen before the water is expelled through the lateral atrial siphon.

Biology & behavior

Halocynthia papillosa begins its life as a free-swimming larva before settling permanently onto a substrate to transition into its adult, sessile form. Unlike many other tunicates, its reproductive activity peaks in late summer and early autumn, suggesting that seasonal environmental factors are not the primary drivers of its breeding cycle.