Margarita Worm Snail, Thylacodes margaritaceus

Margarita Worm Snail, Thylacodes margaritaceus. Underwater photograph taken in Zihuantanejo Bay, Guerrero, March 2020. Photographs courtesy of Ron Woheau, Zihuantanejo, January 2020. Identifications courtesy of Greg Timms, youtube.com/@CoralmasterdivesPuertoVallarta/videos.

Phylogeny: The Margarita Worm Snail, Thylacodes margaritaceus (Chenu, 1844), is a gastropod mollusk that is a member of the Vermetidae Family of Worm Snails. The genus Thylacodes is one of twelve genera in this family, and there are forty species in this genus. The genus name Thylacodes comes from the Greek word and means “pouch-like” or “sack-like”. This refers to the shell in which the animal lives. The species name margaritaceus comes from the Latin and means “pearl-like” or “pertaining to pearls”. This describes the pearly interior of the shell. They are also known as the Margarita Tube Shell and in Mexico as Caracol Gusano and Caracol de Tubos.

Morphology: Margarita Worm Snail shells are tubular, and usually have a flat profile and a flattened base. They grow in a fairly regular spiral with irregular patterns occurring when surrounding obstructions or overcrowding restrict their growth space. The shells are marked with strong lengthwise ridges and lack an operculum. The aperture margin is dark brown to black in color. The most visible portions of the animal are the mantle and the foot. The mantle is orange and is often extended to the margin, but it can be retracted. The foot has a pebbly or granular texture. The foot ranges from whitish to brown in color, with numerous lighter colored spots. The shells are gray to brownish in color. Margarita Worm Shells reach a maximum of 10 cm (3.9 inches) in length; they have apertures that are 1.0 cm (0.4 inches) to 1.3 cm (0.5 inches) in diameter. Margarita Worm Snails can be distinguished from the Scaled Worm Snail Thylacodes squamigerus, which has a scaly shell, a light and dark dashed margin, and is usually clustered together in a rock-like colony.

Habitat and Distribution: Margarita Worm Snails are found firmly attached to rocks and dead shells from the intertidal zone to depths up to 45 m (115 feet). They are normally found as solitary animals or in weak aggregations. They are a subtropical to tropical Eastern Pacific species that are found in all Mexican waters of the Pacific Ocean with the exceptions that they are absent from along the west coast of the Baja Peninsula and north of Bahía San Carlos, Sonora in the Sea of Cortez.

Diet: Margarita Worm Snails have abandoned the typical mobile life of a gastropod. To feed, they deploy mucus nets as external, dynamic, fishing structures to capture plankton and detritus from the surrounding water. This process acts as an underwater “spider web” or “flypaper” system. To accomplish this, the snail has a massive pedal gland located just underneath its mouth. This gland pumps out a highly specialized, water-resistant, elastic mucus. The snail then utilizes its pedal tentacles (the small, highly maneuverable appendages on either side of its head) to draw out the mucus. The mucus travels down specialized grooves in these tentacles, allowing the snail to actively manipulate the slime into long, cohesive strings. Unlike other filter feeders that passively strain water internally, Margarita Worm Snails rely heavily on surge and ambient water currents to deploy its net. The surge catches the slime threads and expands them outward like a drifting parachute or flat mesh. In moving water, a single snail can unfurl a web covering 4 to 7 square inches in just a few minutes. The net is left suspended in the current for roughly 20 to 40 minutes. As the water washes through it, microscopic zooplankton, phytoplankton, algae, and organic debris get stuck to the viscous mesh. To harvest the food, the snail uses its rasplike tongue (the radula) to grab the central strands of the net. It physically reels the entire structure back into its mouth, ingesting the trapped food and eating the mucus itself. This prevents the animal from wasting the massive amounts of metabolic energy required to synthesize the proteins in the slime.

Predation: Predation of Margarita Worm Snails is poorly studied and documented. They rely on their hard tubes for protection. Other species in this genus are eaten by fish, crabs, and other carnivorous mollusks.

Reproduction: Margarita Worm Snails are gonochoric ( male or female for life). Because Thylacodes species are permanently cemented to the reef, they cannot move to find a mate. They adaptively use their feeding nets to solve this problem. Males broadcast packets of sperm into the open water. Down current females deploy their mucus nets to “fish” the sperm packets out of the water and once caught, the net is pulled back into the female’s tube to internally fertilize and brood her eggs safely inside the shell. The eggs develop into crawling hatchlings.

Ecosystem Interactions: Margarita Worm Snails are indirectly parasitized by opportunistic nudibranchs who target the fishing web and steal plankton particulates from the web to feed on. Otherwise, there is no mention in the available literature of them engaging in any types of parasitic, commensal, or symbiotic relationships, though their empty shells are used for housing by the Holmes Hermit Crab, Pylopagurus holmesi.

Human Interactions: Margarita Worm Snails have a very limited impact on human activities. From a conservation perspective they have not been formally evaluated. However, they are fairly common with a relatively wide distribution and should be considered to be of Least Concern.

Synonyms: Serpulorbis margaritaceus, Vermetus margaritaceus, and Vermetus margaritarum.