Digitata Sand Castle Worm, Phramatopoma digitata

Digitata Sand Castle Worm, Phramatopoma digitata. Underwater photographs taken in Bahía de La Ventana, La Paz, Baja California Sur, December 2025 in three foot water. Photographs and partial identifications courtesy of Aaron Woodrow, Juneau, Alaska.

Phylogeny: The Digitata Sand Castle Worm, Phramatopoma digitata (Rioja, 1962), is a member of the Sabellariidae Family of Marine Polychaete Worms. The Sabellariidae Family has twelve genera and there are ten species in the Phramatopoma Genus. They are known in Mexico as Gusano Castillo de Arena. Note: there is a remote possibility that this is the similar California Sand Castle Worm or Honeycomb Tube Worm, Phramatopoma californicus that has only been documented from Guerrero Negro, Baja California, northward along the west coast of the Baja Peninsula.

Morphology: The Digitata Sand Castle Worm has radial symmetry form an encrusting mass characterized by cylindrical shaped projections. Each individual worm makes an encrusting tube out of sand that are 2.5 cm (1.0 inch) in diameter. The Digitata Sand Castle Worm has sand-colored tubes and dark brown tentaticles that extend up to 7.5 cm (3.0 inches). The colonies generally cover a surface area of up to 1.0 meters (3 feet 3 inches) in diameter.

Habitat and Distribution: The Digitata Sand Castle Worm are reef building marine polychaetae worms that inhabit the intertidal and shallow subtidal zones. They can be found in close proximity to mussel beds and prefer areas with some shelter, such as rock faces and overhanging ledges. They are colonial constructing sand tubes on hard substrate and often form large sand mounds in areas with moderately high turbulence. They are often seen on rocky beaches and medium and low tide. They have an honeycomb-like outward appearance and can be up to 2 meters (6 feet 6 inches) in size. They are not mobile. The worms remain in their tubes and are seldom seen by humans. During low tidal episodes they close the entrance to their tubes with an operculum made of setae. The range of the Digitata Sand Castle Worm is poorly documented in Mexican waters of the Pacific Ocean with its northernly range limited to coastal waters of Oaxaca.

Diet: The Digitata Sand Castle Worm are filter feeders that extend their tentacles when submerged, primarily consuming plankton and organic detritus from the water column. They also collect sand grains for tube building and tube repair.

Predation: Predation of the Digitata Sand Castle Worm has not been formally documented.

Reproduction: The Digitata Sand Castle Worm sexually reproduce via broadcast spawning that result in trochopore larvae that passively drift in the in the plankton for two to twenty weeks until they settle to the bottom where they metamorphose into juveniles. They mature quickly and have life spans of one to two years.

Ecosystem Interactions: The Digiata Sand Castle Worm is thought to significantly enhance near-shore hard-bottom habitat.

Human Interactions: The Sand Castle Worm have no direct impact on human activities. From a conservation perspective the Sand Castle Worm has not been formally evaluated. They possess a bioadhesive glue that is utilized to build its protecting tube that is made of polyphenolic proteins worms secrete and utilize to stick grains of sand together underwater. This glue has been reproduced in the laboratory and is currently being evaluated as a biocompatible medical adhesive in human bone repair, corneal incision repair and sealing skin cuts.

Synonyms: Phragmatopoma moerchi digitata and Phragmatopoma virgini.