''Urechis caupo'' is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

of spoon worm in the family

Urechidae Urechidae (commonly known as "fat innkeeper" or "penis fish") is a family of spoonworms in the subclass Echiura. The only genus in the family is ''Urechis'', which has four species. Species The World Register of Marine Species includes these sp ...

, commonly known as the innkeeper echiuran, the fat innkeeper worm (because the tunnels often contain other animals), the innkeeper worm, or the penis fish. It is found in shallow water on the west coast of North America, between southern

Oregon Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...

and

Baja California Baja California (; 'Lower California'), officially the Free and Sovereign State of Baja California ( es, Estado Libre y Soberano de Baja California), is a state in Mexico. It is the northernmost and westernmost of the 32 federal entities of Mex ...

, where it forms a U-shaped burrow in the sediment and feeds on plankton using a mucus net.

Description

''Urechis caupo'' is a plump, unsegmented, cylindrical pink worm growing to a length of up to , with being a more typical length. There are a pair of

seta In biology, setae (singular seta ; from the Latin word for "bristle") are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Annelid setae are stiff bristles present on the body. Th ...

e (bristles) on the ventral surface at the anterior end, and a distinctive ring of about ten setae around the

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

at the posterior end. The

proboscis A proboscis () is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In invertebrates, the term usually refers to tubular mouthparts used for feeding and sucking. In vertebrates, a proboscis is an elong ...

is short.

Distribution and habitat

Shallow water in the northeastern Pacific Ocean is the habitat of ''U. caupo''; its range extends from southern

to northern

. It lives in a burrow in muddy sand in the lower

intertidal The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of Marine habitat, habitats with var ...

and the shallow

subtidal zone The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illuminated ...

. Thousands of these worms occasionally wash up on beaches in northern California. One such stranding took place on December 6, 2019, at Drake's Beach near Pt. Reyes. This was likely the result of recent storm weather which disturbed the worms' burrows.

Ecology

This spoon worm is a

detritivore Detritivores (also known as detrivores, detritophages, detritus feeders, or detritus eaters) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). There are many kinds of invertebrates, ...

and creates a U-shaped burrow in the soft sediment of the seabed. When feeding, it presses a ring of glands at the front of the proboscis against the burrow wall and secretes

mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...

which sticks to the burrow wall. The worm continues to exude mucus as it moves backwards in the burrow, thus creating a mucus net. The worm draws water through its burrow by peristaltic contractions of its body and as food particles pass through the net they adhere to it. When enough food is gathered, the worm moves forward in its burrow and swallows the net and entangled food. This process is repeated, and in an area with plenty of detritus, may be completed in only a few minutes. Faecal pellets accumulate around the worm's anus, and periodically the worm contracts its body sharply to produce a stream of water from the anus that blasts the pellets and loose sediment from the tube, creating a casting on the surface of the sand. Larger food particles are rejected and discarded in the burrow where they provide food for the many different

commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...

organisms which share the burrow, resulting in this spoon worm being known as the "innkeeper worm". These include the California softshell clam (''Cryptomya californica''),

pea crab The pea crab, ''Pinnotheres pisum'', is a small crab in the family Pinnotheridae that lives as a parasite in oysters, clams, mussels, and other species of bivalves. Description Pea crabs are small crustaceans about the size of a pea or dime, wit ...

shrimp Shrimp are crustaceans (a form of shellfish) with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata of the decapod order, although some crustaceans outside of this order are refer ...

s and

scaleworm Polynoidae is a family of marine Polychaete worms known as "scale worms" due to the scale-like elytra on the dorsal surface. Almost 900 species are currently recognised belonging to 9 subfamilies and 167 genera. They are active hunters, but gene ...

s. The arrow goby (''Clevelandia ios'') uses the entrance of the burrow as a refuge into which it can dash if danger threatens. The gut of the spoon worm often contains many

trophozoites A trophozoite (G. ''trope'', nourishment + ''zoon'', animal) is the activated, feeding stage in the life cycle of certain protozoa such as malaria-causing '' Plasmodium falciparum'' and those of the '' Giardia'' group. (The complement of the trophoz ...

of the protozoan '' Zygosoma globosum''. The sexes are separate and fertilisation is external. The eggs are pinkish or yellowish and the sperm are white, being liberated into the water through a pair of modified nephridia. The larvae are

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

ic for about sixty days before settling on the seabed; they are strongly attracted to settle in the vicinity of other spoon worms by a chemical released from the castings.

References

{{Taxonbar, from=Q2242916 Animals described in 1928 Echiurans Fauna of the Pacific Ocean