The Acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. They are closely related to the echinoderms. There are about 90 species of acorn worm in the world. Most acorn worms range from 9 to 45 centimetres in length, with the largest species, Balanoglossus gigas, reaching 1.5 metres (5 ft) or more. The body is made up of three main parts: an acorn-shaped proboscis, a short fleshy collar that lies behind it, and a long, worm-like trunk.
Acorn worms (Hemichordata, Enteropneusta) are more closely related to starfish and sea cucumbers than to other worms and are best known for their acorn-shaped front ends.
Acorn worms are rarely seen by humans because of their lifestyle. They live in U-shaped burrows on the sea-bed, from the shoreline down to a depth of 10,000 ft. (3,050 m). The worms lie there with the proboscis sticking out of one opening in the burrow. Until 1965, it was believed that acorn worms lived only in shallow waters. However, recent work using deep-sea vehicles in conjunction with DNA sequencing has uncovered a wide diversity of vividly colored deep-sea acorn worms, belonging to a new family, which was named Torquaratoridae in 2005.
In 2011, a research team from the P. P. Shirshov Institute of Oceanology recovered a near-complete specimen of a torquaratorid from the floor of the Kara Sea, north of Siberia. Although the specimen was found nearly a mile shallower than any previously known torquaratorid, it turned out to be yet another undescribed member of this family and was recently named Coleodesmium karaensis. This new species is the first acorn worm known to brood its young. Brood pockets consist of a thin membrane on the surface of the mother’s body and each contains a single embryo. Approximately one dozen embryos at various stages of development were identified from the single adult female worm found.
Glossobalanus sarniensis is an acorn worm, belonging to a group of hemichordates called Enteropneusta. They are considered to be intermediary between invertebrates and vertebrates. They have soft, fragile and worm-like bodies and live buried in sand. They are filter-feeders, have an acorn-like proboscis and they breathe by means of gills.
Earliest enteropneust (acorn worm) fossil found, 500 million years old. Fossils of these soft-bodied acorn worms were found in mud dated back to the Cambrian period, placing them a further 200 million years back in time than previously thought.
Christopher Cameron of the University of Montreal’s Department of Biological Sciences and his colleagues have unearthed a major scientific discovery – a strange phallus-shaped creature they found in Canada’s Burgess Shale fossil beds, located in Yoho National Park. The fossils were found in an area of shale beds that are 505 million years old.
Spartobranchus tenuis (Walcott) from the Burgess Shale. Top — individual specimens within and outside their tubes. Bottom — Close-up of a specimen within its tube. Credit: Marianne Collins