Acorn worms are marine invertebrates belonging to the phylum Hemichordata. Their common name is derived from the shape of their front end, which features a muscular proboscis and collar that resemble an acorn. These animals represent a lineage for understanding animal evolution, providing a window into the development of deuterostomes, the major group to which vertebrates belong.
Anatomy and Environment of Acorn Worms
An acorn worm’s body has a three-part structure: a proboscis used for burrowing and collecting food, a collar, and a long trunk. Immediately behind the proboscis is the collar, a short fleshy ring of tissue that can contract to close over the mouth. The trunk, which makes up the majority of the animal’s length, contains the digestive tract and other organs.
These soft-bodied animals are fragile but can regenerate. Their size can vary, with some species measuring only a few centimeters, while others can grow to lengths of over 1.8 meters (about 6 feet). Acorn worms are benthic, meaning they live on or within the seafloor. They are found globally in marine habitats from shallow intertidal mudflats to the deep sea, and many species create U-shaped burrows in soft sand or mud.
Daily Life and Reproduction
The daily activities of an acorn worm center on feeding and maintaining its burrow. They are either deposit feeders, consuming sediment to digest the organic matter within it, or suspension feeders that capture particles from the water. In both strategies, food is trapped in a layer of mucus on the proboscis and then transported to the mouth. An observer might spot the coiled, ropelike castings of sediment left behind by these animals as they slowly tunnel through the substrate.
Reproduction in most acorn worm species is a sexual process that occurs externally. Individuals are either male or female and release their gametes—sperm and eggs—directly into the seawater for fertilization. From this external fertilization, a free-swimming tornaria larva develops. This larva disperses in the water column before settling and metamorphosing into the adult form, and it bears a notable resemblance to the larvae of echinoderms.
The Evolutionary Link to Vertebrates
Acorn worms hold a position in the study of evolution, bridging a gap between invertebrates and chordates. As hemichordates, they are part of the deuterostome superphylum, sharing a common ancestry with both echinoderms and chordates. Genetic studies suggest that hemichordates are more closely related to echinoderms, like starfish, than to chordates.
Despite their closer relation to echinoderms, acorn worms possess several features that are also found in chordates. They have pharyngeal gill slits, which are openings along the anterior part of the gut used for filter-feeding and respiration. These structures are considered evolutionary precursors to the gills found in fish. Furthermore, they have a dorsal nerve cord, a feature of the chordate nervous system, although they also have a ventral nerve cord. Study of these animals offers insights into the ancient body plans of vertebrate ancestors.