Acorn worms are worm-like invertebrates belonging to the class Enteropneusta within the phylum Hemichordata. These soft-bodied animals derive their name from the acorn-like shape of their muscular proboscis. The proboscis, along with a collar and an elongated trunk, forms the three distinct regions of their body.
They primarily inhabit the ocean floor, where they create U-shaped burrows in the sediment. Although traditionally thought to be sediment dwellers, some species have been found in the deep sea, exhibiting different behaviors, such as crawling along the ocean bottom or drifting in the water column.
With over 100 known species, including the well-studied Saccoglossus kowalevskii, these animals play a crucial role in the marine food web, filtering nutrients from seawater or ingesting sediment as they burrow.
Although most acorn worms range between 9 to 45 cm (3.5 to 17.7 in) in length, the largest, Balanoglossus gigas, measures over 150 cm (5 ft). In contrast, one of the smallest species, Meioglossus psammophilus, reaches only 0.024 in (0.6 mm) in length.
As hemichordates, acorn worms have a tripartite body arrangement comprising an acorn-shaped proboscis, a fleshy collar, and a long, worm-like trunk. Their skin is covered with cilia and numerous mucus-secreting glands.
Acorn worms have an open circulatory system characterized by a muscular, sac-like heart located in the proboscis that pumps blood throughout their bodies. A dorsal blood vessel runs along the top of the gut, draining into a central sinus in the proboscis.
Unlike in most animals, the heart of acorn worms is not directly connected to their circulatory system; instead, blood flows through tissue sinuses. The central sinus in the collar region connects to a complex network of sinuses and peritoneal folds in the proboscis. The blood of acorn worms is colorless and lacks cells.
These hemichordates respire through gills, continuously forming new gill slits as they grow. For instance, species like Balanoglossus aurantiacus can develop over a hundred-gill slits on each side of their body, whereas Meioglossus psammophilus has only a single slit. Each gill slit includes a branchial chamber that connects to the pharynx through a U-shaped cleft and opens to the exterior through a dorsolateral pore.
The digestive tract of acorn worms begins with a tubular mouth located in the collar region, which houses a narrow, hollow diverticulum known as the stomochord that extends into the proboscis. This stomochord was once thought to be homologous to the notochord found in chordates, leading to their classification as hemichordates, or ‘half chordates.’ From the mouth, food passes into the pharynx, which is lined with rows of gill slits on either side. The pharynx then leads to the esophagus and finally to the intestine. Unlike most invertebrates, acorn worms do not have a stomach.
They lack a distinct excretory system. Instead, the network of sinuses and peritoneal folds in the proboscis is believed to function as a glomerulus, which functions in excreting metabolic wastes. During low tides, acorn worms expel casts or coils of processed sediments, which are eliminated as waste.
A dorsal and a ventral nerve cord runs along the body of acorn worms, spreading a network of subepidermal nerves (plexus). While the extent of the ventral cord is restricted to the collar, the dorsal cord extends to the proboscis as a hollow structure, probably homologous to the vertebrate brain.
Being invertebrates, they completely lack typical sense organs like eyes or ears. The multiple nerve endings that innervate the skin function as sensory units. Additionally, a ciliated organ lies in front of the mouth, which plays a gustatory role, apart from aiding in filter feeding.
Currently, around 111 known species of acorn worms are grouped under the following four families:
Acorn worms are found in a wide variety of marine environments, primarily as benthic organisms living on or within the ocean floor and often extending to abyssal depths of over 10,000 feet (3,050 meters).
While many acorn worms eat sand or mud to obtain organic detritus, others feed on organic matter suspended in water.
Acorn worms move slowly by contracting their body muscles through peristalsis and using the cilia on the underside of their bodies. The mucous glands on their skin secrete a substance that lubricates their path, making movement easier. Typically, they burrow into the seabed, creating U-shaped, tube-like tunnels in the sediment. Once settled, they often extend their proboscis through the openings of these burrows.
Acorn worms primarily obtain food through deposit feeding, where they ingest sediments such as sand or mud to extract organic matter and microorganisms. In addition, they sometimes practice suspension feeding, where they consume organic matter and microbes suspended in the water.
These hemichordates are dioecious (exist in separate male and female forms) and typically reproduce sexually. However, some species, such as Balanoglossus simodensis, can also reproduce asexually through fragmentation. The female lays multiple eggs encased in gelatinous mucus, which are externally fertilized by sperm from the male before the gelatinous mass disintegrates into individual eggs. Both males and females release their gametes through small pores near the gill slits.
In species like Saccoglossus kowalevskii, fertilized eggs develop directly into adults. However, in most acorn worms, the eggs go through an intermediate planktonic larval stage known as the tornaria. This oval-shaped, translucent larva features convoluted ciliary bands encircling its body, bearing a resemblance to the bipinnaria larvae of starfish. After several days or even weeks, a groove forms in the middle of the larval body, distinguishing the future anterior proboscis from the posterior collar and trunk. The larva finally settles on the sea floor and matures into an adult.
Some species, like Ptychodera flava, produce a bromide compound that smells medicinal and helps them evade bacteria and potential predators.
Research indicates that certain acorn worms, like Ptychodera flava, possess remarkable regenerative abilities, allowing them to fully restore their heads and bodies after sustaining injuries.
Welcome to AnimalFact - your go-to source for fascinating insights into the amazing world of animals!
