Marine Worms: Polychaetes, Annelids and Flat, Fire and Ribbon Worms

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Toupe worm

There are many kinds of worms in ocean. They come in many colors, shapes, and sizes. Some are so small that they live between grains of sand, while others other are longer than humans and eat fish! Scientists are still discovering new worm species, and there are still many mysteries about the ones that are well-known. [Source: Kara J. Gadeken, Erin Kiskaddon, Jenna M. Moore, Kelly M. Dorgan Frontiers, November 14, 2022]

What is a worm anyway? When scientists first began classifying animals they created a group called “Vermes,” which means “worms” in Latin. “Vermes” included “all animals that are longer than wide,” without backbones Later, scientists began to recognize and describe many distinct groups of worms. One of these groups, which includes earthworms, is called Annelida. Most worms fall into this group. “Annelida” means “ringed animals.” They are characterized by having rings of repeating segments along their bodies. Earthworms are annelid worms. Having many segments means flexibility, having different body parts to perform different and being able to survive if large chunks of your body are chopped off.

Annelids are ancient creatures, with ancestors going back to the time when the very first complex animals were created 600 million to 500 million years ago. Fossils of whole worms are very rare because their bodies are soft and do not preserve very well. Currently there are about 14,000 known species of annelid worms but many new species are discovered and described every year by taxonomists. More than 120 species of Annelida were described in 2021 alone.

One of the most important jobs performed by many worms is helping to recycle detritus (pieces of organic material such as dead plankton, algae, and fish poop that are an important food source for many organisms). Much of the actual work of recycling detritus is done by single-celled organisms invisible to the naked eye but these microbes have a hard time moving around in the sediments. That is where worms come in. When worms burrow through sediment, they move detritus around so the microbes can gain access to it.

Worms also provide habitats for other animals. When worms build tubes or burrows, these can be utilized by other creatures as homes or places to escape predators. This is why we often find larger and more diverse animal communities in areas with tube-building worms. Worms can also serve as an important food source for fish, birds, crabs, and other predators. In the grand scheme of things, by eating, burrowing, and being eaten, worms transform the detritus that sinks to the ocean floor into nutrients and energy that are passed up the food chain to provide nourishment to others and keep the circle of life going.

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase; Encyclopedia of Life; Smithsonian Oceans Portal ; Monterey Bay Aquarium ; MarineBio; Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) ; International Coral Reef Initiative ; Coral Reef Alliance ; Global Coral reef Alliance ; Global Coral Reef Monitoring Network

Marine Worm Diversity

According to Frontiers: Worms are found in every ocean habitat, from shallow mud to coral reefs, and even in the deepest parts of the ocean. They are common in warm, tropical places, the coldest polar places like Antarctica, and everywhere in between. [Source: Kara J. Gadeken, Erin Kiskaddon, Jenna M. Moore, Kelly M. Dorgan Frontiers, November 14, 2022]

Worms come in a huge array of shapes and sizes, from microscopic species to the fearsome-jawed “sand-striker” worm that eats fish and can grow longer than a human. Some worms live in very harsh environments, like the super-hot water of hydrothermal vents. Some worms are parasitic and live on or inside other animals, for example, on lobster gills or on feather stars, in sponges or corals, or even inside other worms. Some worms bore holes into solid rock; some are so tiny that they crawl between sand grains as if they were giant boulders; and some swim freely in open water. If you look carefully anywhere in the ocean, you will find a worm.

Many marine worms live in sediment — the mixture of inorganic (particles of rock and shell) and organic (living microbes and dead stuff from the overlying water) materials that accumulates on the seafloor., such as mud or sand. If you have ever gone to a muddy seashore, you might have noticed that the mud is black and smelly just under the surface. This is because the sticky, goopy mud prevents oxygen from getting very far under the surface. So, if you are a worm that lives in mud, getting oxygen to breathe is a big challenge. Some worms stick feathery external gills up into the water where there is more oxygen, and others pump water into their burrows to bring oxygen down to them Some worms have blood just like ours that can store oxygen, which gives them their red color.

Marine Worm Feeding, Predator Defenses and Reproduction

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According to Frontiers: Just like the earthworms in a garden that eat dirt, many worms on the ocean floor eat sediment. Some of the best worm food on the ocean floor comes from dead plankton, fish poop, and other goo produced by organisms in the water that sinks to the sediment surface creating detritus. Worms have special guts with a kind of soap that helps them digest the detritus that coats the sand grains. However, marine worms do not eat just detritus—some of them are carnivores or scavengers, some are herbivores, and some are parasites. Some worms have teeth, jaws, or even venomous fangs for capturing prey or grazing on plants. Some vegetarian worms scrape algae from rocks with special teeth. Compared to the earthworms in a garden, worms in the ocean have evolved a spectacular variety of body parts used to find, grab, and hold onto food. [Source: Kara J. Gadeken, Erin Kiskaddon, Jenna M. Moore, Kelly M. Dorgan Frontiers, November 14, 2022]

Many animals like to eat worms, so worms have evolved numerous ways to avoid predators. Worms that swim in the water or crawl around on the seafloor surface are more exposed, so they often have venomous or irritating spines or use stinky or poisonous chemicals to be less tasty. Some worms hide from predators by building burrows or tubes in sediment. To resist getting pulled or sucked out of their homes, worms use stiff hairs on their bodies to hold onto their tubes or burrows. It is common to find worms that are growing back their tails or even their heads after having those parts bitten off by a passing fish!

In terms of reproduction, Some worms just throw their eggs and sperm into the water, but others turn their tails into a zombie mini-worm that carries eggs or sperm, complete with a new head and eyes. Sometimes worms keep their babies inside or on their bodies or inside their burrows until they are almost grown up. Sometimes worms can clone themselves by splitting in half and regrowing a new head and tail.

Annelida: Segmented Worms

Most marine worms — and most worms period — as we said before belong to the Annelida (segmented worm) phylum of animals. Also known as ringed worms, the annelids include earthworms, polychaete worms, and leeches. Most worms have brains but they are not particularly complex. Each worm's brain sits next to its other organs, and connects the nerves from the worm's skin and muscles, controlling how it feels and moves.

According to Animal Diversity Web: All members of the group are to some extent segmented, in other words, made up of segments that are formed by subdivisions that partially transect the body cavity. Segmentation is also called metamerism. Segments each contain elements of such body systems as circulatory, nervous, and excretory tracts. Metamerism increases the efficiency of body movement by allowing the effect of muscle contraction to be extremely localized, and it makes possible the development of greater complexity in general body organization. /=\ [Source: Phil Myers, Animal Diversity Web (ADW) /=]

Besides being segmented, the body wall of annelids is characterized by being made up of both circular and longitudinal muscle fibers surrounded by a moist, acellular cuticle that is secreted by an epidermal epithelium (type of tissue that forms the covering on internal and external body surfaces). All annelids except leeches also have chitonous lhair-like structures, called setae, projecting from their cuticle (“skin”). Chitonous means made of chiton — a horny substance that forms part of the hard outer body covering especially of insects and crustaceans. Sometimes the setae are located on paddle-like appendages called parapodia. /=\

Annelids have large and well-developed true coelom (the fluid-filled body cavity of an animal that contains the internal organs). Except in leeches, the coelom is partially subdivided by septa. Hydrostatic pressure is maintained across segments and helps maintain body rigidity, allowing muscle contractions to bend the body without collapsing it. /=\

The internal organs of annelids are well developed. They include a closed, segmentally-arranged circulatory system. The digestive system is a complete tube with mouth and anus. Gases are exchanged through the skin, or sometimes through specialized gills or modified parapodia. Each segment typically contains a pair of nephridia (rear openings that serve excretion organs). The nervous system includes a pair of cephalic ganglia attached to double nerve cords that run the length of the animal along the ventral body wall, with ganglia and branches in each segment. Annelids have some combination of tactile organs, chemoreceptors, balance receptors, and photoreceptors; some forms have fairly well developed eyes, including lenses. /=\

Annelids may be monoecious (hermaphroditic, individuals having both male and female reproductive organs) or dioecious (separate male and female individuals). Larva may or may not be present; if present they are of the trochophore type. Some forms also reproduce asexually. They are protostomes, with spiral cleavage. /=\


Deep Sea Polychaete

Many marine worms fall into the Polychaetes class, which include sand worms, tube worms, and clam worms. According to Animal Diversity Web: Most have well developed, paired, paddle-like appendages (parapodia), well developed sense organs, and numerous setae (usually on the parapodia; "polychaete" means "many hairs"). Polychaetes usually have a well-developed head, often complete with well-developed eyes, antennae, and sensory palps. They lack any permanent sex organs (in contrast to other kinds of annelids); sex organs appear as swellings during the breeding season. Eggs and sperm are shed into the coelom and carried outside the body through the nephridia or as a result of the body wall actually rupturing. Fertilization is external, and development proceeds indirectly through a trochophore larva. [Source: Phil Myers, Animal Diversity Web (ADW) /=]

Polychaetes are marine annelids. They are a large and extremely diverse group. Around 10,000 species have been described. Most are marine. Some, such as featherduster worms, are sedentary (remain in the same area), , living in tubes buried in sand or mud, and feed by trapping food particles in mucus or by ciliary action. Others, such as the clam worm, are active, mobile predators that capture prey in jaws attached to their pharynges. Still others, such as fireworms, graze on gorgonians and stony corals. /=\

Polychaetes are extremely abundant in some areas. They play essential ecological roles, serving on one hand as predators on small invertebrates, and on the other as food for fish and large invertebrates. Dr. Mark Siddall, Curator of the Division of Invertebrate Zoology at the American Museum of Natural History, said polychaete worms, which have bristles made of chitin, are an “an enormous, terrifically diverse” group of worms.”


Flatworms (also called platyhelminth, members of the phylum Platyhelminthes) are regarded as the simplest and most basic creature found in the sea. Most but not all live the sea. Many are found in reefs, clinging under rocks and hidden in crevasses. Some of those found in coral reefs are quite colorful. Like jellyfish, flatworms have a single opening to their gut which is used to take in food and excrete waste but unlike jellyfish they have a solid body. Flatworms have no gills and breath directly through their skin. Their undersides are covered by cilia, which beat and allow them to move slowly over surfaces. They have a network of nerve fibers but nothing that would qualify as a brain and they don’t have a circulatory system.

More than 20,000 flatworm species have been described. A number of flatworm species are free-living, but about 80 percent of all flatworms are parasitic (living on or in another organism and securing nourishment from it). Some flatworms cause serious illnesses in humans. Tapeworms and flukes are parasitic flatworms. Despite their simplicity, flatworms have amazing powers. Some have been taught to negotiate their way through a maze. Not only that if they are killed and their flesh is feed to another flatworm they too can negotiate the maze.

Turbellarians are a kind of flatworm. They come in a number of different shapes. Although most are grey, black or translucent. Some found in coral reefs are brightly colored. Most are free-living rather than parasitic. The can vary in size from less than a centimeter to over 50 centimeters. Many large one are also very flat. They have primitive sense organs; move around by creeping or rippling their bodies; and feed on invertebrates.

Christmas Tree Worms

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Christmas tree worms
Christmas tree worm (scientific name: Spirobranchus giganteus) is the common name for a marine worm that lives on tropical coral reefs around the world. They come in a variety of bright colors and aren’t very big — averaging about 3.8 centimeters (1.5 inches) in length. Each worm has two brightly colored crowns that protrude from its tube-like body. These Christmas tree-like crowns are composed of radioles, or hair-like appendages radiating from the worm’s central spine. These appendages are used for respiration and to catch dinner, which typically consists of microscopic plants, or phytoplankton, floating in the water. [Source: NOAA]

Christmas tree worms live in tropical environments at depths between three to 50 meters (10 and 165 feet), Because of their distinctive shape, beauty, and color, these worms are easily spotted. They are some of the most widely recognized polychaetes, or marine burrowing, segmented worms out there. For the most part, Christmas tree worms are sedentary, meaning that once they find a place they like, they don’t move much. In fact, while the colorful crowns of these worms are visible, most of their bodies are anchored in burrows that they bore into live coral. “The worms are buried in the coral,” Siddall says “What you’re seeing is the worm’s filter feeding [and breathing] apparatus.” When startled, Christmas tree worms rapidly retract into their burrows, hiding from would-be predators.

Erin McCarthy wrote in Mental Floss: The Christmas tree " is made of two spiraled plumes, complete with feather-like appendages called radioles, that extend upwards. In the center is a cover, or operculum; when the worm is startled, it pulls its plumes inside its tube and plugs the entrance with the operculum for protection. The worms feed on phytoplankton in the water, using the hair-like cilia on the radioles to capture the animals and work them down to the worm’s mouth. One study suggests they may live for at least a decade, and potentially as long as 40 years. [Source: Erin McCarthy, Mental Floss, April 9, 2015]

Ribbon Worms (Nemerteans)

Ribbon worms are also known as proboscis worms. They occupy their own Phylum — Nemertea, which contains approximately 1,300 species of bilaterally symmetrical, coelomate, unsegmented worms, which are divided into two classes, each of which is divided into two orders. 1) Anopla: A) Palaeonemertea, B) Heteronemertea and 2) Enopla: A) Hoplonemertea, B)Bdellonemertea. [Source: Jeremy Wright, Animal Diversity Web (ADW) /=]

Distinctions between the classes and orders lie in the presence of body armoring (Anopla has no armor and Enopla is typically armed with stylets), proboscis construction, mouth position relative to the cerebral ganglion, and body morphology (Enopla is morphologically specialized into three regions (with the exception of Bdellonemerta), while Anopla is not), as well as gut shape, body wall muscle layering, and positioning of longitudinal nerve cords.

Ribbon worms have widely varied distributions and habitats; most species are free-swimming, benthic (bottom-dwelling), marine organisms, but approximately 100 species are known from deep-sea environments, and planktonic, symbiotic, freshwater and even terrestrial species exist. They typically prey on small invertebrates and their eggs, using a uniquely structured proboscis, but some species are known to feed on plant material and, in some commensal species, phytoplankton captured by their hosts.

Ribbon Worm Characteristics

Drawing of ribbon worms:
1) Carinella bayulensis (=Tubulanus banyulensis)
2) Carinella aragoi (=Tubulanus annulatus)
3–4) Poliopsis lacazei
5) Cerebratulus geniculatus (=Notospermus geniculatus)
6–8) Amphiporus marmoratus (=Amphiporus arcticus; Note: Amphiporus marmoratus is a valid species, but Joubin's description has been synonymised with Amphiporus arcticus)
9) Drepanophorus rubrostriatus (=Brinkmannia mediterranea; Note: Drepanophorus rubrostriatus is a valid species, but Joubin's description has been synonymised with Brinkmannia mediterranea)
10) Nemertes duoni (=Emplectonema duoni)
11–12) Tetrastemma rustica (=Oerstedia rustica)
13) Tetrastemma dorsalis (=Oerstedia dorsalis).

Nemerteans may be very brightly colored or drab. They range in length from a few millimeters (a fraction of an inch) to 30 meters (100 feet) (stretched) in length. Most commonly they are 20 centimeters (8 inches) or less. They are unsegmented, without a differentiated head, and are most often flattened (occasionally convex dorsally or cylindrical). They have very thickly muscled body walls with some sensory and mucous gland cells. [Source: Jeremy Wright, Animal Diversity Web (ADW) /=]

Ribbon worms have a simple circulatory system consisting of vessels and thin walled cavities. . Although this system can vary in complexity depending on species, the most basic arrangement is a pair of longitudinal vessels that extend the entire length of the body and connect to mouth-like cephalic and anal cavities. The blood is a colorless fluid in which cells such as corpuscles (some of which may contain hemoglobin), lymphocytes and leukocytes can be found; the blood does not circulate in any particular direction. This system is mainly involved with transporting nutrients, gases, secretions and wastes through the animal’s body and may aid in regulating hydrostatic pressure. Oxygen and carbon dioxide diffuse across the animal’s body surface.

Nemerteans have a central nervous system consisting of a complex cerebral ganglion with four connected lobes, giving rise to a pair of longitudinal, gangliated nerve cords from the ventral lobes. The nerve cords connect to each other at points throughout the worm’s body and give rise to peripheral sensory and motor nerves. The proboscis apparatus of these animals is unique to the phylum. The proboscis is an elongate, eversible, blind tube, surrounded by the rhynchocoel (a fluid filled, hydrostatic cavity) and additional muscle layers, including the proboscis retractor muscle at the posterior wall of the rhynchocoel. It is either connected directly to the foregut or opens through a proboscis pore. In many species, the proboscis bears stylets, structures typically 50 to 200 micrometers in length, each shaped like a nail. Nemerteans may use their proboscises for locomotion, though its primary function is in prey capture. Stylets are often lost during prey capture and must be replaced as the animal grows; they are continuously formed by specialized cells (stelotocytes) within reverse stylet sacs, where they are stored. Sticky, sometimes even toxic substances are produced by the proboscis in order to trap and immobilize prey./=\

Ribbon Worms Behavior and Feeding

Ribbon worms are highly sensitive to touch, due to ciliated epidermal cells scattered over their surfaces. They also have anywhere from two to hundreds of anterior eyes, usually pigment-cup occeli, although a few species have lensed eyes that are highly sensitive to light intensity and direction. [Source: Jeremy Wright, Animal Diversity Web (ADW) /=]

These animals are also highly chemosensitive and it is likely that structures including cephalic slits, cerebral organs, and frontal glands act as chemoreceptors. Some smaller nemerteans move using their epithelial cilia while many species, particularly terrestrial ones, use mucus to create a smooth, gliding surface; some larger animals swim or drift through the water. Outside of breeding, when they can be found in breeding "knots," these worms tend to be solitary. /=\

Nemerteans are hunters and scavengers. Hunting methods vary between species and depend on prey type. Sticky, sometimes toxic substances are produced by the proboscis in order to trap and immobilize prey. In some cases, the proboscis is everted when the worm comes into contact with prey; it then coils around the prey, which is swallowed whole. In other instances, prey may be stabbed by a stylet, with toxins delivered through the resulting wound. The prey is then either swallowed, using peristaltic action of the body wall as well as ciliary currents within the gut, or injected with digestive enzymes and consumed suctorially. Typical prey includes small invertebrates, including bivalves, polychaetes, crustaceans, insects, and their eggs; some species feed primarily on plant material and, in commensal species, on phytoplankton captured from hosts. /=\

Once food has been swallowed, it leads to a highly ciliated foregut, comprised of a buccal cavity, often a short esophagus, and a stomach, which occasionally possesses enzymatic gland cells. The foregut also has a number of mucus producing cells or glands. The stomach leads to a long, straight midgut with many lateral diverticulae; these are lined with vacuolated, ciliated, phagocytic cells, which also bear microvilli. This greatly increases the surface area available for nutrient absorption.

Ribbon Worms Reproduction

The sex organs of sexually reproductive nemerterans are specialized patches of tissue with specialized cells arranged along each side of the intestine. Prior to breeding, the sex organs enlarge and become hollow, while specialized cells in the testes and ovaries generate sperm and eggs into the newly created lumina, filling the area between the gut and body wall. [Source: Jeremy Wright, Animal Diversity Web (ADW) /=]

The worms become increasingly active once they are almost ready to mate and initiate mating behavior. Spawning is triggered by tactile and chemical cues. Worms join together in a mating mass, releasing eggs and sperm through temporary pores or body wall ruptures, and fertilization is typically external, sometimes in a mass of mucus. Some species exhibit internal fertilization: released sperm move through mucus surrounding the mating worms, entering females’ ovaries.

Most nemerteans are dioecious (with separate male and female individuals), although both sequential and protandric hermaphrodites are known, as is asexual reproduction via transverse fission. Fertilization may be external or internal. There is evidence that sexual maturity is reached after the release of neurosecretory hormones, possibly released by the cerebral organ complex. /=\

Iridescent Fireworms

Iridescent fireworms (Scientific name: Eurythoe complanata) range in length from 5.3 to 35 centimeters (2 to 14 inches), with their average length being around 10 centimeters (4 inches). They come in a variety of colors — orange, salmon pink, blue, green, or even black bodies with a dark midventral line. The common name "fire worm" is derived from the bright red gill tufts along the sides of the body. There are the numerous white bristly appendages that are highly toxic. Their stings can be very painful to humans, hence their name — fireworms.[Source: Molly Mascow, Animal Diversity Web (ADW)]

Iridescent fireworms are polychaetes. They were once considered a circumtropical species occurring in the Atlantic, Pacific and Indian Oceans and the Mediterranean and Red Seas but recent morphological, allozyme and DNA analyses suggest that the iridescent fireworms are limited to the Atlantic Ocean from the Caribbean Sea to southern Brazil. Worms in others places are similar but not the same. A number of species are called fireworms.

Iridescent fireworms have a flattened, rectangular body. They are about twice as wide as they are tall. Their prostomiums, or head segments, are oval-shaped and have four eyes and short sensory tentacles. Three three or four buccal segments make up their “mouth.” They have no jaws. Instead they have papillae with structurally simple parapodia that come in two types: a straight serrate type and a more bristly branched type. The relatively simple structure and lack of identifying features of these polychaetes makes them difficult to distinguish from other closely related species. /=\

The preferred food source for Iridescent fireworms is carrion, which is found in almost any ecosystem, allowing the worms to occupy a variety of habitats. Iridescent fireworms will eat almost any organic matter as long it can fitted into their mouth. In laboratory observations, iridescent fireworms, actively feed when food is present but hide in groups under rocks when no food is available and stay there until food is presented again. Individuals competing for the same piece of food were observed pushing and shoving each other with their bodies. In the wild, iridescent fireworms remains hidden during the day under rocks, into corals, in any type of shelter, emerging at dusk to forage throughout the night and into the early morning. The parapodia of Iridescent fireworms are well developed and the creatures can move rapidly over the ocean bottom they search for food.

Like many earthworms, iridescent fireworms are capable of reproducing both asexually through body fragmentation, and sexually through mating. Mostly they are broadcast spawnesr, releasing eggs and sperm into the water column. In captivity, they spawn one week after the full moon in September and have been observed spontaneously fragmenting and regenerating — a form of asexual reproduction. Due to its widespread distribution and regenerative capabilities, iridescent fireworms are frequently studied by scientists, interested in polychaete regeneration and nervous systems.

Iridescent Fireworm Stings and Toxins

Iridescent fireworms are covered with poisonous bristles containing a neurotoxin that can cause pain when touched or handled. The "sting" of the fireworm has been described as a burning sensation followed by itching and inflammation. [Source: Molly Mascow, Animal Diversity Web (ADW)]

When iridescent fireworms feel threatened the curls themselves up into a ball like a hedgehog and expose their poisonous bristles. The worm’s bright red gill tufts are a form of aposematism (warning coloration that informs potential predators that an animal is poisonous, venomous, or otherwise dangerous). In addition to aposematism, the worms’ bright colored provide camouflaged and help them blend in with the corals in reefs and sediments it inhabits.

Most marine predators avoid the painful poison of the worm. However, the imperial cone (Conus imperialis), a kind of cone snail, has been observed feeding on iridescent fireworms. These snails may incorporate the fireworms venom into their own defence system and appear to serotonin in their attacks on the worms. According to Animal Diversity Web: In leeches, serotonin is shown to increase the permeability of chloride ions in muscle tissue, causing relaxation of the muscle. It is reasonable to conclude that it would have similar effects in another annelid. Since the defense mechanism of the worm depends partly on being able to curl into a ball to expose poisonous barbs, a forced relaxation of body muscles would decrease the efficacy of the defense. This lowered defense makes Iridescent fireworms easier prey for the imperial cones.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023

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