The sea, whose depths hold great enigmas, is home to an extraordinary diversity of animals, with characteristics as fascinating as those displayed by “marine worms,” the most varied group within the Phylum of Annelids (little rings).
Diversity in sizes and shapes, striking colors, ranging from warning signals to iridescent (rainbow colors), and species capable of paralyzing their prey with highly toxic venom, are just some of their qualities.
So far, more than 15,000 species have been discovered, grouped into 70 families with 1,600 genera distributed all over the world and adapted to mostly marine environments, including sand dwellers, the fire worm, feather dusters, and the Bobbit worm.
Want to learn more about them? Here you’ll find everything about marine worms!
What are marine worms?
They are animals belonging to the Phylum “Annelids” and the class “Polychaetes (Polychaeta)”, meaning their body is segmented, divided into segments, and possesses many chaetae (bristles) or hair-like structures.
They are also known as “marine polychaetes” and in their form, are related to earthworms and leeches.
Types of marine worms
Based on their lifestyle, marine worms are traditionally classified into two types or subclasses: errant and sedentary.
Errant Marine Worms
They have a free life, with great mobility thanks to very fine formations (similar to hairs) that project outside the body (chaetae) through structures called parapodia, allowing them to swim, crawl, anchor…
They are active predators and live under rocks, in coral reefs, or in excavations they make in the sand or mud.
Among the wandering specimens are: fireworm (Hermodice carunculata), Bobbit worm (Eunice aphroditois), sea mouse (Aphrodita aculeata), Alitta virens, Ophiodromus flexuosus…
Did you know…? When the fireworm (Hermodice carunculata) feels threatened, it bristles its chaetae (filaments) which contain a neurotoxic venom, and upon contact with the skin, they penetrate it, breaking and causing pain, redness, and swelling.
Sedentary marine worms
They are characterized because they do not move, but live in tubes or buried in burrows in the sand, and their parapodia (body structures that carry the chaetae or hairs) tend to be small.
They are sedimentivores, meaning they feed by filtering the particles present in the water, moving it thanks to specialized structures like crowns with radioles (expansions on the head) or long and mobile palps (tentacles).
They are divided into: tubicolous and burrowing worms.
Tubicolous marine worms
They live inside a solid tube that they themselves construct made of mucus (chitin), from the mixture of mucus with materials they find around them (fine sediment, shell fragments, and algae…) or from calcium carbonate.
The tubes can be located on the surface of rocks, be buried, or placed among the sediments, and serve as a means of protection both from predators and from pollutants.
Certain species (serpulids, spirorbids) may occupy other structures (shells of other organisms).
Tubicolous marine worms have very limited mobility, and project outside of their tube only the feeding and/or respiratory structures.
Some tubicolous specimens are: feather duster worm (Sabellastarte spectabilis), Christmas tree worm (Spirobranchus giganteus), spiral tube worm (Sabella spallanzanii), sand mason worm (Lanice conchilega), fan worm (Serpula vermicularis)…
Burrowing marine worms
They live permanently in galleries that they dig in sandy bottoms or in the mud, commonly in the shape of “J”, open at one end, and “U”, open at both ends, and lined with the worm’s own secretions (chitin).
Among the burrowing specimens, are: lugworm (Arenicola marina), Abarenicola affinis, parchment worm (Chaetopterus), Mesochaetopterus rogeri, Orbinia latreillii…
There are other types of marine worms called marine planarians, but they do not belong to the phylum Annelids; they are mostly parasitic, and are part of the Platyhelminthes, differentiated because their body is not segmented (divided).
Did you know…? In 2020, the “Monaco” project was developed in France, which consists of a clinical trial to administer to COVID-19 patients a solution based on the blood of the “sandworm” for its highly oxygenating properties.
Characteristics of marine worms
External morphology
- Soft body, of variable shape (cylindrical, elongated, oval, ribbon…) divided into repeating segments (metameres).
- Generally, the head (prostomium), trunk (metastomium), and posterior region (pygidium) are distinguished.
If it is wandering (mobile marine worm), the head displays sensory organs such as a pair of short antennae, four small eyes, and a pair of palps (tentacles) associated with the mouth, in addition to a pharynx with jaws and hard teeth (proboscis).
If it is sedentary (immobile marine worm), the head is reduced and they do not have antennas or eyes.
In some species of marine worms, the top part of the head extends backward, forming a shield or neck plate (nuchal). Likewise, they may have venom glands coupled to their jaws.
The trunk contains a large number of segments, and on each side, small appendages or clusters that come out, called “parapodia”, formed by many threads or silks (chaetae), useful for moving and breathing, whose number, shape, and size is variable.
In certain species, the trunk may be divided into thorax and abdomen (sedentary or immobile marine worms).
In the posterior region (or pygidium), the anus and other accessory structures are located.
- Diverse coloration: transparent or white, blue, green, red, yellow, orange, purple, or combined with bright tones. Some show bioluminescence (they create a chemical reaction that produces light).
- Average size: 5 to 10 centimeters; some are tiny, and others, reach several meters in length.
Did you know…? The giant marine worm “Eunice aphroditois or Bobbit worm” inhabits oceans around the planet and can reach up to 3 meters in length! It is equipped with jaws and toxins that make it a dangerous and fearsome predator.
Internal Morphology
Tegumentary System (Skin)
The body of the marine worm is covered by a very thin, transparent cuticle, and an epidermis (outer skin layer), which has mucus-producing glands.
Nervous System
Comprised of a central ganglion (brain) and a pair of nervous cords running through the entire body, receiving information from the sensory organs (antennas, eyes, tentacles…) present. It is more developed in wandering (mobile) marine worms.
Some marine worms have equilibrium organs (statocysts).
Muscular System
The body of the marine worm has muscular layers (longitudinal and circular) that facilitate mobility.
Circulatory System
Closed circulatory system; consists of two blood vessels connected to each other: a dorsal vessel (equivalent to a heart), responsible for pumping the internal fluid throughout the body, and a return blood vessel, running through all the segments or rings.
The blood (internal fluid) is usually colorless, although it may contain respiratory pigments (hemoglobin).
Respiratory System
They breathe through gills, which are respiratory structures formed by expansions distributed throughout the body that act as filters, or through cutaneous respiration, through the body wall.
Certain species store oxygen or can switch to anaerobic respiration (does not require oxygen) for hours or days.
Digestive System
Composed of a tube that runs the length of the body, extending through: mouth, pharynx, esophagus, stomach, intestine, culminating in the anus.
Some species (wandering or mobile) possess specialized structures, like proboscises or pharynges equipped with strong jaws; others (Osedax) lack both mouth and stomach, instead having a kind of root for absorbing nutrients.
Excretory System
Marine worms eliminate waste through “nephridia”, small pores distributed throughout the body, with the help of a tissue called chloragogen located in the digestive tube.
Reproductive System
The reproductive glands (gonads) are simple; nearly all species have separate sexes, and some are hermaphrodites (both sex cells).
Their reproductive organs are not permanent; in more evolved species, the gametes (sex cells) are usually found in the abdominal area as small swollen bumps.
Did you know…? The green bomber worm “Swima bombiviridis” gets its name from launching small luminous bombs with an intense green color to defend against predators.
How do marine worms reproduce?
Most marine worms reproduce sexually; the gametes (oocytes and sperm) are released into the water column, and external fertilization occurs.
From the fertilized egg, a larva (trochophore) develops, which will transform into an adult, adopting an elongated body divided into segments or small compartments.
In many cases, the eggs are protected by females, remaining attached to their body, within the tube where they live, or in gelatinous masses. Certain species (Cirratulidae) retain the egg and embryo within their body (viviparity).
They can also reproduce asexually; in this case, thanks to their regenerative ability, from a single body fragment, the development of a new organism is possible (budding or fragmentation).
Certain species (Syllidae, Eunicidae) undergo an incredible process called “epitoky”, through which the body or a part of it becomes a reproductive structure (epitoke), filled with eggs or sperm, leaving its habitat to reach the surface and reproduce.
Did you know…? The species “Antarctonemertes riesgoae” has a unique reproductive strategy: the female creates a wrapper and incubates her eggs like chickens! Not only that, but it exhibits defensive behavior when the cocoons are disturbed.
Where do marine worms live?
They are distributed worldwide in all marine environments (very few in freshwater), from lagoonal and intertidal and beach areas to the deepest, including areas near underwater volcanic sources.
They can also be found in unusual environments such as hydrocarbon seepages, bottoms of polar seas covered by ice, whale bones that fall to the bottom, floating remains of inorganic matter…
Some species are capable of living in moist terrestrial environments.
They live freely (wandering) in excavations on the sediment and in tubes they construct themselves (sedentary).
Did you know…? The species “Osedax” is known for being a marine worm that devours the carcasses of gray whales, burrowing into their bones to reach the fat they feed on.
What do marine worms eat?
Marine worms have different feeding habits, depending on their way of life: some are predators, scavengers (consume dead animals) or omnivores (feed on both animals and plants).
Others are sedimentivores, meaning they filter the sand or mud they consume directly with their mouth while digging the substrate, or stir the water thanks to tentacles and radioles (structures on the head) to collect suspended food.
Did you know…? The marine worm “Leocratides kimuraorum” emits the loudest underwater sounds known (157 decibels) through a simple muscular contraction, and is so powerful it can break small glass jars.
Ecological importance
Due to their abundance, way of life, and feeding, marine worms play a fundamental role in the recycling of marine organic matter, and help to renew the water present in the pores of the rocks, oxygenating the sediment by stirring it.
Certain species (capitellids, terebellids) are bioindicators of water quality; they also represent an important food source in the diet of other organisms (fish and marine invertebrates), and are used as bait for fishing.