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Marine Annelids Facts and Characteristics

[Kingdom: Animalia] [Clade: Bilateria] [Superphylum: Lophotrochozoa] [Phylum: Annelida]

The broad phylum Annelida contains around 22,000 different species. The group includes several thousand marine annelids (segmented worms) that live in various aquatic environments.

This segment describes the main characteristics of annelida, including what they eat and how they reproduce, with a list of the most common annelid species (soft-bodied marine worms).

Five Key Characteristics of Phylum Annelida

Besides their segmentation (known as metamerism in zoology), almost all polychaete worms share several traits, such as:

An elongated body that divides into a number of small segments.
Being true coelomate animals means they have an internal cavity and they display bilateral symmetry.
Excretion of waste occurs through metanephridia and protonephridia.
They are hermaphroditic organisms using external fertilisation.
Their nervous system is developed and very efficient, but they have a "closed" circulatory system (same as humans).

Despite several revisions, most research refers to Annelida subdivisions as being Polychaetes (marine organisms), oligochaetes (e.g. earthworms), and certain species of leeches.

Polychaetes are bristle worms that live in various types of oceanic environments. There are about twelve thousand (12,000) Polychaete species still in existence.

Here's the thing:

By and large, basic annelid morphology is a combination of a head, multiple segments (that contain their internal organs), an anus located at the end of the digestive tract (gut), and a single nuchal organ (used in mating and for food detection).

Most polychaetes (poly meaning many and chaetae meaning hair) also have a pair of unjointed lateral outgrowths (parapodia) on each segment - limbs used for locomotion.

What Do Annelids Eat?

Being nocturnal creatures and a heterotrophic species means annelids need to find food for nourishment. Even so, many of the Polychaete species vary in their actual feeding habits.

Some are natural predators that eat flatworms, small crustaceans, and mollusks. Whereas, others are scavengers that feed on dead or decaying organisms on the ocean floor. Others are filter-feeders that eat plankton and most of the different species of zooplankton.

Annelid Worms Species List

Bearded Fireworm (Hermodice carunculata)

The bearded fireworm (Hermodice carunculata) is a fascinating creature that looks like a pinky-brown furry caterpillar (pictured below).

This segment describes the major characteristics of the bristle worm species, including what they eat, and how this hairy sea worm invertebrate reproduces.

Bloodworms (Glycera)

Unlike earthworms (which live in soil), seaworms are aquatic bristle worms that live in muddy sediments at coastal seafloors.

In fact, marine bloodworms (Glycera annelids) are an important food source for most bottom-feeding fish families and some saltwater crustaceans.

Bobbit Worm (Eunice aphroditois)

It has several nicknames, including sand striker and trap-jaw worm, but the most common name derives from the infamous John and Lorena Bobbitt case in 1993.

This guide contains some chilling facts about bobbit worms, including how these lethal ambush predators kill their prey, where they live, and how these benthic bristle worms reproduce.

Christmas Tree Worm

Christmas tree worms are sedentary tube-building polychaetes of the phylum annelida, and live on large stony brain corals and Porites compressa (finger coral).

This segment contains facts and information about Spirobranchus giganteus, including where they thrive best, what they eat, and how these invertebrate tube-dwellers breed and reproduce.

Cirratulus Cirratus

Clam Worm (Alitta succinea)

Alitta succinea has several common names, including the clam worm, cinder worm, and pile worm. These segmented worms are most commonly found in shallow estuarine and marine environments.

Like almost all of the annelid species, the body of clam worms is segmented and elongated with numerous fleshy appendages (parapodia and setae). These structures contain bristles that assist with movement and respiration.

Moving on...

Besides being an important predator, they also become prey for crustaceans and bottom-feeding fish. But, one of the lesser-known annelida characteristics is that the wriggling clam worm species is widely used as fishing bait.

Clam worms rarely grow more than fifteen (15) centimetres long (6 inches). Their body is a reddish-brown colour with a lighter brown section near the head. They have four eyes, two main sensory organs (feelers), and four pairs of elongated fleshy tentacles.

Pro Tip: This particular marine annelid has become a popular research subject studied in marine biology to get accurate information about annelid anatomy and their reproductive strategies.

Dodecaceria

Eurythoe Complanata

Feather Duster Worms

In general, feather duster worms (Sabellidae) are comparatively large segmented sedentary marine tube worms.

Its common name of "feather duster" refers to its giant size fan-like coloured crown tentacles situated on each side of its head.

Flatworms

The phylum Platyhelminthes contains a group of soft-bodied invertebrate flatworms that are closely related to annelid worms and some snails.

Giant Tubeworm (Riftia pachyptila)

Golden Fireworm (Chloeia flava)

Hypania Invalida

Palolo Worm (Palola viridis)

Parchment Tube Worm

Polydora Ciliata

Pompeii Worm (Alvinella pompejana)

Only a few of these annelids examples can tolerate the extreme habitats like those found on the walls of hydrothermal vents in the deep Pacific Ocean.

Yet, the pompeii worm lives in a marine environment where the water temperatures range between near freezing (2°) and 80° Celsius. As a matter of fact, they are one of the most heat-tolerant members in the entire animal kingdom.

Key features...

Alvinella pompejana has a fleece-like coating of bacteria on its back. The presence of a symbiotic bacteria may insulate the worm from extreme heat and toxins.

In return, pompeii worms secrete a sugary mucus to provide nutrients for the chemolithotrophic bacteria.

Like almost all of the segmented worms phylum, they have a "jointed" body that can grow up to thirteen (13) centimetres long (5 inches).

Despite a general paleness in skin colour, adult pompeii worms use a bright red plume of gills located on the head for gas exchange.

Red-tipped Fireworm (Chloeia viridis)

Streblospio Gynobranchiata

How Do Annelids Reproduce?

For the most part, Polychaetes reproduce via asexual reproduction. They divide into two or more pieces to produce a completely new individual. Even so, the parent will remain as a complete organism.

A similar regeneration mechanism also applies if the organism suffers severe damage. For example, some species are able to regenerate from a single segment - even if their head is cut off.

Pro Tip: Marine annelids and polychaetes are pivotal contributors to the food chains in the oceans because these burrowing worms recycle detritus organic material and help to aerate sediment and substrate.