Annelids (phylum Annelida) are bilaterally symmetrical worms which may or may not be segmented. The name “Annelida” comes from the Latin word "annellus", which means "little rings", as traditionally this phylum only includes segmented worms which mostly have a body made up of a series of identical segments, separate by ring-like constrictions (annuli). Recent phylogenetic studies, however, have shown that many unsegmented worms are genetically very similar to the annelids, and are in fact more closely related to annedids like earthworms than many marine annelids. These unsegmented worms are hence now placed under this phylum as well. As a result, there is no single external feature that can be used to differentiate annelids from other phyla.
Generally, the segmented annelids will have a body comprising identical segments (excluding the head and tail) containing the same set of organs, and in some cases, external structures used for locomotion. They should not be confused with arthropods like millipedes and centipedes, which will have segment legs that annelids lack. The above picture features an unidentified annelid worm.
The unsegmented annelids are believed to have lost the segments through evolution to better adapt to survive in their habitats.
Many annelids, except leeches, are known to be able to regenerate lost body parts, even their heads. A number of species can also reproduce asexually by splitting into two or several parts and regrow the lost parts. Some annelids can even regrow from severe damages, such as from a single segment! Most also reproduce sexually, and are mostly hermaphrodites with both male and female reproductive organs.
A) Peanut Worms (Order Sipuncula)
Peanut worms are previously regarded as a phylum of their own as well, but again, recent studies showed that they are also annelids. Their body comprises an unsegmented trunk and a retractable structure called an introvert. When disturbed, they can retract their body into a shape resembling a peanut kernel. They are usually deposit feeders. When made into a jelly, peanut worms are considered a delicacy in some parts of China. They are usually found on soft substrates, such as sandy or muddy bottoms. Depending on the species, peanut worms seen locally grow to about 5–20 cm long. They are hard to identify based on external features.
The above picture features an unidentified peanut worm found on a sandy shore.
Generally, the segmented annelids will have a body comprising identical segments (excluding the head and tail) containing the same set of organs, and in some cases, external structures used for locomotion. They should not be confused with arthropods like millipedes and centipedes, which will have segment legs that annelids lack. The above picture features an unidentified annelid worm.
The unsegmented annelids are believed to have lost the segments through evolution to better adapt to survive in their habitats.
Many annelids, except leeches, are known to be able to regenerate lost body parts, even their heads. A number of species can also reproduce asexually by splitting into two or several parts and regrow the lost parts. Some annelids can even regrow from severe damages, such as from a single segment! Most also reproduce sexually, and are mostly hermaphrodites with both male and female reproductive organs.
A) Peanut Worms (Order Sipuncula)
Peanut worms are previously regarded as a phylum of their own as well, but again, recent studies showed that they are also annelids. Their body comprises an unsegmented trunk and a retractable structure called an introvert. When disturbed, they can retract their body into a shape resembling a peanut kernel. They are usually deposit feeders. When made into a jelly, peanut worms are considered a delicacy in some parts of China. They are usually found on soft substrates, such as sandy or muddy bottoms. Depending on the species, peanut worms seen locally grow to about 5–20 cm long. They are hard to identify based on external features.
The above picture features an unidentified peanut worm found on a sandy shore.
B) Myzostomids (Order Myzostomida)
Myzostomids are small parasitic annelids that mostly live on feather stars and sometimes on other echinoderms. They come in a variety of colours and shapes, which allow them to camouflage with their hosts. Most of them feed on food particles collected by their host, although a few are known to feed on their hosts' tissues. Most myzostomids have a smooth upper side and five pairs of parapodia with hooks on their underside, which allow them to cling to their hosts. Many species lack obvious segments.
These unidentified myzostomids on another feather star has horn-like structures and appear flattened, much like a flatworm.
C) Earthworms (Order Haplotaxida)
Earthworms are possible the group of annelids that most Singaporeans are familiar with. They are burrowing annelids that lack eyes but have specialised cells in their skin to convert light into signals. They mostly breathe through their moist skin. Earthworms play important ecological roles in many terrestrial ecosystems. They aerate the soil as they burrow and bring nutrients from underground to the surface. They also break down organic matter into humus, improving soil fertility, and are prey for many animals, making them an important part of the food web. Their ability to regenerate lost body parts varies among species. Some earthworms can grow into two new worms after being bisected, while for others only one half will survive.
The above picture features an unidentified earthworm. Earthworms are generally hard to identified in the field, as specimens usually need to be examined under the microscope to check for the arrangement of the tiny hair-like structures to determine the species.
D) Leeches (Order Arhynchobdellida)
Three freshwater leeches with very similar appearances—Hirudinaria bpling, Hirudinaria manillensis and Limnatis dissimulata—have been recorded from the region, and it is hard to differentiate them from photographs. Leeches feed on blood and detect their hosts by their odour, body heat or vibrations from their movements. They attach themselves to their hosts using the sharp jaws in their mouths and the strong suckers at their rear ends. They then secrete an enzyme to prevent the blood from clotting and drop off when they are full. The species featured here is yellowish-brown, with black spots on its sides.
The above photo features a Buffalo Leech (Hirudinaria sp.), which is sometimes encountered in freshwater habitats such as streams and ponds. Some marine leech species can also be found in Singapore, but the terrestrial species may have disappeared already due to the low mammal population in our forest.
E) Polychaetes (Class Polychaeta)
Polychaetes are segmented worms found in marine environments and usually have many bristles ("poly" meaning many, and "chaeta" meaning bristle). They are also commonly called bristleworms due to the bristles projecting from a pair of paddle-like structures called parapodia, which are attached to the sides of each body segment. Compared with other annelids, polychaetes have a well-developed head with two to four pairs of eyes, although some species are blind.
i) Spoon Worms (Order Echiuroidea)
Spoon worms were previously regarded as a phylum of their own, but recent phylogenetic studies have shown that they are annelids. They are usually filter feeders, feeding by raising their proboscis out of their burrows to collect plankton and tiny organic matter. This species, the innkeeper spoon worm, has a reddish, sausage-shaped body with longitudinal grooves that form white stripes along its length. Its proboscis can be fully white, or light green above and yellow below. It is used for burrowing and for feeding on detritus in marine sediments. This species inhabits sandy shores and creates U-shaped burrows, which often host other small animals such as crabs and other annelids.
The above picture features an Innkeeper Spoon Worm (Ochetostoma erythrogrammon), which was found on a sandy-muddy shore.
ii) Collar Worms (Order Eunicida)
Many members of the family Eunicidae are called collar worms, as they have a segment of a different colour around the neck. Many species are omnivorous, but some are aggressive predators with sharp teeth that can cut small fish or other small prey in two. The Giant Reef Worm (Eunice sp.) featured above is usually seen on coral reefs, snapping at algae. Its body is brown with an iridescent sheen, and the collar is whitish with brown spots. The antennae are smooth.
Previously grouped under the genus Eunice, recent studies suggest that Leodice species should be considered a group of their own genetically. They can be recognised by their ringed or beaded antennae. The Red Reef Worm (Leodice sp.) featured here has blue eyes and ringed antennae with candy cane markings. Its body is red, with a white collar. It inhabits coral reefs.
iii) Fire Worms (Order Leodice)
The Iridescent Fireworm (Eurythoe complanata) is commonly found under rocks. Many of these worms are able to reproduce asexually by breaking itself apart, and each fragment can grow into a viable individual worm. Members of the family Amphinomidae are often called fireworms, as they are armed with sharp, venom-filled bristles that break off upon contact, giving painful stings to the victim. This orange worm is often found hiding under rocks during the day and emerges after dark. It is an omnivorous scavenger that also preys on corals, sponges and small invertebrates.
The Small Fireworm (Chloeia incerta) can swim by flapping its fluffy parapodia. In the middle of each segment is a T-shaped to triangular marking that differentiates it from another local species, Chloeia flava, which has circular markings.
The onuphids (family Onuphidae) often incorporate materials from the surroundings, such as leaves and shells, to their tubes. Most species live in fixed tubes, but some are known to move around carrying their tubes, or even leave their tube to build new ones. They are commonly found on sandy or muddy shores in shallow waters. They are believed to be opportunistic omnivores, feeding on anything readily available, such as algae, decaying organic matter, small animals and even dead animals. They are characterised by tiny little bottle-brush-like structures on their sides, which appears like tiny fir trees when magnified. The above is likely to be the tube of a Diopatra sp.
iv) Tube Worms (Order Chaetopterida)
Members of this order build permanent, fragile tubes. The body has two to three distinct regions, each with highly modified appendages.
The chaetopeterids (family Chaetopteridae) built permanent fragile tubes and can be found in shallow waters on sandy or muddy shores, sometimes attached to hard surfaces. They usually occur in clusters, sometimes carpeting over wide areas. Many of these worms feed by flapping fan-like structures they possess to generate a water current, so as to trap plankton and other organic matter with the mucus nets they have constructed within their tube.
v) Phyllodocid Worms (Order Phyllodocida)
Members of this order possess a pair of paddle-like parapodia on each body segment. The head usually has one or two pairs of eyes and two antennae.
This large Mangrove Ragworm (family Nereididae) is usually found in mangrove forests and mudflats. It is not commonly seen, as it is usually hidden in the muddy substrate. However, during the breeding season, this species can be seen in large numbers. The local species is not well studied, but in related species from other countries, the females spawn in their burrows, and only the males emerge. They develop paddle-like structures for swimming and release their sperm while swarming in the water above the females' burrows.
Scale worms (families Polynoidae and Iphionidae) are polychaete worms with plate-like scales on their back, believed to help them camouflage with the rocky habitat they live in. They mostly feed on small or sessile invertebrates. On a rocky shore, they normally forage during high tide, and hide in cracks or under rocks during low tide.
Blood worms (family Glyceridae) possess a long eversible proboscis, and are usually red in colour, hence the common name. They are usually found burrowing in mudflats or hiding under rocks and crevices in shallow waters, and may appear like earthworms due to the less conspicuous bristles when they are in the mud. They mostly feed on small invertebrates, though some also feed on detritus To hunt, the carnivorous species will track the prey in their burrow systems by sensing the changes in water pressure caused by its movements and shoot out their proboscis to grab it. The photo shows a Glycera rouxii.
vi) Spaghetti Worms (Order Terebellida)
Members of this order possess long, filamentous tentacles to collect and feed on detritus. They are commonly called spaghetti worms, as the tentacles resemble noodles. Most local species have yet to be identified.
Spaghetti Worms (family Terebellidae) have very long tentacles radiating from their front ends, resembling noodles, and hence the common name. They are deposit feeders - collecting detritus by spreading their long tentacles. A few other annelid families, such as Cirratulidae, Ampharetidae and Trichobranchidae, may also have species with similar long tentacles. Cirratulids will have tentacles occuring on most segments, not just the frontend, and they have a wedge-shaped head. Ampharetids have retractable tentacles (but not Terebellids). Trichobranchids are very similar but have long-handled hooks in the anterior region, and never in double rows. Unfortunately the photo above which I took was not clear enough to tell whether it's a terebellid or trichobranchid. It has tentacles with yellow and red bands.
Sometimes, several individuals can occur in the same tiny tidal pool. These unidentified worms have red tentacles.
vii) Sabellid Worms (Order Sabellida)
Members of this order possess a ring of feathery tentacles to capture and feed on plankton. They live in tubes made from their own mucus and materials from their environment, such as sand and shell fragments.
The sabellariids (family Sabellariidae) build curved, tusk-shaped tubes, usually in colonies on rocks other other hard structures. They sometimes form extensive reefs, exhibiting a honeycomb-like appearance when viewed from the top, and hence they are also called honeycomb worms. They are suspension feeders, and use their tentacles to gather plankton to feed on. Unfortunately, I do not have good photos of them as yet.
The sabellids (family Sabellidae) have feather-like tentacles on their heads to filter plankton, hence giving them the common names "fan worm" or "featherduster worm". They are usually solitary and live in rough tubes. While most species live in fixed tubes, a few are known to be able to move around. When their body is exposed, the segments can be seen. Various species can be seen on our shores, and they come in different colours. The most commonly seen species is the Reef Fan Worm (Sabellastarte sp.), as shown in the largest inset above. The white fan worm is a Bispira sp. while the black fan worm is yet to be identified.
The serpulids (family Serpulidae) possess plume-like tentacular crowns, and hence are also called "plume worms". They live in calcareous tubes and filter plankton from the water to feed on with their tentacles. Examples include the keel worms (see above photo), which are commonly found cemented to rocks and other hard surfaces. The "keel" refers to the shape of the tube, which somewhat resembles the keel of a boat.
References
- Chan, W.M.F. 2009. New nereidid records (Annelida, Polychaeta) from mangroves and sediment flats of Singapore. Raffles Bulletin of Zoology, Supplement No. 22: 159-172
- Glasby, C.J., 1999. The Namanereidinae (Polychaeta: Nereididae). Part 1, Taxonomy and Phylogeny. Records of the Australian Museum, Supplement 25: 1-129.
- Ruppert, E.E. and R.D. Barnes. 1991. Invertebrate Zoology (International Edition). Saunders College Publishing. U.S.A. 1056 pp.
- Tan, L.T. & L.M. Chou, 1993. Checklist of polychaete species from Singapore waters (Annelida). Raffles Bulletin of Zoology, 41(2): 279-295.
- Wilson, R.S., P.A. Hutchings & C.J.Glasby (eds), 2003. Polychaetes: An Interactive Identification Guide. CISRO Publishing, Melbourne.
- Fauchald, K., & P. A. Jumars. 1979. The diet of worms: a study of polychaete feeding guilds. Oceanography and Marine Biology: an Annual Review, 17: 193-284.
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