Eurythoe complanata

Eurythoe complanata was considered a circumtropical species, occurring in the Atlantic, Pacific and Indian oceans and the Mediterranean and Red seas. However, a recent study by Barroso et al. (2010) looking at morphological features, allozyme analyses, DNA analysis and molecular divergence suggests that the true range of E. complanata is limited to the Atlantic Ocean, specifically from the Caribbean Sea to southern Brazil. Worms found on the coasts of Atlantic islands often show distinct morphological features of the parapodia, leading to debates about the species status. Most classifications include the coastal Atlantic island worms since the differences are small and generally resemble variations found in any species. Polychaetes identified as E. complanata around the globe actually consist of three or more closely related species sharing many morphological traits. (Barroso and Paiva, 2007; Barroso, et al., 2010)

The preferred food source for Eurythoe complanata is carrion, found in almost any ecosystem, allowing the worm to occupy a variety of habitats. Eurythoe complanata occurs in intertidal zones and on sandy beaches in tropical regions of the Atlantic Ocean. This species thrives in warm, shallow waters, so it is found exclusively on coastlines. Eurythoe complanata is nocturnal and needs places to hide during the day to avoid predation and dessication. Individuals can be found under rocks, inside calcareous algae, inside corals or burrowed in soft sediments. Larval forms are free-floating and pelagic. (Barroso, et al., 2010; Pardo and Amaral, 2006)

Eurythoe complanata has a variety of colors. These worms can have an orange, salmon pink, blue, green, or even black body and can often have a dark midventral line. The common name "fire worm" is derived from the bright red gill tufts along the sides of the body and the numerous white bristly appendages, as well as the painful sensation caused by its sting. Eurythoe complanata has a flattened, rectangular body, the cross-section of which is about twice as wide as it is tall. The fire worm prostomium, or head segment, is oval-shaped and bears four eyes, short sensory tentacles, and three or four buccal segments on its eversible mouthparts. This species has no jaws, papillae, or teeth. Eurythoe complanata has two types of structurally simple parapodia: a straight serrate type and a more bristly branched type. The relatively simple structure and lack of identifying features of this polychaete makes it difficult to distinguish it from other closely related species. This problem of "low morphology" is probably what led to the inferred cosmopolitan distribution of the animal. (Barroso, et al., 2010; Chamberlin, 1919)

Eurythoe complanata begins life as a fertilized egg and has indirect development. The egg hatches into a trochophore larval stage, which is a feeding larva with a prototroch. A prototroch is a characteristic band of cilia that beat in unison and create a current used both for locomotion and feeding. The trochophore larva is free-floating and pelagic. The larva develops into a juvenile worm which matures into an adult. Asexual reproduction of E. complanata is referred to as fragmentation. The fire worm has regenerative capabilities and when broken into pieces, each piece develops a head and tail and becomes a complete worm. (Reish, et al., 2009; Rouse, 2000)

Like many oligochaetes and polychaetes, Eurythoe complanata is capable of reproducing both asexually through body fragmentation, and sexually through mating. (Müller, et al., 2003)

Eurythoe complanata is a broadcast spawner, releasing eggs and sperm into the water column. Fertilization is external. There has been little or no studies of E. complanata spawning in the wild. In captivity, this species spawned one week after the full moon in September. In laboratory studies, individuals have been observed spontaneously fragmenting and regenerating, demonstrating asexual reproduction. (Pardo and Amaral, 2006; Van Veghel, 1993)

Broadcast spawning assures that offspring will not compete with parents for space or resources, as the young are swept into the water column and carried away. However, this physical separation of parents and offspring also makes any sort of parental care impossible. Large amounts of sperm and eggs are released to compensate for the lack of parental investment. Even though the pelagic larvae are easy prey for many marine predators and do not benefit from any sort of protection, the great numbers ensure survival of some to adulthood. (Van Veghel, 1993)

Little is known about the lifespan or longevity of E. complanata.

In laboratory observations, E. complanata displayed active feeding behaviors in the presence of food. However, when food was not present in the habitat, it hid in groups under rocks and remained still until food was presented again. Individuals competing for the same piece of food were observed pushing each other with their bodies. In the wild, E. complanata remains hidden during the day to protect itself from dessication and predation. It burrows under rocks, into corals, or hides in any type of shelter, emerging at dusk to forage throughout the night and into the dawn. The parapodia of E. complanata are well developed and it moves rapidly over the substrate as it actively seeks food. (Pardo and Amaral, 2006)

No information was found for the home range.

Eurythoe complanata detects prey and food primarily by sensing chemical changes in the water. When exposed to a fish maceration in their tank, individuals quickly show a feeding response, moving their bodies and everting their mouthparts. Eurythoe complanata has a chemosensory nuchal organ, present on a structure called the caruncle, which is a bulge formed from the prostomium containing bands of sensory cilia. Although distinctive to the family Amphinomidae, the caruncle is not a unique structure since it occurs in several other families of polychaetes.

Individuals have two pairs of eyes, which are sensitive to light. The species is mostly nocturnal and crepuscular. Thus, E. complanata becomes uncomfortable and seeks shelter when exposed to bright lights. Eurythoe complanata curls up into a ball and exposes poisonous bristles when touched. This polychaete's distinctive white bristles and bright bands of red gills are a form of aposematism to help potential predators recognize it as a poisonous animal. (Pardo and Amaral, 2006; Purschke, 2005)

Eurythoe complanata is an omnivorous scavenger. Larger pieces of food (such as two mm pieces of fish provided in one laboratory observation) are ingested by the pharynx, which can increase in size to handle different sizes of food. Smaller food particles are plucked from the water by the everted pharynx. In laboratory observations, individuals only ate fish offered to them. In the wild, this polychaete has been observed feeding on corals and algae as well. Eurythoe complanata seems to consume about any organic matter it can fit into its mouth. (Pardo and Amaral, 2006)

The bright red gill tufts of E. complanata are a form of aposematism, warning potential predators of its neurotoxin-bearing bristles. Most marine predators avoid the painful poison of the worm. However, Conus imperialis has been observed feeding on E. complanata. This is thought to be due to the presence of high levels of serotonin in the venom of C. imperialis. 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 E. complanata easier prey for C. imperialis. In addition to its aposematism, the often brightly colored worm is camouflaged to blend in with the corals and sediments it inhabits. (McIntosh, et al., 1993)

Eurythoe complanata is an important scavenger and detritivore. It feeds on the carcasses of dead animals and helps keep dead organic matter from building up in the ecosystem. (Pardo and Amaral, 2006)

Due to its widespread distribution and regenerative capabilities, E. complanata is a frequently studied organism. It has been used in the laboratory to establish a model of polychaete regeneration and to expand knowledge of the polychaete nervous system. The widespread distribution of this polychaete has also made it useful as an indicator species, and there have been many studies conducted regarding the concentrations of heavy metals and other pollutants in the worms. (Müller, et al., 2003)

Eurythoe complanata is 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. (Eckert, 1985; Pardo and Amaral, 2006)

Perhaps due to its generalist food habits and widespread distribution, this animal is not currently of any special concern on any conservation list.

Eurythoe complanata is often a surprise visitor in salt water aquaria. New corals added to tanks often have worms hiding in them. In captivity, E. complanata can reach up to three feet in length and is known for startling aquarium owners when it suddenly pops out of hiding during a tank cleaning. The worms breed well in tanks and can be quite prolific. Its pelagic larvae are a rich food source for corals and other small invertebrates. Eurythoe complanata can be beneficial to tank health as it is a generalist scavenger and will eat any extra organic matter laying about the tank. (Shimek, 2010)