Anopla

(Anoplans)

Phylum Nemertea

Class Anopla

Number of families 11

Thumbnail description
Worms that have an unarmed proboscis; mouth situated below or posterior to the cerebral ganglia; and central nervous system situated within the body wall, or between the body musculature and epidermis

Evolution and systematics

The fossil record of nemertines is extremely sparse, as would be expected from a soft-bodied animal. There is a trace fossil (genus Archisymplectes) from the Pennsylvania-age from central Illinois that may represent an anoplan nemertine. The anoplan palaeonemerteans have been regarded as the phylogenetically basal nemerteans based on a simpler nervous system and cerebral organs, but recent molecular studies are inconclusive.

Class Anopla used to be divided in three subclasses, Archinemertea, Palaeonemertea, and Heteronemertea, but Archinemertea has been shown to be paraphyletic and the name is not used by most authors. Recent studies based on nucleotide sequences established that the paleonemertea is a non-monophyletic group but did not formally reclassify the order. The term palaeonemerteans and Paleonemertea thus refer to a presumed paraphyletic assemblage and are used for convenience and tradition without reflecting a monophyletic group. The Palaeonemertea includes those species with two (outer circular and inner longitudinal) or three (outer circular, middle longitudinal, and inner circular) body muscle layers. The subclass Heteronemertea includes those species with primarily three body wall muscles, although there exist species with an additional inner circular layer. The central nervous system is situated between the outer longitudinal and the middle circular muscular layers in heteronemerteans, and in the inner longitudinal muscle layer, or external to body wall muscles, in the palaeonemerteans.

The class is currently divided into 11 families and 93 genera comprising approximately 500 described species. The largest genus, Cerebratulus contains 116 species, and the second largest, Lineus, has 80 species.

Physical characteristics

Anoplans are differentiated from nemerteans in the class Enopla by the proboscis, which is armed (i.e., there is a stylet attached to it) in enoplans but unarmed in anoplans. Other differences between the two groups include the position of the lateral nervous system and the relationship between the mouth and proboscis; enoplans have a common opening for the mouth and proboscis, while anoplans have separate openings for the two structures. Anopla contains the largest nemerteans. The longest species, Lineus longissimus, is described as being up to 98 ft (30 m) long, and several other hetero-and palaeonemerteans are big and solid worms. Parborlasia corrugatus, for example, is abundant and widely distributed in the Antarctic and can reach lengths up to 6.5 ft (2 m). While most nemerteans are rather drab in color, the most colorful and strikingly pigmented are found among the anoplans. There have not been many experiments with nemerteans, but a few studies indicate that the coloration and pigmentation are cases of warning coloration. Most nemerteans have toxic and noxious substances in their body—undoubtedly to deter predators. When these substances are accompanied with bright colors and a lifestyle (day active, no hiding, etc.) that does not encourage them to hide from predators, the interpretation of aposematisms seems well founded. In the field, a nemertean is generally recognized by the way it moves. The normal movement is gliding over the surface by help of cilia on the ventral side in combination with mucus produced by the worm. Some species may, under certain circumstances, swim with undulating movements, but only for a short period of time.

Distribution

Anoplan nemerteans are known from all continents and all seas.

Habitat

Anoplan nemerteans are benthic and found in the littoral as well as in deeper waters. In general, heteronemerteans are more common in the littoral, while palaeonemerteans seems to be more abundant at deeper soft bottoms. However, the ecology of nemerteans is, in general, poorly known, as is their actual spatial distribution. While larger species may be found simply by turning over boulders, smaller species are not found unless special techniques are utilized. An easy way of collecting nemerteans is to place seaweed and smaller algae in a bucket of seawater and let it stand for a few hours, and up to a couple of days, depending on weather and temperature. The worms will crawl to the sides of the bucket, where they are easily observed and collected, as the oxygen concentration decreases in the water. Although nemerteans are abundant, especially in temperate waters, their presence is often overlooked because they are not easily observed.

Behavior

Most nemerteans are solitary, free-living animals. No anoplan nemerteans are known to be parasitic or commensal.

Feeding ecology and diet

Nemerteans are common predators in a variety of habitats, but there is little published information on food and feeding of anoplans. Palaeonemerteans appear to feed on annelids that are compatible with their size. What is known about heteronemerteans indicates that their food is living or dead polychaetes, but they also feed on mollusks, crustaceans, and other nemerteans. Scavenging may be a way of life for many species, but clear-cut evidence exists only for the Antarctic Parborlasia corrugatus. There are no parasitic or commensal known hetero-or palaeonemerteans.

Reproductive biology

Most nemerteans are dioecious, although there are a few hermaphroditic hoplonemerteans (class Enopla). Palaeonemerteans are traditionally regarded as direct developers, whereas heteronemerteans all produce pilidium larvae (free-swimming, planktotropic larvae), or larvae that seem to be derived from a pilidium. These heteronemerteans are broadcast spawners, but there are also heteronemerteans (Lineus rubber and L. viridis) known to reproduce by a kind of pseudocopulation in which the

fertilized eggs are deposited in a gelatinous cocoon producing pilidium-like larvae known as Schmidt's and Desor's larvae, respectively. Only a few species are known to reproduce vegetatively by fission (a few species in genus Myoisophagus).

Conservation status

Very little is known about nemertean ecology, distribution, or abundance. It is clear, however, that certain species are the most abundant invertebrate group in some habitats/some localities. Whether other species are threatened is almost impossible to say. The most recent IUCN Red List of Threatened Species includes only six nemerteans, all belonging to class Enopla.

Significance to humans

It is documented that the heteronemertean, Cerebratulus lacteus, (tapeworm) was frequently collected and used as bait by sport fishermen in the United States, at least into the middle of the twentieth century. Another large heteronemertean used as bait in South Africa and Mozambique is Polybrachiorhynchus dayi, also referred to as tapeworm by fishermen. Other uses of anoplan nemerteans are not known.

Species accounts

List of Species

No common name

Baseodiscus delineatus

order

Heteronemertea

family

Baseodiscidae

taxonomy

Baseodiscus delineatus (Delle Chiaje, 1825), Italy.

other common names

None known.

physical characteristics

May attain lengths of 3.3 ft (1 m) or more, but rarely more than 0.07–0.11 in (2–3 mm) wide. Head slightly bi-lobed, demarcated from rest of body, with numerous black or dark brown eyes, mainly distributed along the cephalic margins, and form two large dorsolateral groups near back of head. Background color uniform dull yellowish fawn to light brown, marked by reddish brown, interrupted longitudinal stripes extending full body length.

distribution

From Iceland eastwards to the Atlantic and North Sea coasts of Europe.

habitat

Lower shore beneath boulders or sublittorally on coarse shell or gravel substrata containing some mud or sand.

behavior

Nothing is known.

feeding ecology and diet

Nothing is known.

reproductive biology

Separate sexes, external fertilization with pelagic larvae.

conservation status

Not threatened.

significance to humans

None known.

No common name

Lineus longissimus

order

Heteronemertea

family

Lineidae

taxonomy

Lineus longissimus (Gunnerus, 1770), Norway.

other common names

None known.

physical characteristics

The longest nemertean known, individuals of 16.4–32.8 ft (5–10m) length are common and specimens of up to 98.4 ft (30 m) are reported. Flaccid body contracts and extends in series of irregular muscular waves when disturbed. Colors range from dark olive-brown or rich chocolate brown in smaller specimens to blackish brown to black in larger animals. Flickering purplish iridescence often evident, resulting from activity of epidermal cilia. Ventral color may be paler than dorsal. Tip of head pale or whitish and usually appearing bi-lobed. There are 10–40 reddish brown or black eyes in a row on each side of the snout.

distribution

From Iceland eastwards to the Atlantic and North Sea coasts of Europe.

habitat

Typically found on the lower shore beneath boulders on muddy sands, but also in rockpools or in deeper sublittoral locations on muddy, sandy, stony, or shelly bottoms.

behavior

Epidermis contains toxic/noxious substances; probably a defense mechanism.

feeding ecology and diet

Nothing is known.

reproductive biology

Separate sexes, external fertilization with pelagic larvae.

conservation status

Not threatened.

significance to humans

None known.

No common name

Tubulanus annulatus

order

Palaeonemertea

family

Tubulanidae

taxonomy

Tubulanus annulatus (Montagu, 1804), England.

other common names

None known.

physical characteristics

Strikingly colored, vivid brick-red, orange-red, garnet-red, or brownish red marked with white longitudinal stripes and rings. Mid-dorsal stripe extends on to the cephalic lobe and terminates at a transverse white band on head. The two lateral stripes do not reach head. Ventral surface is paler than dorsal. Length 29.5 in (75 cm) or more, but rarely exceeds 0.11–0.15 in (3–4 mm).

distribution

In Northern Hemisphere from Pacific coast of North America eastwards to the Atlantic, North Sea, and Mediterranean coasts of Europe.

habitat

Found intertidally beneath stones or on sand or mud near low water level, but more common sublittorally on a wide variety of substrata at depths down to 130 ft (40 m) or more.

behavior

Epidermis contains toxic/noxious substances and the worm is avoided by fishes. In view of the conspicuous appearance in shallow waters, this species may be a case of aposematic coloration.

feeding ecology and diet

Nothing is known.

reproductive biology

Separate sexes.

conservation status

Not threatened.

significance to humans

None known.

Resources

Books

Gibson, Ray. Nemerteans. London: Hutchinson University Library, 1972.

——. Nemerteans. Synopses of the British Fauna. Dorchester: The Linnean Society of London, 1994.

Gibson, Ray, Janet Moore, and Per Sundberg. Advances in Nemertean Biology. Dordrecht: Kluwer Academic Publishers, 1993.

Sundberg, Per, Ray Gibson, and Gunnar Berg. Recent Advances in Nemertean Biology. Dordrecht: Dr. W. Junk Publishers, 1988.

Periodicals

Gibson, Ray. "The Invertebrate Fauna of New Zealand: Nemertea (Ribbon Worms)." NIWA Biodiversity Memoir 118 (2002): 1–87.

Sundberg, Per, Turbeville J. McClintock, and Susanne Lindh. "Phylogenetic Relationships among Higher Nemertean (Nemertea) Taxa Inferred from 18S rDNA Sequences." Molecular Phylogenetics and Evolution. 20 (2001): 327–334.

Thollesson, Mikael, and Jon L. Norenburg. "Ribbon Worm Relationships: A Phylogeny of the Phylum Nemertea." Proceedings of the Royal Society of London Series B—Biological Sciences. 270 (2003): 407–415.

Per A. Sundberg, PhD