title for a learn-about section in A SNAIL'S ODYSSEY
  Predators & defenses
  Considered here are defenses of ribbon worms, while PREDATORS & DEFENSES OF FLATWORMS are dealt with in their own section. Defenses of nemerteans include hiding away and possibly possession of toxic chemicals. As in flatworms, aposematic or warning coloration may be involved, but no research on this subject appears to have been done on any west-coast nemertean species.
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  Defenses of ribbon worms
  This section deals with defenses of ribbon worms, while the topic of PREDATORS OF RIBBONWORMS is considered elsewhere.
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Research study 1
 

photograph of fragmented nemertean Cerebratulus sp.Some species of Cerebratulus nemerteans are known to fragment upon handlng. This strategy may act to distract a predator while one or more of the fragments crawl away and regenerate. Fragmentation and subsequent regeneration as a defensive strategy appears not to have been studied in west-coast nemerteans.

A fragmented nemertean Cerebratulus sp. collected
intact in Barkley Sound, British Columbia. When
handled in the laboratory it broke into 2 pieces 1X

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Research study 2
 

Free-living nermerteans such as Paranemertes spp. bear eyespots or ocelli on their anterior, dorsal surfaces.  The ocelli are not image-forming.  They possibly allow the worm to seek out dark areas such as its burrow and crevices in which to hide away. Stricker et al. 2001 Zoologica Scripta 11: 107.

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Research study 3
 

One of the earliest possible defensive secondary metabolite isolated from a nemertean is anabaseine, identified in Paranemertes peregrina from the San Juan Islands, Washington.  The authors note that this is the first reported occurrence of this alkaloid as a natural product.  In preliminary tests it appears not to have an effect on the functioning of crustacean nerves.  Kem et al. 1971 Toxicon 9: 15.

NOTE  an alkaloid with the structure 2-(3-pyridyl)-3,4,5,6-tetrahydropyridine

NOTE however, the authors add that its structure is similar to anabaseine reported in certain plants. The plant form of anabaseine has been synthesised and is found to have insecticidal properties

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Research study 4
 

More recent studies have identified several types of possible toxins in nemerteans.  Two of these, amphiporine, found only in hoplonemerteans, a group characterised by possessing stylets, and nemertine, found more generally throughout the phylum, cause convulsions when injected into crabs.  These 2 toxins are localised in the epithelial tissues of the body wall and proboscis, and may function mostly in defense against predators.  A third type, anabaseine, described in the previous Research Study 2, is a pyridine alkaloid with similar chemical and pharmacological properties to amphiporine.  Anabaseines, of which several different types have been described, are found only in species with stylets, and may therefore function both in subduing prey and in defense.  In the more than 2 decades since the writing of the paper cited here, little more is known of the ecological role of the toxins in west-coast species, including even whether nemerteans are generally unpalatable to other invertebrates.  Kem 1985 Am Zool 25: 99.

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Research study 5
  The presence of bright coloration in an invertebrate immediately evokes the possibility of aposematic or warning coloration.  A quick and completely unscientific survey of common west-coast nemerteans suggests that the most brightly coloured or patterned species to the human eye are ones that lack stylets, such as Tubulanus spp. (Palaeonemertea) and Cerebratulus spp. (Heteronemertea; see photos below).  Many species of Tubulanus are diurnal in habit and do not hide when approached by potential predators.  In contrast, the least brightly coloured or patterned species are hoplonemerteans, in other words, ones that possess stylets, such as Amphiporus spp.  What all this means, if anything, is anyone's guess.
 
photograph of nemertean Tubulanus polymorphus courtesy Pamela Roe, California State University Stanislaus, California
Tubulanus polymorphus (head end is at bottom Right) 0.5X
Examples of colorful west-coast nemerteans.

 

photograph of a nemertean Cerebratulus montgomeryi courtesy Pamela Roe, California State University Stanislaus, California
Cerebratulus montgomeryi (a single individual) 0.4X

photograph of anterior end of nemertean Tubulanus sexlineatus courtesy Pamela Roe, California State University Stanislaus, California
Head end of the colorful Tubulanus sexlineatus, with a full body view shown in photograph on Right 3X
photograph of nemertean Tubulanus sexlineatus courtesy Pamela Roe, California State University Stanislaus, California
Tubulanus sexlineatus 1X
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Research study 6
 

Most nemerteans have simple ocelli of the inverted pigment-cup type and lack any focusing structures.  They seem morphologically capable only of differentiating light from dark.  In general, then, the bright coloration in nemerteans is likely not used for intraspecific communicatino but, as noted in the previous Research Study, could be a form of aposematic coloration.  Sundberg 1987 Oikos 48: 3.

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Research study 7
 

drawings of two Japanese nemerteans known to contain the potent neurotoxin tetrodotoxin, courtesy Gakken Illustrated Nature Encyclopedia, Gakken Co., TokyoA 4th type of toxin, described in the Japanese species Lineus fuscoviridis and Tubulanus punctatus, is the neurotoxin tetrodotoxin.  Neither species lives on the west coast, but several other species of both genera do occur here, and may possibly contain the toxin.  Tetrodotoxin is known from pufferfishes, newts, gobies, frogs, blue-ring octopuses, trumpet shells, flatworms, and others.  Extracts from the Japanese species cited are lethal when injected into mice. Miyazawa et al. 1988 Toxicon 26: 867. Drawings courtesy Gakken illustrated nature encyclopedia, Gakken Co., Tokyo.

NOTE  additionally, several polypeptide neurotoxins are described from the Atlantic-coast Cerebratulus lacteus, a species that lacks a stylet. Some of these neurotoxins are lethal to mammals; others, to crustaceans.  Several species of Cerebratulus inhabit the west coast and may possess similar neurotoxins.  Kem 1976 J Biol Chem 251: 4184; Blumenthal & Kem 1976 J Biol Chem 251 (19): 6025.

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Research study 8
 

colour plate of nemerteans from unknown sourceThis study cited here is not on west-coast nemerteans, but it does provide a thoughtful discussion on a mechanism for evolution of aposematic coloration in a hypothetical nemertean species, possibly a "must read" for anyone interested in doing research on the subject. The model is a 2-locus one and assumes: 1) the worm is toxic; 2) 3 colour forms exist in the population coded by one of the 2 loci1: red, intermediate, and cryptic; 3) the fitness of each morph depends on water depth, as red wavelengths attenuate2 quickly with increasing depth; 4) possible predators are fishes with good visual acuity and colour vision; 5) the worms are at first sublittoral, but evolve to shallower depths, where they become more visible to predators; and 6) the cryptic and red forms have higher fitness than the intermediate forms.  This is because the cryptic ones are hidden and the red ones, while at first inviting proportionally more attacks, survive better because of an enhanced avoidance learning on the part of the predators which reduces the number of future attacks by them.  A threshold value of conspicuousness is assumed below which the predators fail to differentiate their potential prey and thus fail to learn.  The intermediate forms have reduced fitness because their degree of conspicuousness is below this threshold, but they are still easier to detect than the cryptic forms.  Thus, the predators may find and eat the intermediate forms, but are incapable of associating their distastefulness with any obvious feature of colour as they do with the red morph.  The author’s model of evolution shows that warning coloration (or, the opposite: cryptic coloration) can evolve in a hypothetically distasteful nemertean through individual selection3, although the author adds that it is not necessarily the only way for this to happen. Sundberg 1987 Oikos 48: 3. The drawings show colorful representatives of world nemerteans, from an unidentified source.

NOTE1  in this 2-locus model the red conspicuous form is designated aa, the intermediate form Aa, and cryptic form AA; the sublittoral form is designated BB and the littoral forms as either Bb or bb.  Thus, Locus A determines colour while Locus B determines behaviour (habitat preference)

NOTE2  in clear oceanic water red wavelengths in sunlight decrease by about 99% for every 1m of depth.  Thus, even at a comparatively shallow depth of 3m, red wavelengths are theoretically reduced to 1-millionth of their intensity at the sea surface.  A red or orange nemertean at depth will appear black to a predator with colour vision

NOTE3  the classical view is that aposematic coloration has evolved through kin selection, or promoting the passing on of an individual’s genes by aiding the survival or reproduction of its close kin 

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