title for a learn-about section in A SNAIL'S ODYSSEY
  Foods, feeding, & growth
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  Prey capture by nemerteans
  Prey capture by nemerteans is considered here, while other topics relating to FOODS, FEEDING, & GROWTH of flatworms & nemerteans can be found in another section.
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Research study 1
 

drawings showing mouth/gut and proboscis morphology in 2 major groups of nemerteans courtesy Brusca & Brusca 2002 Invertebrates, Sinauer Assoc, Inc, Sunderland, MassPrey capture by carnivorous nemerteans involves use of a proboscis that can be extended outwards for some distance to wrap around and immobilise the prey. The proboscis system functions independently of the gut and, depending upon species, may be physically separate from the gut. In species of Paranemertes, Carcinonemertes, and Emplectonema, to name some common west-coast representatives of Hoplonemerteans, the proboscis has an associated stylet that is used to stab the prey, to inject toxin, or both.  In these stylet-bearing species the proboscis is everted from the mouth, located at the anterior end of the body (see lower drawing). In other common west-coast genera, such as Tubulanus (Palaeonemertea), and Cerebratulus and Micrura (Heteronemertea) the proboscis lacks a stylet.  In both of these groups the proboscis moves in and out of the body via its own special anterior opening, while the mouth is separate and located on the ventral surface (upper drawing). Drawings courtesy Brusca & Brusca 2002 Invertebrates Sinauer Assoc, Inc. Sunderland, Mass.

NOTE  not all nemerteans are carvivorous; some species eat detritus and others, algae

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Research study 2
 

In Paranemertes peregrina, a mud-burrowing, polychaete-eating carnivorous nemertean, toxins for immobilising the prey are produced in gland cells located at the back of the proboscis sac.  Other studies classify the toxins of nemerteans as potent neurotoxinsGibson 1970 Biol Bull 139: 92.

NOTE  the toxin in P. peregrina is a bipyridyl neurotoxin called anabaseine.  It appears to be distributed throughout the body but mostly in the anterior proboscis region

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Research study 3
 

table showing polychaete species eaten by nemerteans Paranemertes peregrina out of several types offeredLaboratory observations of feeding of the hoplonemertean Paranemertes peregrina at Friday Harbor Laboratories, Washington provide this account of capture and feeding on polychaete prey.  A prey is located by contact and not, apparently, by distance chemosensory means.  Contact invariably leads to an initial recoiling by the predator, followed by rapid eversion of the proboscis, winding it several times around the prey, and accompanying this with stabbing thrusts of the stylet.  Stylet thrusts can occur at several location along the prey worm’s body. Within a few seconds even a large prey is paralised by venom pumped into it via the stylet.  Although accessory stylets are available to Paranemertes in adjacent sacs, the central stylet is tough and can be used for multiple stabbings before being replaced. If a worm is stabbed but not eaten, the effect of the venom wears off in about 20min.  Accompanying the muscular squeezing of the proboscis is secretion of sticky mucus that appears also to bind the prey.  At this time the predator backs off, locates an area of the prey’s body that it can envelope in its expanding mouth, and begins to suck in the prey with peristaltic contractions of the musculature around the mouth.  Small-sized prey can be attacked and consumed in 2-3min. 

In another part of the study, different species of polychaetes living in the immediate habitat of P. peregrina in Garrison Bay, Washington are offered to P. peregrina, then scored on whether the nemertean recoils or shows no response, everts its proboscis, or eats the prey. Only the the last is shown in the Table on right).  Note that most species are ignored.  Of 24 polychaetes eaten, 92% are nereids. Roe 1970 Biol Bull 139: 80.Photograph courtesy Rebecca Kordas, Biodiversity Research Centre, UBC, Vancouver.

NOTE  in the absence of sensory tentacles in nemerteans, track-following is presumably done by contact of skin receptors in the oral area with molecular residues left in the track of the intended polychaete prey.  The author does not think that Paranemertes peregrina has the ability for distance chemoreception but, as most, or many, other species of nemerteans appear capable of this, perhaps it should be re-investigated for this species

NOTE the central main or functional stylet in the closely related P. sanjuanensis is about 90µm in length and is composed of a single grooved shaft (see photo in Research Study 5 below).  There is no duct through which toxin can be pumped into a prey.  In this species there are 5 reserve sacs each containing about 3 stylets, photograph of a nemertean Paranemertes peregrina capturing a nereid polychaete with its proboscis courtesy Rebecca Kordas, Biodiversity Research Centre, UBC, Vancouvermaking 15 reserve stylets in total.  Neither frequency nor mechanism of replacement of stylets is known for any species of nemerteans.  Stricker et al. 2001 Zoologica Scripta 11: 107.


A Paranemertes peregrina has snared a
much larger polychaete with its extended
proboscis. The proboscis is sticky and, as
evidenced by the white secretions in the
photograph, makes a firm attachment even
on the slippery epithelium of the polychaete

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Research study 4
 

photograph of nemertean Paranemertes peregrina capturing a polychaete worm courtesy Rebecca Kordas, Department of Zoology, UBCIn San Juan Islands, Washington nemerteans Paranemertes peregrina inhabit burrows in mud-flat regions.  When the burrow is exposed by the receding tide, the nemertean emerges and searches for polychaete prey, with preference for the nereid Platynereis bicanaliculata.  Overall, Paranemertes preys on representatives of 5 polychaete families.  A nemertean wanders the mud flat until it crosses the track of a nereid worm.  It follows the track, half the time in the wrong direction, and will even enter a polychaete's burrow in pursuit.  After contacting, enveloping, and consuming the prey, which may take less than 10min, the nemertean returns to its burrow by following its own mucous trail.  If it has lost the trail, or if the integrity of the trail is disrupted during the attack, the nemertean will dig another burrow.  Feces of P. peregrina are easily recognised by their content of mucus-enclosed bundles of polychaete setae and jaws.  Roe 1976 Biol Bull 150: 80. Photograph courtesy Rebecca Kordas, Biodiversity Research Centre, UBC, Vancouver.


photograph of a nereid polychaete soon after being attacked by a nemertean worm courtesy Rebecca Kordas, Biodiversity Research Centre, UBCMore views of Paranemertes
peregrina
capturing a nereid (see Research Study 2 above). In this encounter the intended prey was able to escape, but note the deflated appearance of its body, suggesting toxin injection by the predator

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Research study 5
 

drawings showing details of stylet apparatus in a hoplonemertean ribbon-worm Paranemertes peregrinaThe stylet apparatus and its function is described in detail for Paranemertes peregrina, collected from mudflats in San Juan Islands, Washington.  The apparatus consists of a single central, functional stylet cemented firmly at the base of the proboscis sheath (see drawings on Left). The stylet is 85µm in average length and consists of a porous base tapering lengthwise into a shaft bearing helically-wound ridges and grooves. The functional stylet is everted, presumably by hydrostatic pressure, and is used to stab prey, often repeatedly.  Glandular areas located in the epithelium immediately anterior to the functional stylet may secrete adhesives that help attach the everted proboscis to the prey and keep the stylet embedded in the prey during injection of toxin.  The stylet base is continuous with an ejaculatory duct leading to a glandular region, the secretions of which are thought to be the toxins used to immobilise prey.  The authors suggest that the toxins are injected into the prey by way of the grooves on the stylet. Adjacent reserve-stylet sacs, up to 5 in number, contain additional stylets in progressive states of formation, and these may number up to 60 or more in a single individual.  A duct joins each reserve-stylet sac to the lumen of the proboscis near the photograph of front end of nemertean Paranemertes peregrina showing common mouth & proboscis opening courtesy Dave Cowles, Walla Walla University, Washingtonfunctional stylet and is the means of transfer of replacement stylets when the functional one wears out or is lost. Although not mentioned by the authors, the fact that all save a single stylet in the reserve sacs of the worm featured appear to be fully formed, suggests that their formation is either quick or replacement needs are small. The authors, however, suggest that stylet replacement may occur frequently enough following prey attack to cause depeletion in number of reserve stylets.  Each reserve sac is essentially a single cell, greatly expanded, and the stylets are produced within membrane-bound vacuoles associated with the Golgi apparatus of the cell.  A stylet begins as an organic thread around which mineral crystals, thought to be phosphates of calcium and strontium, are precipitated.  The authors provide the first detailed histological and ultrastructural account of the stylet apparatus in relation to its function in capturing prey.  Stricker & Cloney 1981 Zoomorphology 97: 205; see also Stricker & Cloney 1982 Biol Bull 162: 387. Photograph courtesy Dave Cowles, Walla Walla University, Washington. wallawalla.edu.

NOTE  the species is named (lit. “wander about” L.) for its conspicuous habit of wandering over the intertidal substratum seeking out polychaete prey

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Research study 6
 

Field and laboratory studies on Paranemertes peregrina at Friday Harbor Laboratories, Washington and Bamfield Marine Sciences Centre, British Columbia suggest that perception of polychaete prey is mediated through so-called cerebral organs located on the upper surface of the head. The organs are composed of nerves and glandular elements surrounding a ciliated canal that opens to the outside.  The function of the organs has never been established, but the fact that the beat frequency of the cilia within the canal increases in the presence of food suggests a chemoreceptive role.  Paranemertes occupies temporary burrows and emerges at low tide to forage for polychaete prey on the mud surface.  Contact with the mucous trail of a prey elicits searching behaviour.  In one of their experiments the authors remove the cerebral organs in some specimens and leave them intact in others.  Following removal, the ability of Paranemertes to recognise prey trails is abolished.  Is this simply a by-product of the operation?  No, because these same operated worms are still able to follow their own mucus trails, and sham-operated individuals are still able to track prey.  Moreover, the worms without cerebral organs will eat a dead or injured polychaete worm if presented to them.  The authors conclude that the cerebral organs are responsible for recognition of a prey species. Amerongen & Chia 1982 J Exp Mar Biol Ecol 64: 11.

NOTE a cut made parallel to, and to the same depth, as ones used to extirpate the organs in other test individuals.  Both types of cuts heal within 12h

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Research study 7
  Behaviour of the nemertean Tetrastemma sp. when attacking and consuming an amphipod in a laboratory dish includes stabbing several times by the stylet to the ventral side of the amphipod.  After immobilisation, the nemertean sucks out the soft issues and leaves behind a cleaned exoskeleton.  This takes about 30min. Stricker & Cloney 1982 Biol Bull 162: 387.
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Research study 8
 

photographs of stylets of 9 species of Puget-Sound, Washington nemerteans courtesy Stricker 1983 J Morph 175: 153.Each stylet is a solid, nail-like structure, ranging in length from 50-300µm depending upon species.  The stylet consists of 2 components: a central organic matrix that serves as a template during calcification, and an outer cortex made up of crystalline calcium phosphate.  The shape of the cortex corresponds closely with the shape of the organic matrix.  In stylets of Paranemertes spp., coloured pink in the photograph, the helically wound grooves and ridges in the cortex are complementary to twists in the organic matrix.  Shown in the figure are stylets from 9 nemertean species collected from Puget Sound, Washington, with lengths indicated for each.  Stricker 1983 J Morph 175: 153. 

NOTE  calcium phosphate is a principal component of vertebrate teeth

Top row (L to R): Amphiporous angulatus, A. formidabilis, A. imparispinosus, Emplectonema gracile

 

 

 

 

Bottom row: Emplectonema purpuratum, Paranemertes paregrina, P. sanjuanensis, Tetrastemma sp., Zygonemertes virescens

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