title for learn-about section of A SNAIL'S ODYSSEY
  Foods, feeding, & growth
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Research study 1
 


photo complex showing details of nematocyst clusters on the capitate tentacles of a hydroid Coryne sp.All cnidarians rely on their nematocysts for capture of food and for defense.  The knobby ends of the hydroid polyp Coryne sp. contain potent stinging cells, ones that can cause a persistent and sometimes painful rash when touched by humans.  In cross-section, each small bump on the knob is a single nematocyst contained within a larger cell or nematocyte.  This type of nematocyst is a penetrating type, or stenotele. It has a bulbous capsule filled with a toxic protein material that is hygroscopic, that is, it readily takes up water.  Discharge of the nematocyst generally requires 2 stimuli, one chemical and one physical, the latter being a touch to the small trigger or cnidocil at the protruding end of the nematocyst.  When a small crustacean or other prey contacts the cnidocil, the capsule becomes instantly permeable to water, pressure builds, the lid pops open, and the internally coiled thread explosively everts.  The thread unravels like a finger of a rubber glove being blown outwards.  Spines and bristles that were formerly inside the thread are now outside, and the bristles aid in penetration of the thread.  Although chemical taste of a prey and touch are the proximal stimuli leading to discharge, it is thought that ultimately the nematocyst is under some degree of nervous control.  Nematocyst discharge takes only a few msec and is one of the fastest responses known in the animal kingdom.  The thread tip accelerates at 40,000g.  Reviewed in Kass-Simon & Scappaticci 2002 Can J Zool 80: 1772.

 

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  Polyp
  Foods, feeding, & growth of the polyp are considered in this section, while that of the MEDUSA is considered in another section.
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Research study 1
 

drawings of developmental stages of the hydroid polyp Clytia gregaria showing distribution of non-starch polysaccharidesThe polyps of thecate hydroids (such as Obelia spp). are supported by an exoskeleton of polysaccharides secreted by the underlying epidermis.  A study at Friday Harbor Laboratories, Washington on early embryonic growth in the polyp of the colonial hydroid Clytia gregaria using histochemical staining techniques shows that during early development considerable amounts of polysaccharides are contained within egg, blastula, gastrula, and the motile planula. As seen in the drawings, the epidermal cells comprising the planula have a fairly even distribution of polysaccharides, but there is a notable concentration of these polysaccharide-containing cells at the blunt anterior end.  During settlement, the planula attaches at its anterior end, flattens like a pancake, and begins to secrete the hard chitinous material that will eventually form the perisarc.  An opening appears at the location that was previously the posterior end.  The perisarc elongates, develops annulations, and  becomes the first, or primary, feeding polyp (hydranth).  At this time all the epithelial cells have lost their intensely staining polysaccharides.  Bonner 1955 Biol Bull 108: 18.

NOTE  formerly Phialidium gregarium; a colonial thecate species bearing some resemblance to Obelia spp.

NOTE it is unfortunate that the author does not include illustrations of this interesting event in early development of the perisarc

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Research study 2
 

The 2-tentacled hydroid Proboscidactyla flavicirrata lives as a colony on the rims of leathery tubes of several species of sabellid worms.  Studies on growth of P. flavicirrata collected from Friday Harbor, Washington show that at the distal or anterior part of the tube the stolons of the colony drawings of hydroid Proboscidactyla flavicirrata on its host wormrun parallel to the axis of the tube, while schematic showing growth of the hydroid Proboscidactyla flavicirrata as its host's tube lengthens with growthproximally they form an anastomosing network.  Gastrozooids are located at the tube rim, gonozooids behind the gastrozooids, and dactylozooids in the proximal colony regions.  The gonozooids bear medusa buds in varying stages of development.  The defensive polyps, or dactylozooids, are each crowned by a bundle of nematocysts.  As the worm adds to its tube, the whorl of gastrozooids moves along with it, propagated by anastomosing stolons. How these polyps avoid being covered over by the tube accretion is not discussed by the author. 

As the polyps move to keep up with tube-building activities of their host, the stolons stretch out behind, forming the parallel system seen in the drawing on the Right.  If the gonozooids that are normally sited close to their “mother” gastrozooids fall behind in this movement, they lose their medusa buds, diminish in size, and become dactylozooids.  Campbell 1968 Biol Bull 135: 96.

NOTE these are the defensive polyps

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Research study 3
 

photograph of the solitary hydroid Coryphella palmaphotograph of a growing stolon of the hydroid Corymorpha palmaThe solitary hydroid polyp Corymorpha palma anchors to mud substrata by means of a holdfast consisting of elongated stolons. Growth of stolons in hydroids has usually been thought to result from cell division, but studies on Corymorpha specimens collected at Newport Bay, California show that, rather than through cell proliferation, the stolons elongate by being stretched.  The tips appear to move actively across the substratum by means of contractile waves generated in the epithelium and the tissues are pulled along behind them.  The epidermal and gastrodermal tissues behind the tip become stretched into a narrow column only 2-3 cells thick (see photo above). Counts of cells in stolons are the same over large differences in length.  Campbell 1968 Biol Bull 134: 26.

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Research study 4
 

drawing of hydroid Proboscidactyla flavicirrata showing polyps with host worm tube removedThe small polyp Proboscidactyla flavicirrata is an obligate symbiont on the outside rims of tubes of sabellid worms.  As shown in the drawings the colonies consist of 4 types of individuals, gastrozooids, dactylozooids, gonozooids, and attached medusae.  There are 8 medusa buds to each gonozooid, each bud being at a different stage of development.  Gastrozooids bear only 2 tentacles and they generally sit on the tube rim with their mouths facing towards the host worm.  A gastrozooid feeds by collecting material from the food grooves of the host worm on its tentacles, then wiping the tentacles through its mouth.  Tentacles can extend up to 25 times their contracted length and so can reach some distance into the worm’s food grooves.  Food obtained in this way consists mostly of unicellular algae and some larval crustaceans.  While nematocysts are not used in the type of feeding just described, the polyps are able to capture larger prey such a copepods, nematodes, and eggs, sometimes of the host worm, using their nematocysts.  Spencer 1974 Biol Bull 146: 100.

NOTE  common hosts in the collection area of southern Vancouver Island, British Columbia and San Juan Island, Washington are the sabellid worms Schizobranchia insignis and Pseudopotamilla ocellata

NOTE  counts of medusa buds on P. flavicirrata seem to vary...either 4 or 8...depending on the author

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