title for learn-about section of A SNAIL'S ODYSSEY
  Predators & defenses
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Research study 1
 

Despite the presence of both distasteful chemicals and nematocysts in hydroids, many species of aeolid nudibranchs find them quite tasty as prey.  Are the hydroid’s defenses completely ineffective?  The answer seems to be ‘yes’ for the hydroid’s regular prey-catching tentacles which contain small-sized nematocysts, but ‘no’ for special tentacles known as cnidophores, which contain especially large-sized and potent nematocysts.  Studies on several photo of a polyp of the colonial hydroid Eudendrium ramosum showing highly toxic cnidophore tentacleMediterranean species show that if the cnidophores are touched by the cerata of a nudibranch, within seconds there is a massive discharge of nematocysts.  These adhere to the skin of the ceras and may cause localised destruction of tissues. Martin & Walther 2002 J Mar Biol Assn UK 82: 455. Photo courtesy the authors.

NOTE  lit. “nettle” “carry”. Cnidophores are special large tentacles originating from the bases of about one in 5 polyps of the colonial hydroid Eudendrium ramosum (see photo on Right). The cnidophores bear extremely potent nematocysts, holotrichous isorhizas, in densities about 3 times more than regular tentacle nematocysts. The study cited above deals with E. ramosum colonies in Europe, but the same species is attacked and consumed by aeolid nudibranchs Flabellina iodinea on the west coast. It is not known whether west-coast E. ramosum colonies have cnidophores

NOTE  lit. “horns”; sing. ceras, elongate structures on the upper surface of aeolid nudibranchs that contain extensions of the digestive gland.  The multiplication of the gland in this way provides relatively more surface area for digestion and absorption of nutrients

   
  Examination of a nudibranch's feces after a meal of hydrods discloses large numbers of both discharged and undischarged nematocysts, so the predator is definitely exposed to firing nematocysts as it attacks and eats its prey. The photographs below show several nudibranch species preying on or interacting with west-coast hydroids:
 
photograph of aeolid nudibranch feeding on a Corymorpha sp. hydroid
Aeolid with hydroid Corymorpha sp. The predator's mouth is at 11 o'clock. The white dots are the aeolid's eggs 4X
photograph of Flabellina picta eating hydroids Tubularia sp. and Eudendrium californicum courtesy Jeff Goddard, UC Santa Barbara
Flabellina picta eats hydroids Tubularia and Eudendrium 2X. Photo courtesy Jeff Goddard, UC Santa Barbara
photograph showing nudibranch Doto columbiana crawling on a hydroid Aglaophenia sp. courtesy Jeff Goddard, UC Santa Barbara
Doto columbiana crawls on hydroid Aglaophenia sp. 13X. Photo courtesy Jeff Goddard, UC Santa Barbara
 
photograph showing sunflower star Pycnopodia helianthoides avoiding the stings of a hydroid
Sunflower star Pycnopodia helianthoides avoids the stings of a hydroid (located Left of the arm located at 6 o'clock)
photograph of nudibranchs Flabellina eating a solitary hydroid Corymorpha sp. courtesy Kevin Lee, Fullerton, CA
Several Flabellina eat a solitary hydroid, possibly Corymorpha palma 1X. Photo by Kevin Lee, Fullerton, CA
photograph of aeolid nudibranch Flabellin about to eat a hydroid polyp
Aeolid nudibranch Flabellina about to eat a hydroid polyp 3X
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Research study 2
 

photograph of the hydromedusan Aequorea victoriaThe anthomedusan Stomotoca atra and leptomedusan Aequorea aequorea are common components of the plankton in Puget Sound, Washington and each is predatory on other medusae.  Experiments at Friday Harbor Laboratories, Washington show that S. atra, the smaller of the 2 species, when contained with A. aequorea, will cease swimming and drop downwards in the water column.  The inhibition of swimming can also be produced by pieces of isolated mesoglea and mesogleal extractsfrom A. aequorea.  The causative agent is heat-stable, non-toxic, and is present in other medusae.  These other medusae seem not to be inhibited by extracts from A. aequorea. When living A. aequorea are used, the inhibition occurs after several minutes, but when a piece of mesoglea or mesogleal extract is used, the inhibition takes only a few seconds, the response time depending upon size of piece and extent of dilution, respectively.  In the last instances, the response may last for 8h or more.  Generally, the inhibition is completely reversible.  Although the author is unwilling to assign a function to the response until more is known about the behaviour of these organisms in nature, it seems possible that it is defensive.  Lenhoff 1964 Biol Bull 126: 115. Photograph courtesy Chris Gunn, North Island Explorer, Campbell River, British Columbia.

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Research study 3
 

photograph of hydromedusan Aglantha digitatle with its manubrium stuffed with preyThe medusa of Aglantha digitale exhibits 2 swimming modes, one a typically slow pulsing; the other, a much faster mode that may be an escape response.  Studies in Victoria, British Columbia show that the nervous system of Aglantha is divided into 3 parts: 1) an inner nerve ring, 2) an outer nerve ring separated from the inner one by a layer of mesoglea, and 3) a group of giant axons each 20µm in diameter that joins the 2 rings and extends over the surface of the contractile tissues used in swimming.  The contractile tissues, or myoepithelia, arae present in the velum, subumbrella surface, and along the radial canals, and are made up of unusually large cells and large mitochondria.  The large diameters of the axons suggests a possible 10-fold faster conduction velocity of impulses involved in the escape response.  A slowly pulsing individual responds to tactile stimulation by a sudden and rapid shooting forward equivalent to several body lengths.  A repeat response can be evoked by a similar stimulus after only a few seconds. The author considers that normal slow swimming is controlled by a mechanism similar to that found in other medusae, while the escape response results from action of the giant axons.  Singla 1978 Cell Tiss Res 188: 317.

NOTE  this author and others use the terms muscles and myoepithelia (contractile tissues) interchangeably, but “muscles” in their strictest definition are derived from mesoderm and are found only in advanced animals, or triploblasts.  Mesoderm is not found in hydroids, which are diploblasts

 

 

Hydromedusa Aglantha digitale (?) with
its manubrium stuffed with prey 40X

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Research study 4
 

drawing of hydromedusan Aglanthe digitale showing locations of giant axons used in escape swimmingFurther detail on fast swimming of medusae of Aglantha digitale is provided by researchers at Friday Harbor Laboratories, Washington.  Their findings reveal that the escape response is mediated by a giant ring axon of 35µm diameter that circles the bell margin.  Eight giant axons run from the margin up the inside of the bell.  Additionally, large axons run the length of each tentacle and within the tentacles are unique longitudinally oriented myoepithelial cells. The effectiveness of the response is explained by the speed of the nerve impulses, which produce a nearly synchronous contraction of the series of drawings showing a swimming pulse in the hydromedusan Aglantha digitale as might be ilicited from contact with a predatorcircular contractile tissues lining the inner surface of the bell and velum. The series of drawings on the Right are taken from stroboscopic cinematography, and depict an escape contraction and part of a subsequent refilling phase following contact with a larger medusa Aequorea aequorea, a species known to capture and eat AglanthaRoberts & Mackie 1980 J Exp Biol 84: 303; Donaldson et al. 1980 Can J Zool 58: 549

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