title for learn-about section on cnidae  
  Cnidae (nematocysts & spirocysts)
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Research study 1

Cnidae are the means by which sea anemones and other cnidarians capture prey and defend against predators.  Nematocysts have double-walled capsules and can be stinging or tangling depending on the type of cnidarian.  Another type of “adhesive organelle” found only in sea anemones is the spirocyst.  Spirocysts have single-wall capsules, discharge branched or single threads, are non-penetrating and non-venomous, and contribute mainly to the adhesive stickiness of a sea-anemone tentacle to both animate and inanimate objects.

NOTE  lit. “nettles” G.  There are many types of cnidae: up to 30 or more if you are a “splitter”, or as few as 17 or so if you are a “lumper”.  There may also be several different names used for the same basic cnida type.  Until someone sorts it all out, the ODYSSEY will be faithful to names used in a specific publication

NOTE  the most toxic of several types of nematocysts in sea anemones is a type known as microbasic-p-mastigophores. They are often called penetrants because their discharging threads pierce the flesh of prey rather than wrapping around and tangling

drawings of different types of cnidae of cnidarians
Cnidae of cnidaria. The 2 main types found in sea anemones are: spirocyst (top, yellow) and microbasic-p-mastigophore (bottom, blue). The first type is sticky and non-toxic; the second, penetrating and toxic 1000X. From Hyman 1940 The invertebrates: Protozoa through Ctenophora p384. McGraw-Hill Book Co., Inc. NY

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Research study 2


Cnidae are not distributed evenly in the body of a sea anemone, nor are they restricted to the tentacles or mesenterial filaments.  Some types are located on the body column and lining of the pharynx.  Cnidae in Urticina piscivora and U. lofotensis include: 1) spirocysts, located abundantly on the tentacles; 2) penetrants (microbasic-p-mastigophores nematocysts), found commonly in the pharynx and mesenterial filaments, but absent in the tentacles and rare on the body column; and 3) 5 types of beta-rhabdoides (basotrich nematocysts), found on the tentacles, pharynx, mesenterial filaments, and body column.  The basotrichs, shown in the central section of the diagram on the Right, are all penetrating types and are toxic.

Capture of food by the tentacles relies mainly on the basotrichs and the spirocysts.  The authors make no mention of any significant difference in cnidae allocations in the 2 species that would explain the fish-catching capability of U. piscivora but not U. lofotensis.  They do note that the mesenterial filaments of the latter species contain larger microbasic-p-mastigophores than the former species.  Final subduing of live prey relies on highly toxic nematocysts in the mesenterial filaments. Sebens & Laakso 1977 The Wasmann J Biol 35: 152.

drawings of cnidae found in sea anemones Urticina lofotensis and U. piscivora
photograph of a sea anemone Urticina lofotensis

Sea anemone Urticina lofotensis,
with tentacles partly contracted 0.3X


Fish-eating anemone
Urticina piscivora

photograph of a sea anemone Urticina piscivora
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Locations of nematocysts and spirocysts on a sea-anemone tentacle are often visible as raised bumps or white spots.  They are present on the regular capture tentacles and (in some species) on special “warrior” tentacles (see drawing far Right), and on mesenterial filaments inside the gut cavity (see plaster model near Right). 

NOTE  these are thread-like, extended edges of the mesenteries that hang free within the gastrovascular cavity in some anemones, also known as acontia. They function to entangle and kill ingested prey. In some species the acontia can be extruded via pores in the body column or mouth where they serve in defense. Such sea anemones are often referred to as being acontiate (in the Order Actiniaria). Acontiate forms make up only a small portion of west-coast species, with Metridium being the most commonly encountered genus

plaster-cast representation of the inner parts of a sea anemone

drawing of warrior tentacles or acrorhagi of a sea anemone Anthopleura elegantissima

"Warrior" tentacles, or acrorhagi, being displayed by Anthopleura elegantissima
NOTE  these are more like swollen bumps than tentacles and are found in only a few species (e.g., Anthopleura elegantissima).  They are located just under the row of longer capture tentacles. They are also known as acrorhagi (lit. “topmost” “grape” G.)

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The cnidae are produced in special cells of the body column known as cnidoblasts. Once the nematocysts are fully formed the cells, now known as a cnidocytes, migrate to locations on the tentacles or mesenterial filaments for use.  Manufacture of a sea-anemone microbasic-p-mastigophore nematocyst takes about 5h and starts with formation of a vacuole within which a capsule is secreted. This is followed by formation of a thread from elements of the Golgi apparatus. The thread is formed outside the capsule, then somehow enters it and becomes coiled. The capsule then elongates to its mature form. On discharge of a penetrant nematocyst, which usually requires simultaneous exposure to both chemical and physical stimuli, the internal thread explodes through a rupture in the end of the capsule, and the thread rapidly everts until it penetrates the flesh of a prey or predator.  The pressure of water taken up by a protein material within the capsule is the force driving the thread outwards.  The toxin is driven down the hollow thread until it blows out the tip within the prey’s flesh.

first in a series of drawings showing the operation of spirocysts and nematocysts of a sea anemone to capture a small crustacean
drawings of stages involved in the formation of a nematocyst in a sea anemone second in a series of drawings showing the operation of spirocysts and nematocysts of a sea anemone to capture a small crustacean

The toxin is what creates the sting and, if not leading to immediate death of the prey, may incapacitate it enough to allow it to be eaten.  Many thousands of nematocysts may discharge into a single prey.  Once discharged, the nematocysts are useless and are resorbed by the sea anemone. Kass-Simon & Scappaticci 2002 Can J Zool 80: 1772.

NOTE  lit. “nettle bud” G.
NOTE a more detailed description of nematocyst discharge is found elsewhere in the ODYSSEY: LEARN ABOUT HYDROIDS: PREDATORS & DEFENSES

third in a series of drawings showing the operation of spirocysts and nematocysts of a sea anemone to capture a small crustacean
In the sequence on the Right, the dragging leg of a passing amphipod provides the stimuli necessary
for discharge of 2 spirocysts. The threads of the spirocysts contract, drawing the amphipod
close enough to trigger discharge of a penetrant nematocyst. This indicates a level of cooperativity
between the 2 types of cnidae: the spirocysts assist the nematocysts in discharging into the prey
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Research study 5

Adhesion of a nematocyst-penetrable object, such as a crustacean prey, to the tentacle of a sea anemone appears to be a combination of 1) adhesion of the nematocyst threads to the prey, and 2) adhesion of the nematocyst capsule to the tentacle.  However, another source of stickiness is the mucus covering the tentacle.  So, with all these elements operating, what principally accounts for the adhesion of a prey to the tentacle?  Tests using nylon beads coated with gelatin and direct measurements of stickiness of each of the above factors in sea anemones Aiptasia pallida in Corona del Mar, California show 2 interesting things.  First, the contribution of spirocysts to overall adhesion of a test bead is insignificant, and second, the main adhesive force of tentacle to a penetrable prey is a combination of discharged nematocysts and mucous stickiness.  

schematic drawing of mechanism of attachment of an object to a nematocyst thread


Schematic showing the 3 main elements causing a prey
(or gelatin-coated nylon bead) to stick to an anemone's
tentacle. Of these, nematocyst-thread adhesion and mucous
stickiness are the main attachment forces. Of course, the
adhesive strength of capsule attachment to the tentacle
may ultimately govern whether a prey will rip free. Each cnida
is shown with its attendant companion (supporting) cells

drawing showing numbers of nematocysts required to kill crustacean prey of different sizes The authors suggest that the nematocysts and spirocysts have complementary roles, but that their level of interactivity depends on the nature of the prey. Thus, nematocysts predominate with soft-bodied prey that they can penetrate, while spirocysts predominate when the target surface is hard enough for the spirocysts to adhere, but too hard for the nematocysts to penetrate.  The authors also speculate that the spirocysts may assist in prey capture by tangling and interfering with the swimming appendages of the prey. How much force is exerted by a small swimming crustacean like the nauplius of a brine shrimp Artemia salina, and how many nematocysts and/or spirocysts would it take to secure a single, struggling individual?  Based on motion analysis of swimming Artemia nauplii (each 0.01mg live mass), the researchers estimate that a force of 0.2 µN is exerted per power stroke.  Since a single microbasic-p-mastigophore (penetrant) nematocyst can withstand forces of 1.5-4.4 µN (authors’ estimates for Aiptasia), then only one nematocyst is necessary (see top row in cartoon on Right).  In comparison, one spirocyst is not strong enough to tether a struggling nauplius unless it also fouls a primary swimming appendage.  For capture and retention of a more massive prey, such as Cyclops (1mg live mass), up to 5 nematocysts may be necessary (see bottom row in cartoon).  However, it is unlikely that spirocysts alone would be capable of doing the job without the involvement of nematocysts. . Thorington & Hessinger 1990 Biol Bull 178: 74; Thorington & Hessinger 1996 Biol Bull 190: 125.

NOTE  in this species spirocysts are about 3 times more numerous than nematocysts on the tentacles

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Research study 6
schematic drawing showing possible mode of action of receptors on companion cells of nematocysts leading to discharge of the nematocysts
Stimulation of receptors on the supporting (companion) cells by molecules of N-acetylated sugars from appropriate crustacean prey initiates lengthening of the stereocilia. These touch and "tune" the mechanoreceptor on the cnidocyte (the central cell in the above array) to the particular vibration frequency of the prey. The nematocyst is now ready to discharge when the prey touches the mechanoreceptor

Studies on the process of nematocyst discharge in southern California anemones Haliplanella luciae show preferential discharge into targets vibrating at specific frequencies (30, 55, and 65-75 Hertz1).  In the presence of sub-micromolar concentrations of N-acetylated sugars2 and other nematocyst “sensitisers”, these vibration optima shift to lower frequencies (5, 15, 30, and 40 Herz) corresponding approximately to vibrations of small, edible swimming planktonic crustaceans.  graph showing freqency of vibrations of small, struggling crustacean prey perceived by a sea-anemone's nematocystsThe graph shows a power spectrum3 of a swimming adult brine shrimp with these 4discharge frequencies indicated by arrows.  To explain the shift in discharge optima, the authors hypothesise that each cnidocyte has adjacent companion (supporting) cells with surface receptors sensitive to chemical emanations specifically from crustacean exoskeletons (see drawing on Right).  Stimulation of the receptors initiates extension of stereocilia from the companion cells (up to 70% increase in length).  These are thought to “tune” (by touch or close proximity) the mechanoreceptor (a single cilium) on the cnidocyte to be responsive to frequencies that match movements of small (i.e., edible) swimming crustaceans. If the “wrong” crustacean happens by, for example, one that is crawling or swimming but too large to eat, it may set off the proper chemoreceptor cascade, but the mechanoreceptor will be tuned to the wrong frequency to elicit discharge.  It is not clear what time scale is proposed for these transformations, but it must be relatively short if the anemone is to take advantage of the transient presence of an edible swimming prey.  The theory is a nice one because it raises the possibility that cnidocytes of other cnidarians are “in tune” with the special swimming vibrations of their own preferred prey.  Moreover, for a given cnidarian, different chemoreceptor cascades may be involved in tuning their mechanoreceptors to the swimming frequencies of different prey species. Watson & Hessinger 1989 Science 243: 1589.

NOTE1  1 Hz is one cycle (vibration) per second

NOTE2  N-acetylated sugars are components of crustacean cuticles; other sensitisers known include mucin and certain amino acids

NOTE3  the power spectrum for a swimming brine shrimp shows maxima at 2, 12, 19, 30, 38, and 60 Hz (the last an artifact caused by electrical noise), approximating the discharge frequencies measured experimentally (listed above)

Research study 7
graph comparing scaling of cnidae capsule lengths with body mass in 3 species of sea anemones, Anthopleura elegantissima, A. xanthogrammica, and Urticina crassicornis

Interestingly, larger sea anemones produce larger cnidae.  The effect is shown for populations of Anthopleura elegantissima, A. xanthogrammica, and Urticina crassicornis examined in locations in Charleston, Oregon and in San Juan Islands, Washington.  In each case, significant scaling of spirocyst-capsule length versus live mass of body is shown for preparations from tentacles and acrorhagi1 (see accompanying graph).  The effects are relatively small2 , ranging only from 0.2-0.7-fold increases in capsule length over 100-fold increases in live mass in the 3 species, but are nonetheless significant. The author explains the change in size as a natural outcome of increased cell size generally as overall body size increases, leading to production of larger-sized cnidae.  Thus, cell size in sea anemones is reported3 to scale with exponents of 0.02-0.2, while spirocyst size as shown in the present study is shown to scale with exponents ranging from 0.02-0.09, which is in close agreement.  As noted by the author, cnidae cannot be larger than the cells enclosing them.  Is there any functional significance to possessing larger-sized cnidae?  Larger anemones are capable of catching larger-sized prey or defending against larger-sized competitors, and perhaps enhanced weaponry (more toxin per discharge) is useful.  The author notes that this is the first detailed study of microscaling of sea-anemone cnidae.  Francis 2004 Biol Bull 207: 116.

NOTE1  this only occurs in the tentacles for U. crassicornis, as acrorhagi are apparently lacking

NOTE2   the predicted isometric scaling relationship for log length over log mass is 0.33, but note that the author has not presented the ordinant axis data (capsule length) on a log scale.  Slope values obtained in the present study range from 0.02-0.09, which are reported by the author to be close to those found for scaling of cell size against body size in sea anemones, but it is not known whether these other studies also used a semi-log plot as done here

NOTE3   works cited by the author of this Research Study

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