Research Study 1: Acontia/nematocysts

Fig. 1.  Metridium senile extruding acontia from pores in the body wall 

Fig. 2.   Comparison of acontial lengths in Metridium senile and Metridium farcimen

Fig. 3.  Comparison of nematocysts in acontia and tentacles in the anemones Metridium senile and M. farcimen

Studies in Anacortes, Washington show that lengths of both acontial threads (Fig. 1) and acontial nematocysts in the anemones Metridium farcimen and M. senile scale significantly with increasing body size, indicating a selection for more damaging acontial defenses in larger-sized anemones (Fig. 2). The magnitude of size increase is not large, only about 27% over a 750-fold increase in body mass for M. farcimen, and 18% over a 70-fold increase for M. senile.  The authors additionally note that scaling exponents are significantly smaller for nematocysts in the capture tentacles than in the contia.  They explain this by the fact that food size of Metridium does not increase substantially with body size, and so a comparable scaling in nematocyst size would not be expected in the capture tentacles. Note also the considerably larger size of acontial nematocysts as compared with tentacle nematocysts (Fig. 3). When the authors compare same-sized individuals of M. farcimen (a subtidal species) and M. senile (intertidal/shallow subtidal), they find significantly larger acontial nematocysts in the former species. They explain this on the basis of greater predation pressure on M. farcimen in its subtidal habitat, although the justification for this argument is not strong.  However, as noted by the authors, more acontial threads, longer acontia, more nematocysts, and larger acontial nematocysts in larger individuals should improve deterrence potential against predators.

NOTE apparently, holding the anemone upside-down out of water causes it to extrude its acontia, which then hang down by gravity and can be measured

NOTE the scaling exponents for acontia length in the two species range from 0.45-0.60, which is significantly greater than the value of 0.33 expected for isometric scaling of length against body size, and thus is positively allometric.  In comparison, the scaling exponents for nematocyst length against body size in the acontia range from 0.04-0.05, which is far below an expectation based on isometric scaling.  Presence of allometric scaling in biology usually means that something significant is going on, although what this is may not always be obvious

Research Study 2: Acontia/nematocysts

A study with possible relevance to defenses of sea anemones notes that aqueous dialysed extracts of several west-coast species when tested by intraperitoneal injection into laboratory mice have the following levels of toxicity:

Higer toxicity: Anthopleura elegantissima and A. xanthogrammica.  A dose of 2ml extract per 100g of mouse kills the mouse in10-20min.  
Lower toxicity: Metridium senile, Corynactis californica (actually a corallimorpharian), Urticina crassicornis, U. lofotensis, and U. coriacea.  The same dose of the first species listed (M. senile), in comparison with the “higher toxicity” data above, kills the mouse in about 170min.  Other than demonstrating that different sea anemones have different toxicities to mice, the functional significance of the study is not clear.  

NOTE  this removes larger molecules such as proteins, as found in nematocyst toxins, and amines

NOTE  to justify its inclusion in this part of the Odyssey, we can assume that at least some toxins from nematocysts are included in the extracts

Research Study 3: Acontia/nematocysts

Fig. 1.  Predatory nudibranch Aeolidia papillosa crawls near to a sea anemone Metridium senile preparatory to attack

In the Gulf of Maine a common prey of the nudibranch Aeolidia papillosa is Metridium senile.  The anemone defends itself by extrusion of acontia through pores in its body wall.  These entangle with the cerata of Aeolidia resulting sometimes in death.  Laboratory studies where Aeolidia are offered large groups of mixed sizes of Metridium as prey show that preference by Aeolidia is principally for smaller-sizes.  Apparently, the acontial nematocyst defenses in small anemones are much less potent than in larger anemones.  Small anemones are attacked and eaten.  Large anemones are attacked and lose some tissue mass to the predator, but usually survive.  The author suggests that this size-related predation by Aeolidia skews the distribution of natural populations of M. senile to the large end of the size range.

Research Study 1: Crawling/release of attachment

Fig. 1.  Release of attachment by an unidentified anemone species. after being attacked by a leather star Dermasterias imbricata. Time elapsed from 1st to 4th image is 3min.

The leather star Dermasterias imbricata is a predator of sea anemones. Studies at the Bamfield Marine Science Centre, British Columbia show that an anemone polyp (not identified by the authors) will, on contact with the asteroids Dermasterias imbricata and Patiria miniata, expand their oral discs, constrict their columns, and detach their pedal discs (this take about 30ec, on average). Other asteroid species do not elicit this response.  Of five species of Urticina tested by the authors, only U. piscivora shows similar behaviour, and only to Dermasterias.  Re-attachment occurs within minutes when the pedal disc contacts a surface to which it can adhere.  In that the pedal-disc release is mediated by a train of electrical pulses in the slow conduction system, the behaviour appears to be similar to release of attachment and swimming in Stomphia spp. 

NOTE a non-nervous conducting system mediated by cells in the outer epithelial layer of the body.  Anemone species commensal with hermit crabs are induced to relax and detach from the substratum by the potential host gently stroking the column with its chelae.  An identical response can be induced in some west-coast anemones, such as Anthopleura elegantissima, by gentle stroking the column with glass rod or even chopsticks.  This last is a behaviour in a non-commensal species that has been observed but not carefully studied 

NOTE for a description of swimming in Stomphia, see the following section on SWIMMING

Research Study 2: Crawling/release of attachment

Fig. 1.  Because Urticina piscivora is virtually weightless in water it would likely be carried away in even small current velocities

Later studies on Urticina piscivora at the Bamfield Marine Sciences Centre, British Columbia show that small individuals when attacked by a leather star Dermasterias imbricata will release their attachment to the substratum and float away (Fig. 1). Large anemones show no response, while intermediate-sized individuals sometimes respond, but slowly.  If they survive and re-attach, the down-shore tumble may place them in spatial refuge, out of contact with shallow-water-dwelling leather stars.  Tests with 19 other seastar species yield negative results, as do tests with four other Urticina species.

Research Study 3: Crawling/release of attachment

Fig. 1.  Time for detachment in Urticina piscivora in relation to past feeding regime

Interestingly, the detachment response in sea anemones is not simply an “on or off” one, but is modulated by factors such as past feeding history.   Fig. 1 shows that better-fed Urtricina piscivora take longer to detach, but whether this owes to their greater health and resilience, or to a sense by the starved ones that the spot they are in is a poor one anyway, is not known.  Anemones are known to crawl about less if dietary conditions are good.

Research Study 4: Crawling/release of attachment

Fig. 1.  Great green anemone Anthopleura xanthogrammica escapes by detachment on contact with predatory nudibranch Aeolidia papillosa

On contact with the nudibranch Aeolidia papillosa, the green sea anemone Anthopleura xanthogrammica may sometimes inflate its column and detach itself from the substratum to escape being eaten (Fig. 1). 

Research Study 1: Swimming

Fig. 1.  Stomphia coccinea swimming from contact with predatory sea star.  Note the "cone" extending from the pedal disc

Fig. 2.  Swimming response of Stomphia coccinea to contact with predatory sea star Dermasterias imbricata.  Is the cone a swelling that helps lever the pedal disc off the substratum?

The first description of swimming of anemones in response to predator stimulation appears to be that by researchers at the University of Washington, Seattle.  Apparently, an incidental contact of Stomphia coccinea with a sea star, one of Dermasterias imbricata, Hippasteria spinosa, or Crossaster papposus, initiated swimming (Fig. 1) and prompted later study.  First, several other common sea-star predators and various ophiuroids fail to induce swimming.  Contact initiates column extension and whirling within a few seconds (Fig. 2), the last likely a strategy to remove contact with the predator.  Swimming in the laboratory lasts only a few moments.  Note in both figures the appearance of a conspicuous papilla-like structure or cone on the base of an anemone prior to swimming, thought to be important in facilitating quick detachment.  Swimming terminates with the animal resting on its side, followed shortly by re-attachment and righting.  The authors comment that the swimming response is "all or none", never partial. 

Research Study 2: Swimming

Fig. 1.  Stomphia didemon 

One of the more dramatic responses by a sea anemone to a predator is the quick and vigorous swimming induced in Stomphia spp. on contact with sea stars Dermasterias imbricata and Hippasteria spinosa. Later studies at Friday Harbor Laboratories, Washington using individuals collected in Puget Sound and San Juan Islands, Washington reiterate the behavioral sequence in more detail (Figs. 1-6).

NOTE  there are at least two species of Stomphia in Puget Sound, S. coccinea and S. didemon, with a possibly third undescribed species being considered.  The first description of Stomphia coccinea swimming is actually from laboratory observations done in the 1930s, but not in response to sea stars (Stephenson 1935). 

Fig. 1.  Normal appearance is shown by specimen on Left. On stimulation, as from touch of a predator, the body pulls down, and tentacles and oral disc withdraw (specimen on Right)

Fig. 2.  Body then expands slowly, column elongates and becomes turgid

Fig. 3.  Body bends laterally and sometimes rotates, then detaches. Detachment is surprisingly quick as the attachment of the basal disc appears firm.

Fig. 4.  thrashing-, back-and-forth swimming movements for seconds or minutes; distance traveled may be less than a meter

Fig. 5.  Individual comes to rest on its side with oral disc rigid; temporarily inactive and non-excitable

Fig. 6.  Recovery in 1-2min with righting

Courtesy Sund 1958

Research Study 3: Swimming

Fig. 1.  Behavioral sequence involved in swimming in sea anemones Stomphia coccinea.  Start is at the 12 o-clock position

Fig. 2.  'Muscles' involved in swimming in sea anemone Stomphia coccinea.  True 'muscles'  derive in evolution from mesoderm...not yet evolved in these two-layered cnidarians

A researcher at Friday Harbor Laboratories, Washington further describes swimming behaviour in Stomphia coccinea in response to contact stimulation by leather stars Dermasterias imbricata (Fig. 1).  To induce swimming in the laboratory a sea star is touched briefly to Stomphia's tentacles, no longer than 10sec).  This elicits momentary tentacle attachment, followed by swimming behaviour as shown.  The contractile tissues  involved with each main behavioral component are featured in Fig. 2.  The oral sphincter and retractor muscles first contract, shortening the body, then these relax and the body elongates.  Just before the pedal disc detaches, the body may twist vigorously back and forth through alternate contraction and relaxation of opposing sets of retractors.  These same tissues contribute to the side-to-side thrashing during swimming.  They act on the deformable hydrostatic skeleton represented by the water-filled gastrovascular cavity or coelenteron.  At 13^oC the column may bend back-and-forth 30 times in the space of 5min of swimming.  Other tissues involved in swimming are the parietal-basilar and radial (the last not shown in the drawing), also located on the mesenteries.  Contraction of the parietal-basilars raises the pedal disc and provides a physical "kick" to initiate swimming.  These same tissues are primary contributors to the sharp disc raisings so characteristic of the swimming.  Other contractile elements, the circular, also act on the hydrostatic skeleton to cause body elongation.  Some fluid is released from the mouth on initial column contraction, but for the hydrostatic skeleton to work effectively during swimming the oral sphincter remains mostly closed.  The author provides details on these contractions in response to both mechanical touch and contact with sea-star skin extracts, as well as to mild electrical shocks, not included here.  As the muscles described here are present in most or all sea anemones, it is surprising to learn that only a few species actually swim.  Only two west-coast sea-star species induce swimming in Stomphia, with Hippasteria spinosa being the other.  The origin and evolution of swimming in world Stomphia species are unclear.

Research Study 4: Swimming

Fig. 1.  Stomphia coccinea within 12min fully attaches to a live horse mussel Modiolus modiolus in the laboratory.  Note how the pedal disc and tentacles are both involved in the attachment process

Post-swimming Stomphia coccinea appear to settle preferentially onto shells of horse mussels Modiolus modiolus in west-coast areas where this particular substratum is available.  This is evidenced by Stomphia in dredge hauls commonly coming up attached to Modiolus shells.  Indeed, in laboratory tests in which 18 Stomphia are induced to swim by contact with sea stars Dermasterias imbricata and then provided with the choice of flat stones or live Modiolus on which to settle, 11 of the anemones within an hour settle onto the latter (of the others, 5 attach to the glass container and two fail to attach at all).  The results are suggestive, but hardly convincing.  Questions that come to mind are, "was the dredging done in an area well populated with horse mussels?  does the anemone also settle onto other types of living substrata (the scallop Pecten is mentioned in the article as also inducing the same response)?  if so, does it specifically prefer Modiolus over other shelled molluscs?". 

NOTE  in fairness, the report is meant to be just a short note, and were the author to be questioned directly, he might well respond with: "well, now it's up to you to find out...."

Research Study 5: Swimming

In fact, in a later co-researched paper on the same subject, the foregoing author may well have answered these questions, and perhaps others, so let's find out.  This later paper actually concerns both Stomphia coccinea and a new species Actinostola, both of which prefer to attach to shells of horse mussels Modiolus modiolus after being induced to swim in response to stimulation by predatory sea stars Dermasterias or Hippasteria.  Studies at Friday Harbor Laboratories, Washington show that anemones recently settled onto other surfaces will readily reattach to Modiolus shells brought into contact with the tentacles, but not if they have been attached to these other surfaces for longer than about a day.  If the shells are previously boiled in alkali to remove organic matter, the response is lost.  Other new features of the study include low population density of anemones on Modiolus shells, only a handful amongst hundreds dredged up (depth not recorded).  The paper concludes with a reiteration of shell-seeking behaviour in the two sea-anemone species, divided into several phases (Fig. 1 shows the new Actinostola species moving from a tile to a Modiolus shell valve).  In answer to one question raised in the above Research Study, an anemone will move from inanimate substrata to shells of several bivalve molluscs, including mussels, scallops, and cockles quite readily, and also to shells of abalone, limpet, and whelk gastropods, although not so readily.  Despite a lengthy discussion of their results, the authors omit consideration of the anemones' preferences for shell substratum over rock or other inanimate substrata. 

NOTE  this purportedly new species closely resembles the deep water-inhabiting Stomphia didemon, found in the same area of Puget Sound, Washington.  The authors refer to it as a new species of Actinostola, identified as such by Dr. Charles Cutress, at that time a cnidarian specialist in the Department of Marine Sciences, University of Puerto Rico, Mayaguez.  A report seems never to have been published, however, and it was not until 1973 that the new species was officially described as Stomphia didemon (Siebert 1973 Pac Sci 27: 363).

NOTE  although the treated Modiolus shells are neutralised and well cleaned before being tested, can one be sure that it isn't some residual part of the alkali treatment that is destroying a shell's attractiveness?  Should naturally cleaned shells (i.e., free of periostracum) on the shore not have also been tested?

Fig. 1.  Four views of a Stomphia didemon moving from an inanimate object to a living Modiolus sp. shell over a period of about 30min

Research Study 6: Swimming

A sure swimming response by Stomphia coccinea comes from contact with only two species of sea stars, Dermasterias imbricata  and Hippasteria spinosa (Figs. 1 - 2), and from contact with the nudibranch Aeolidia papillosa, where responses are 95-100% positive (see Table).  Sometimes, contact with blood stars Henricia leviuscula (3%; Fig. 3), rose stars Crossaster papposus (2%; Fig. 4), , and sun stars Solaster dawsonii (5%) give positive responses.  Contact with 13 other sea-star species around Puget Sound, Washington elicits no responses.  Of the many sea stars tested, only Dermasterias and Hippasteria are known predators of Stomphia.

Fig. 1.  Leather star Dermasterias imbricata 

Fig. 2.  Sea star Hippasteria spinosa

Courtesy Randy Shuman, Seattle

Fig. 3.  Blood star Henricia leviuscula

Fig. 4.  Rose star Crossaster papposus

Research Study 7: Swimming

Fig. 1.  At rest, the pedal disc is large; while swimming, it is small and has a prominent cone-like extension.

Researchers from the University of Alberta, Edmonton working at Friday Harbor Laboratories, Washington determine that the pedal disc of Stomphia coccinea is by no means passive in the swimming process.  For an organ that attaches firmly to the substratum when at rest, detachment during escape swimming occurs quickly.  Observation of anemones from below through transparent plastic shows at the moment of detachment deep ridges forming along the radial lines of the mesenteries that reduce the contacting surface area by about half.  The mesenteries shorten from above, thus breaking the adhesive sole and lifting the disc.  Simultaneously, in a process not well understood, the adhesive substance binding the remainder of the disc to the substratum dissolves.  At the time of release and preparatory for swimming, the pedal disc narrows considerably and an unusual cone-like extension appears (Fig. 1), presumably from hydrostatic pressure inside the animal.  The authors at first thought the cone may act to spring the pedal disc free of its attachment, but observations with anemones positioned on plastic bearing a central round window to accommodate the extended cone shows that swimming is initiated with no pushing involvement of the cone.  Interestingly, through extensive muscle contraction the basal end of the anemone becomes much denser than the oral end (by a factor of four), perhaps helping to maintain proper orientation during swimming and later re-attachment.  As swimming is initiated, a mucous envelope is shed from the basal region, likely the remains of an adhesive layer.  At the end of a swimming bout the disc becomes quite sticky and, concurrently, there is a general discharge of nematocysts from the disc epithelium.  These are thought by the authors to aid in re-attachment. 

NOTE  this observation, seemingly ignored by later researchers, supports the idea of a neural control of nematocyst discharge in cnidarians.  The idea is reinforced by the fact that tentacular nematocyst-discharge is largely inhibited during swimming.  Surprisingly, the nematocysts released from the basal disc are microbasic-p-mastigophores, a potent stinging type, rather than spirocysts, a non-venomous sticky type.  Rather than being a binding agent as suggested by the authors, then, could the release of these toxic stinging nematocysts during re-attachment be a "landing-area clearing" strategy?

Research Study 8: Swimming

Fig. 1.  Sea anemone Stomphia sp.

The swimming contractions in Stomphia spp. (Fig. 1) are initiated and regulated by a pacemaker system of nerves that form a ring around the column.  Interestingly, excision experiments on the column of Stomphia coccinea show that while the same pacemaker system functions to control swimming after attack by sea stars Hippasteria and sea slugs Aeolidia papillosa, different sensory pathways are involved – each specific to that particular predator. 

NOTE  a European species Hippasteria phrygiana is used in this particular study, which is done on Stomphia specimens in Denmark

Research Study 9: Swimming

Fig. 1.  Leather star Dermasterias imbricata 

Studies at Friday Harbor Laboratories, Washington show that if an object such as a pipe cleaner is rubbed on the aboral surface of the leather sea star Dermasterias imbricata (Fig. 1) and then touched to a single tentacle of Stomphia, it will cause the anemone to swim.  However, if Stomphia’s oral disc is flooded with food extract prior to the touch, the swimming response is inhibited.  Moreover, nematocyst discharge in Stomphia virtually ceases during swimming, from the moment the pedal disc is released to when it re-attaches.  Complete recovery of sensitivity requires about 20-60min. The authors point to their finding as evidence that nematocysts, rather than being independent effectors, are themselves subject to control systems operating elsewhere in the body. 

NOTE in this case an aqueous extract of crushed scallop

NOTE  in addition to this nematocyst response, the authors confirm the findings of other researchers that Stomphia is generally insensitive to handling and other stimulation during, and for some time after, swimming

Research Study 10: Swimming

When different tissues and/or exudations of leather stars Dermasterias imbricata are homogenised and spun down, then injected from a syringe onto Stomphia coccinea, only skin from the aboral surface, mucus, and coelomic fluid initiate swimming.  Later studies show that the effective agent is an alkaloid, given the name imbricatine, which is of a type formerly known only from plants.  Even in extremely low concentrations (e.g., 50ng) it is effective in making S. coccinea swim, but has much less effect on S. didemon and none at all on other anemones such as Urticina piscivora.  Preliminary evidence suggests that the chemical in Hippasteria spinosa that causes Stomphia to swim is actually different from imbricatine, suggesting that the responsive prey anemones must have finely tuned chemical recognition systems.

NOTE  in a later publication, researchers at the University of British Columbia report the structure of imbricatine, a novel benzyltetrahydroisoquinoline alkaloid (Pathirana & Andersen, 1986)

NOTE  nanogram: one billionth of a gram, or 10^-9 g

Research Study 11: Swimming

Fig. 1.  Close view of aboral surface of Dermasterias imbricata showing clusters of dermal papulae and the madreporite (far Right)

Discovery of numerous secretory cells in the dermis layer of the aboral skin-surface of Dermasterias imbricata (Fig. 1), but not on other parts of the body, and a similar chemistry of the secretory substance contained within them with the alkaloid just described, suggests that these cells may be the source of the stimulatory substance imbricatine.  Whether the secretion serves a defensive role in the sea star is not known, but the author notes that the thickened dermis is otherwise lacking in defensive spines and pedicellariae, and so the possibility exists. 

Research Study 12: Swimming

Studies at the Bamfield Marine Sciences Centre, British Columbia (see Table for results) show that a deeper-living species Stomphia didemon (seldom found shallower than 25-30m depth) will swim from sea stars, with three species in the Order Valvatida, namely, Hippasteria spinosa (97% swimming response), Dermasterias imbricata (90), and Patiria (Asterina) miniata (90), inducing strongest responses. Five other sea-star species produce intermediate responses: Poraniopsis inflata (26%), Mediaster aequalis (14), Solaster stimpsoni (33%), S. dawsoni (10), and S. endeca (17). Nine other species induce no or only little response: Pycnopodia helianthoides (3), Crossaster papposus, Pteraster tesselatus, Henricia leviuscula, Evasterias troschelli, Orthasterias koehleri, Stylasterias forreri, Pisaster ochraceus, and P. brevispinus.

NOTE tested in the lab, 10-30 trials each, with “swimming” defined as detaching within 1-2min. "Degree of response" is an arbitrary classification ranging from 0 = no response, to 5 = detachment & swimming. Degree of response differs significantly between the three asteroid Orders, Valvatida (mean of 4.1), Spinulosida (1.8), and Forcipulatida (0.7).   More recent classification places Solaster spp., Crossaster papposus, and Pteraster tesselatus in Order VELATIDA, leaving only the blood star Henricia leviuscula (of the list presented) in Order SPINULOSIDA. Also, the bat star Asterina miniata is now reclassified as Patiria miniata. The phylogenetic affinity of the swimming response is considered later in this section 

Research Study 13: Swimming

Is it possible that repeated contact with the more shallow-living Dermasterias imbricata could force Stomphia spp. to deeper depths?  Staged interactions on rocky slopes in Barkley Sound, B.C. suggest that the answer is "yes", at least for the deeper-living S. didemon.  In one experiment, investigators move four individuals of this species from deeper water to an area at 10m depth inhabited by Dermasterias.  At two intervals over the following week they touch the sea stars to the sea anemones, each time causing the anemones to swim and settle at a lower level.  Although the net vertical displacement is only 2m, the experiment shows that it could effect considerable vertical separation over time.

NOTE  with great respect for the difficulty of conducting in situ experiments using SCUBA, especially at depth, one would have to conclude that this experiment is rather weak.  Apart from small sample size and conducting the experiment in shallow water after disturbing and relocating the four individuals from their normal deep-water habitat, we have to ask the question, "why is the shallow species S. coccinea not included in the experiment?"  Also, the experiment is conducted on an underwater slope that favours downward rolling at the completion of swimming.  What if the same experiment is done on a flat bottom, and/or if water currents are taken into account?  One could just as readily propose that onshore currents during predator-induced swimming in the shallow-water-inhabiting S. coccinea has led over time to them remaining in shallow water...

Research Study 14: Swimming

Is there any common factor about the sea stars that might explain why they induce Stomphia to swim?  Several ideas are proposed in the literature in this regard.  Let’s look at the first of these, namely, phylogenetic affinity.  The 17 species in the study belong to four Orders of Class Asteroidea.  Although changes in classification have since modified species placement in the various ORDERS in the above list, note that the species that strongly induce swimming in Stomphia are VALVATIDS.  Those that only moderately or don’t induce swimming in Stomphia are VELATIDS, FORCIPULATIDS, and SPINULOSIDS.  So, phylogenetic affinity looks promising, and perhaps this could be looked into more fully than already done.

Research Study 15: Swimming

Another idea is dietary affinity – i.e., escape is induced by species that eat a common dietary item, most obviously, sea anemones. The categorisation is made difficult by the wide variety of prey items consumed by the various sea-star species, but immediately we see that one species that strongly induces swimming is Patiria miniata, a sea-star species that consumes a variety of bottom matter including detritus, and encrusting animals and plants, while the others are mostly carnivores. Of the species listed, only the leather star Dermasterias imbricata commonly eats sea anemones. Also, all of the FORCIPULATIDS listed are carnivores, but none regularly eats sea anemones.

MAIN DIETARY ITEMS OF THE SEA STARS FEATURED IN THE STUDY:

Hippasteria spinosa: sea pens
Patiria miniata: omnivorous scavenger
Mediaster aequalis: sea pens, algae, sponges, hydroids
Poraniopsis inflata: sponges?
Dermasterias imbricata: anemones, corallimorpharians, sea cucumbers, sea urchins
Solaster stimpsoni: sea cucumbers
Solaster dawsoni: sea stars
Solaster endeca: sea cucumbers 

So, the idea of dietary affinity does NOT hold promise.

Research Study 16: Swimming

Another possible “commonality” among the sea stars is habitat occupied.  We would not expect Stomphia to swim from a species that it never encounters in in the field.  Stomphia didemon is a deeper-inhabiting species, occurring most commonly from about 10-20m in depth, and sporadically to depths greater than 200m. Stomphia coccinea, in contrast, inhabits more shallow habitats.  Thus, in  shallow depths, most of the asteroids listed above would be expected to encounter Stomphia (a notable exception is the mostly intertidal-inhabiting ochre star Pisaster ochraceus).  At depths of 20m or more, few sea-star species would be present. The authors of the above study specifically note that of Mediaster, Dermasterias, Patiria, and Hippasteria (from the tabulated list), only the last, Hippasteria, would likely co-occur on "deep reefs" (20-25m) with Stomphia.

In summary, the phylogenetic idea looks the best, even though there are parts of the pattern that don’t fit and, in some cases, more questions are raised than answered.  For example: what do the five valvatid species have in common with themselves and with the three velatids (all species of Solaster), and what do the three Solaster species have in common with themselves, but not with the other two velatids?

Research Study 17: Swimming

Another swimming anemone that inhabits deep water in Puget Sound is Actinostola n.sp., apparently closely related to Stomphia (in fact, it may turn out that this "new species" is actually the deep-water-inhabiting S. didemon, or at least a close variant thereof).  An interesting finding by researchers at Friday Harbor Laboratories, Washington is that touch by Stomphia coccinea to a single tentacle of the purported Actinostola species induces it to swim (Fig. 1 series), but not vice versa, nor does one Stomphia coccinea cause another conspecific to swim.  The response of individual Actinostola is highly variable, ranging from 0-100% in multiple tests with 10 individuals (overall, 57% positive responses).  An Actinosola is also mostly unreactive to contact by one of its tentacles to a leather star Dermasterias imbricata.  The authors are uncertain of what function the swimming response to Stomphia might serve, as the two anemone types are widely dispersed in their natural habitats. 

NOTE  in their publication the researchers refer to S. coccinea but not to S. didemon.  However, the situation is made puzzling because WoRMS (World Register of Marine Species) recognises both species of Stomphia as well as Actinostola callosa, found both in the Pacific and Atlantic oceans

Fig. 1 series.  Stomphia coccinea being touched to a single tentacle of Actinostola n.sp. initiates swimming response

1sec later Actrinosola closes up

3sec after closing Actinostola begins to extend its column

Immediately after extending, Actinostola reopens

5sec later, Actinostola detaches and begins to swim

20sec after detaching, Actinostola ceases swimming (note the prominent basal cone)

Research Study 1: Alarm pheromone

Fig. 1.  Sequence of behaviour in a downstream Anthopleura elegantissima following physical damage to an upstream conspecific in the laboratory.  Note the quick response time.

When a sea anemone Anthopleura elegantissima is damaged physically, all nearby and downstream conspecifics exhibit a stereotypical alarm response consisting of several rapid, convulsive, radially symmetrical flexures of tentacles towards the centre of the body column. Eventually, the tentacles pull in and the marginal sphincter muscles close the disc region (see Fig. 1 for an individual of 2cm basal-disc diameter).  The substance released is a quaternary ammonium ion compound termed anthopleurine  by its discoverers.  It is effective in concentrations as low as 3.5 x 10^-10 mole per liter of seawater.  Extracts of the compound elicit identical responses in conspecifics to that of wounding.  The authors note that anthopleurine is only the second pheromone in marine invertebrates to be described – the first being a sex pheromone crustecdysone in crabs.

NOTE (3-carboxy-2,3-dihydroxy-N,N,N-trimethyl)-1-propanaminium

NOTE a chemical released by one individual that affects other individuals of the same species

Research Study 2: Alarm pheromone

Fig. 1.  Effect of anthopleurine on an anemone

Later studies in Pacific Grove, California by the same research group show that anthopleurine is effective only with Anthopleura elegantissima and A. xanthogrammica, but not with A. artemisia or Metridium senile, or the corallimorpharian Corynactis californica (see Table).  The receptor cells for anthopleurine are located primarily in the tentacles. The first response is contraction of the tentacles causing them to draw closer to the body column (Fig. 1). The retractor tissues in the mesenteries then contract quickly, followed by slower contraction of the sphincter tissues in the upper column region. Within a few seconds the anemone is fully contracted.

Research Study 3: Alarm pheromone

Fig. 1.  Close view of head of nudibranch Aeolidia papillosa showing rhinophores (darker in colour: long-distance chemical perception), oral tentacles (lighter in colour: close-in contact chemoreception), and many cerata 

Defenses of the aggregating anemone Anthopleura elegantissima to attack by the nudibranch Aeolidia papillosa (Fig. 1) include crawling away, releasing attachment to the substratum (and then floating off, probably later to die), raising the tentacles, bulging the column and, if wounded by the predator, releasing the pheromone anthopleurine.  Bulging seems to prevent the predator from reaching up and biting the possibly tastier tentacles.  The anemone apparently never defends with its attack tentacles (acrorhagi), yet these are used effectively in interclonal aggression.  

NOTE known pheromones (most from studies of insects, crustaceans, and fishes) are mainly of alarm or sexual types. In the present case, the pheromone causes rapid bending and shortening of the tentacles in neighbouring anemones, as well as constriction of upper margins of the oral discs and raising of the tentacles, additionally preventing the predator from biting at them from below

Research Study 4: Alarm pheromone

Fig. 1.  Aeolid nudibranch Aeolidia papillosa

Fig. 2.  Uptake of alarm substance anthopleurine into the cerata of a predatory nudibranch Aeolidia papillosa after feeding on the sea anemone Anthopleura elegantissima

Fig. 3.  Response of Anthopleura elegantissima to an approaching nudibranch Aeolidia papillosa that has previously fed on the same anemone species

Studies at Hopkins Marine Station, California show that the alarm pheromone anthopleurine in the sea anemone Anthopleura elegantissima is distributed throughout the body, but with highest concentrations in the lower column wall and pedal disc regions (see Table). When a predatory snail Aeolidia papillosa (Fig. 1) eats A. elegantissima it takes up the anthopleurine into its tissues, especially those of the cerata (Fig. 2).  The cerata contain extensions of the digestive gland within which the anemone’s tissues are digested and absorbed. A single meal of Anthopleura after a few days starvation raises the content of anthopleurine in the cerata to peak level within 24h, then concentrations decrease over the next few days presumably as the anthopleurine diffuses out, is metabolised, and/or excreted.  The results show that the anthopleurine is not synthesised by Aeolidia.  More interestingly, if Aeolidia now approaches an Anthopleura to within about 1cm distance, the anthopleurine (either from the snail’s urine or diffusing from the certata tissues) evokes alarm responses in its potential prey (Fig. 3). The anemone raises its column, and the tentacles and oral disc (the regions containing least concentrations of anthopleurine), are partially enclosed. The lower column and pedal disc, containing highest concentrations of anthopleurine, are the parts now left open to contact by the predator. On these bases, the authors speculate that anthopleurine may be functioning both as an alarm pheromone and as a chemical feeding-deterrent.

Research Study 5: Alarm pheromone

Fig. 1.  Interactive effecs of alarm substance anthopleurine on a nudibranch and sculpin

An interesting predator-prey interaction involving the alarm pheromone anthopleurine is explored in studies at the Bodega Marine Laboratory, California. It involves the anemone-eating Aeolidia papillosa, the anemone-eating sculpin Clinocottus globiceps, and several sea-anemone species including Anthopleura xanthogrammica and A. elegantissima. First, if Aeolidia eats A. xanthogrammica: (#1 in Fig. 1) the nudibranch later gives off a substance (likely anthopleurine) that induces an alarm response in downstream A. elegantissima, causing them to close up (#2) but not A. xanthogrammica.  Next, when a sculpin attacks A. xanthogrammica the anemone releases anthopleurine (#3), which could be both alarm-inducing and defensive.  Now, the question is: given the chemical signature being released by Aeolidia, might the sculpin be confused into attacking the nudibranch thinking it to be food?  The answer is "yes".  The sculpin does attack the nudibranch (#4), but immediately rejects it (#5).  The author thinks the rejection is based not on any defensive properties of Aeolidia’s sequestered nematocysts, because the sculpin readily eats the anemones from which Aeolidia gets its anthopleurine, but rather from defensive secretions from epidermal glands in Aeolidia. The study is imaginative, interesting, and thoughty-provoking, and the author is to be congratulated.

NOTE  it is not clear from the study whether the sculpin is simply attacking something (the nudibranch) that may be good to eat, or whether it is attacking what it perceives to be an anemone that it knows from past experience is good to eat.  To answer this we need to know whether the sculpin will attack an Aeolidia that has been feeding on non-anthopleurine-bearing anemones, such as Metridium spp., which should be an interesting project for someone

NOTE  the topic of secondarily-derived nematocyst defenses in aeolid nudibranchs is considered in detail elsewhere in the ODYSSEY:   LEARN ABOUT NUDIBRANCHS & RELATIVES>DEFENSES AGAINST PREDATORS>NEMATOCYSTS