Predators & defenses
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  Behavioral defenses
  Topics relating to predators & defenses include behavioral defenses, considered here, and LARVAL, PHYSICAL, and CHEMICAL defenses, considered in other sections.
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Behavioral responses to an attacking sea-star predator in abalones and relatives include shell-twisting, tentacle movement, fast crawling, and sometimes releasing attachment of the foot and dropping to the sea bottom. photographs showing the twisting response of an abalone Haliotis kamtschatakan in response to an encroaching sunflower star Pycnopodia helianthoides
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  Adult abalones mostly sit openly on rocks, whereas juveniles often hide under rocks.  In Pacific Grove, California, sea otter predation is so intense that surviving abalones Haliotis rufescens are mostly found in crevices.  Do the abalones learn to hide in crevices, or has selection favoured a crevice-dwelling variant?  Neither seems likely.  We do know, however, that sea urchins Strongylocentrotus spp. co-inhabit these crevices and compete with the abalones for space and food.  Lowry & Pearse 1973 Mar Biol 23: 213.
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drawing of an abalone showing external featuresOn contact with tube feet or scent of sea star predator, such as Pycnopodia helianthoides or Pisaster ochraceus, many west-coast species of abalone initiate escape and protective behaviour.  This includes moving of the epipodial tentacles, stiffening and swelling of the epipodium, rapid twisting of the shell back and forth, and rapid crawling.  Accompanying this may be release of a viscid white mucousy secretion.  This escape locomotion differs from ordinary locomotion in that the shell is elevated and the foot is lifted from the substratum in its central portion. Drawing (above) and information from Mongomery 1967 Veliger 9: 359; drawing rRight) from Crofts 1929 HALIOTIS Publ XXIX Liverpool Mar Biol Comm Memoirs, Univ Press Liverpool, 174 pp. drawing of an abalone in fast locomotory speed
Note the raised central portion of the foot of this fast-crawling abalone
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photograph of an abalone Haliotis kamtschatkana escaping from an approaching sunflower star Pycnopodia helianthoides

CLICK HERE to see a video of Haliotis kamtschatkana escaping from a sunflower star Pycnopodia helianthoides. The predator has been placed by a SCUBA-diver near to a quiescent abalone. Note that the abalone easily outdistances the fast-crawling sea star. Video courtesy James Mortimer, Bamfield.

NOTE the video replays automatically

 

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Escape speeds of abalone Haliotis kamtschatkana in response to sea-star predators varies with the type of sea star.  The speeds shown here are ones recorded in laboratory tests.  James & Nolen 1978 Am Zool 18: 675.

NOTE neither Leptasterias nor Pisaster is a natural predator of abalones.

of abalone Haliotis kamtschatkana to various sea stars
 

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  Although adult abalones and sea urchins in California compete for the same algal foods, juvenile abalones may be found sheltering for protection under the spine canopies of adult sea urchins, along with juvenile red and purple sea urchins and many other small invertebrates, including sea stars, ophiuroids, snails, crabs, worms, and amphipods. Laboratory tests in California show that survival of juvenile abalone in the presence of predatory crabs is almost 70% better when red sea-urchins are present than when they are absent.  Rogers-Bennett & Pearse 2001 Conserv Biol 15: 642-647.
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photograph of black turban snails Chlorostoma funebralis in a surf-channel crevice

Larvae of black turban snails Chlorostoma funebralis settle in the higher regions of the intertidal zone and live there for several years before migrating downwards to lower areas.   Here, they contact sea-star predators, most notably ochre stars Pisaster ochraceus.   Defensive response by Chlorostoma to touch or close presence of predatory sea stars is sideways twisting followed by quick escape crawling, or releasing their hold on rocks and dropping to the sea bottom.  Feder 1963 Ecology 44: 505; Paine 1969 Ecology 50: 950.

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Escape responses of Chlorostoma funebralis, involving shell-twisting and sometimes direct table of % strong responses of black turban snails Chlorostoma funebralis to several west-coast sea starscrawling up, onto, and over a potential predator, are graded depending upon whether or not the test species is a known predator.  Physical contact with several sea-star and whelk species in Pacific Grove, California elicits strong escape responses. 

The only test species eliciting no, or only weak, responses in Chlorostoma are the bat star Asterina miniata and leather star Dermasterias imbricata. The first is an omnivorous scavenger, lives on muddy-bottom habitats, and rarely, if ever, encounters C. funebralis. The second, however, Dermasterias, is predatory on a wide variety of invertebrates including sea urchins, sea stars, and spongest.  Its lack of effect on Tegula is therefore somewhat surprising and may warrent further investigation.  Yarnall 1964 Veliger 6 (Suppl): 56.

NOTE  tests involve touching a tube foot (sea star) or piece of sole of foot (whelk) to Chlorostoma, then grading the intensity of response.  The author uses 4 categories of response in the study (strong, medium, weak, and absent) but, for economy of space, only percentages of Chlorostoma individuals (N = 50 for each pairing) showing strong responses are reported here.   “Touch” controls with a clean steel probe give an average of 12% strong responses out of 450 trials (50 with each test species)

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schematic showing extent of overlapping distributions of top shells (Chlorostoma and Promartinia) and their sea-star predators

Of the species of Chlorostoma (Tegula) inhabiting rocky-shore and kelp-forest regions along the west coast two, C. (Tegula) funebralis and C.(Tegula) brunnea, live in mid-low/shallow subtidal regions where they regularly contact predatory sea stars Pisaster spp. and Pycnopodia helianthoides.

Two other species, Promartynia (Tegula) pulligo and C. (Tegula) montereyi, live primarily on the bottom portions of Macrocystis spp. and other large subtidal kelps and have less contact with Pisaster spp.  On some central California shores, C. (Tegula) brunnea, if not on the sea bottom, may be found on the tops of large kelps where it also has no contact with Pisaster. Lowry et al. 1974 Biol Bull 147: 386; Riedman et al. 1981 Veliger 24: 97.

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photograph of snail Chlorostoma brunnea

Chlorostoma brunnea, which has more overlap with Pisaster species than do the other snails, relies on avoidance and fast crawling to escape, often combined with shell-twisting and, on vertical surfaces, release of attachment. 

photograph of snail Promartynia pulligo

Promartynia pulligo, in contrast, flees from contact with Pisaster species but clamps down on contact with Pycnopodia. 

 

photograph of snail Chlorostoma montereyiChlorostoma montereyi flees from contact with Pisaster species but, if caught, retreats deeply into its shell which is relatively larger than the other species.  Most often the sea star rejects what it may perceive as an empty shell.  Unable to outrun Pycnopodia, C. montereyi on capture allows its soft tissues to be touched by the sea star, which often leads to rejection, perhaps because of distasteful chemicals.  Feder 1963 Ecology 44: 505; Watanabe 1983 J Exper Mar Biol Ecol 71: 257;  sea also Watanabe 1984 Ecology 65: 920. 

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photograph of sea star Pisaster giganteus courtesey NOAA Channel Islands National Marine SanctuaryWhen touched by foraging sea stars Pisaster giganteus (see photo on Left) the snails photograph of snail Calliostoma ligatumPromartynia (Tegula) pulligo (see photo in previous Research study) and Calliostoma ligatum (photo on Right) rotate their shells vigorously, then release their attachment and tumble away.  Studies in California show that C. ligatum uniquely coats its shell with slippery mucus from glands on the posterior part of its foot, possibly to aid in this twisting response.  It also uses its radula to “bite” (scrape?) at the sea star.  Are these strategies effective in reducing predation by sea stars?  To test this, 33 individuals of each species are first narcotised, then placed along with untreated control snails (in groups of 4: 2 of each treatment group) 5-10cm in front of the leading arms of foraging P. giganteus in the field.  Under these experimental circumstances, the answer to our question is 'yes'.  Narcotised individuals of both species are significantly more likely to be captured than control individuals (see histogram on Right).  Moreover, significantly fewer unnarcotised C. ligatum are eaten than P. pulligo, possibly owing to its more effective defense (i.e., slippery shell; the author comments that no “biting” is seen in the field study).  Harrold 1982 The Amer Nat 119: 132; photo of P. giganteus courtesey NOAA Channel Islands National Marine Sanctuary.

NOTE  5-10min in seawater containing MgCl2

histogram showing % capture by sea stars Pisaster giganteus of snails narcotised and non-narcotised
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At least 4 trochid-snail species co-inhabit beach areas in Santa Catalina Island, California with several sea-star and whelk predators.  Three of these, Chlorostoma aureotincta, Norrisia norrisi, and Pomaulax undosa will run from contact with Pisaster giganteus and other sea-star and whelk predators, but the fourth, Chlorostoma eiseni, stays put.  The authors credit histograms of growth of Pisaster giganteus fed on different snail species this behaviour with a 65% thicker shell and less digestible tissues in C. eiseni as compared with its congenitor C. aureotincta

If juvenile Pisaster are fed shell-less tissues of the two Chlorostoma species, growth is similar, suggesting that the species do not differ in nutritional value (Left-hand histogram).  However, growth of juvenile sea stars on intact snails is significantly slower on a diet of C. eiseni than C. aureotincta, possibly owing to the longer time taken for sea stars to digest the tissues of the former (Right-hand histogram).  In laboratory comparisons of digestibility, tissues of aureotincta are digested 3-fold quicker than those of eiseni.  Moreover, the overall time for Pisaster to eat a single C. eiseni in the lab is 11h versus 4h for C. aureotinctaSchmitt 1981 J Exp Mar Biol Ecol 54: 251.

NOTE  growth data for another asteroid species, not shown here, is similar to those presented for Pisaster giganteus

NOTE  the tests use 50% H2SO4 over 30min at 40oC

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In the lower intertidal zones of California and Oregon beaches Chlorostoma funebralis co-occurs with a potential predatory crab Cancer antennarius.  Laboratory studies at Bodega Marine Laboratory, California show that adult crabs (7-10cm carapace width) are capable of eating all sizesof funebralis offered, up to 3cm diameter.  When exposed to crab-scented seawater in containers in the lab, a significant proportion of the prey snails crawls upwards in apparent escape in comparison with control snails in clean seawater (76 vs. 22%, respectively).  In running photograph of crab Cancer antennariusseawater, the percentages crawling upward are also significantly different (60 vs. 26%), but smaller in magnitude, probably owing to dilution of the crab scent in the flowing water.  Interestingly, funebralis from sites where C. antennarius is absent do not show the behaviour.  Geller 1982 J Exp Mar Biol Ecol 65: 19.

NOTE  the crabs are positioned in mesh containers upstream from the test snails


Crab Cancer antennarius 0.4X

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photographs of snail Norrisia norrisi to show extent of barnacle foulingThe snail Norrisia norrisi in Santa Catalina, California likes to inhabit large kelps1. In its lofty kelp habitat it has about a 4% chance each day of being dislodged. If this happens and it crawls on the sea bottom, it suffers 10-fold greater mortality than it would in the kelp habitat.  One of its benthic predators is the octopus Octopus bimaculatus, which drills2 and eats it.  If the octopus fails to drill completely through the shell of Norrisia, which happens quite often, the drill holes attract settling larvae of the barnacle Megabalanus californicus3 and the shell may become heavily fouled with these barnacles.  About 30% of the snails have barnacles growing on them.  If heavily fouled the snails have difficulty crawling back to the relative safety of the kelps.  When they attain their perches, they are dislodged twice as frequently and remain longer on the sea bottom than unfouled snails.  On the sea bottom, especially with their load of barnacles, they run the risk of being captured and consumed by benthic predators, such as octopuses, lobsters, and whelks. The author's measurements on locomotory rates indicate that moderately to heavily fouled snails are at least 25% slower than unfouled ones.  Moreover, data from laboratory tests on susceptibility of Norrisia to sea-star predators show that fouled snails have an 8-fold greater risk of being eaten than unfouled ones.  It’s double jeopardy: if the octopuses fail to kill Norrisia outright, their drill-holes become fouled with barnacles and indirectly lead to greater risk of death from other sources.  Schmitt et al. 1983 J Exp Mar Biol Ecol 69: 267.

NOTE1  the kelps are Macrocystis pyrifera and Eisenia arborea.  Counts by the researchers show that 97% of N. norrisi inhabit these kelps, while the remainder live on the sea bottom

NOTE2 the authors report that of 269 live Norrisia examined, 29% have been drilled by an octopus, and many of these are also encrusted with barnacles.  Why the octopuses fail so often to kill a snail is not known

NOTE3 this barnacle is virtually the only epibiont found on Norrisia in the study area

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graph showing upward-climbing response of black turban snails Chlorostoma funebralis dependent upon intensity of sea-star presence increases from belowThe upward crawling behaviour of Chlorostomafunebralis in the presence of potential predators can greatly influence the species’ lower limit of distribution.  In southern California the lower limits of C.funebralis are generally higher than in northern California, possibly because of the greater abundance of predators in the southern regions.  Predators in this area include ochre stars Pisaster ochraceus, several crabs Cancer spp., and octopuses Octopus bimaculoides and O. bimaculatus – the last two of which are lacking at northern sites.  Predator intensity at each site is scored by direct quadrat counts and observation of transient predators (e.g., octopuses and crabs), and by indirect counts using the presence of characteristically crushed and drilled shells as indicative of crab kills and octopus kills, respectively.  Note in the graph on the Left that as the “predator score” increases (i.e., greater predat-ory intensity) at the 13 sites studied, so the lower limit of distribution Chlorostoma rises. 

Reciprocal translocations of snails from one location to another, in some cases to different heights on the shore, provide supportive data.  For example, snails from San Luis Obispo and San Mateo, released on the shore at San Luis Obispo, move quickly upwards, regardless of whether they are released low on the shore or high on the shore (see graph lower Left). Results for the reciprocal transplant, from both locations and released on the shore at San Mateo, are shown in the graph at the graphs showing heights of black turban snails Chlorostoma funebralis at different sites in Californialower Right. Note that snails from the high "predator-score" location (San Luis Obispo) consistently move higher on the graphs showing heights of black turban snails Chlorostoma funebralis at different sites in Californiashore after translocation, in accordance with the author's predictions.

Also noteworthy is the generally lower heights attained by the two sets of funebralis in the translocation to San Mateo (graph on Right). The “predator score” is lower at the San Mateo site than at the more southerly San Luis Obispo site (4 vs. 6, respectively).  This general pattern is repeated in many other translocations.  The author proposes, based on these field experiments and on laboratory observations that the snails may be responding to chemical exudations from the predators.

The explanation for snails translocated from the southern population (San Luis Obispo) always ending up higher on the shore than snails from the northern population (San Mateo) may owe to long-term differences in the intensity of predation photograph of Octopus bimaculoides courtesy of Roger Hanlon, Woods Hole, Massachusettsby octopuses, which are much rarer in the intertidal zone in northern California.  However, an alternative explanation also provided by the author might be that acclimation to cooler, moister conditions at the more northern site (San Mateo) could keep the northern snails lower on the shore when translocated to the southern site.  This may also explain why the southern snails (San Luis Obispo) move higher on the shore than the northern snails (San Mateo) when translocated to the more northerly site.  Finally, funebralis generally is larger in the northerly populations than in the southerly ones, and larger snails are usually found higher on the shore than smaller ones.  Fawcett 1984 Ecology 65: 1214; photo of Octopus bimaculoides courtesy of Roger Hanlon, Woods Hole, Massachusetts.

NOTE  there are 12 sites in California ranging from San Diego County in the south to Sonoma County in the north, and one site at Pt. Anderson in Washington

NOTE  in this example, the 2 sites are separated latitudinally by about 300km

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  Studies in San Juan Island, Washington show that 3 “Margarites” species, M. (Pupillaria?) pupillus, M. (Pupillaria) salmonea, and M. (Pupillaria) rhodia, and Calliostoma ligatum, respond only weakly to the scent of predatory sea stars Leptasterias hexactis and Pisaster ochraceus.  However, if the sea stars are touched to the soft parts of the snails, escape reaction is greatly intensified.  All species twist their shells vigorously, rear up (often losing contact with the substratum), and sometimes somersault.  Hoffman 1980 Pac Sci 34: 233.
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Top shells Pomaulax gibberosa in British Columbia often harbour a commensal polynoid worm that sometimes shows itself outside of the snail’s mantle cavity.  Whether the worm comes out to bite at a potential predator, similar to the behaviour of a mutualistic worm in keyhole limpets Diodora aspera, is not known.

 

 

In this photo the snail has partially emerged from the
seawater in an aquarium tank. The worm may have come
out to investigate the sudden change in living conditions 0.7X

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