Predators & defenses
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  Physical defenses
  Topics relating to predators & defenses include physical defenses, considered here, and LARVAL, CHEMICAL, and BEHAVIORAL defenses, considered in other sections.
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photograph of a sunflower star Pycnopodia helianthoides chasing an abalone Haliotis kamtschatkana

All snails gain at least some protection from their shells, but this is especially true for abalones.  When the large foot muscle of Haliotis clamps down on a rock, it is almost impossible to detach the abalone without damaging it.  At this point, the abalone is likely to be fully protected from attacks by sea otters and octopuses, but not from sunflower stars Pycnopodia helianthoides, which simply digest their prey in situ if the abalone does not crawl away.

NOTE  feeding and digestion in sea stars is considered in detail elsewhere in the ODYSSEY: LEARN ABOUT SEA STARS: FEEDING


Haliotis kamtschatkana moves away from an advancing
sunflower star Pycnopodia helianthoides (the abalone
is at 11.30 o'clock in the photo) 0.25X

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Research study 1
 

photograph of abalone Haliotis kamtschatkana showing camouflaging effect of algal growth on its shellphotograph of abalone Haliotis kamtschatkana showing bilin-pigment coloration in the shellNorthern abalone Haliotis kamtschatkana are often difficult to see against the background of algae on the rocks that they inhabit.  The shell may be covered with a coat of filamentous algae (as shown in the photo on Left: the abalone is at 4 o'clock in the photo), with pink encrusting coralline algae, with infestations of boring sponges, clams, algae, and polychaetes, or the shell itself may be coloured. Is this camouflage against visual predators such as mink, sea otters, fishes, and scoters, or perhaps chemical camouflaging? 

Colour in an abalone’s shell comes from bilin-type pigments derived from seaweed foods that are incorporated into the proteinaceous outer layer of the shell, the periostracum.  Studies on California red abalone H. rufescens kept for up to a year on different diets show that various species of red algae produce a red-coloured shell, while the green alga Ulva sp. produces a pale green shell.  Brown algae (mostly large kelps) produce shells that are coloured white or various shades of green.  Leighton 1961 Veliger 4: 29.

NOTE generally these growths are not life-threatening to the abalone, but heavy infestation, especially on the leading edge of the shell, may interfere with growth and with righting after dislodgement

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Research study 2
 

Discussion of the many internal and external parasites of abalones and other vetigastropods is outside the scope of the ODYSSEY, but mention should be made of limpets and slipper limpets photograph of a slipper limpet Crepidula aduncathat inhabit the shells of Chlorostoma spp.  For example, the limpet Lottia asmi lives on shells of Chlorostoma (Tegula) funebralis (8% infestation in the Monterey Bay region of California). The slipper limpet Crepidula adunca inhabits shells of 3 Chlorostoma species: funebralis (25% infestation in Monterey Bay), brunnea (75%) and Promartynia (Tegula) pulligo (68%).  Slipper limpets are protandrous hermaphrodites and often sit one on top of the other on a Chlorostoma shell.  This can add extra mass (15-18% in the Monterey study) for the host to carry about. There is no defense against these shell-occupiers. Evans 1992 Mar Behav Physiol 19: 241.

NOTE e.g., Crepidula adunca -  these are a type of caenogastropod, related to littorines and mud snails

NOTE lit. “first male” G., referring to the fact that an individual Crepidula starts off as a male, then changes to a female.  In a chain of individuals on top of a host snail, the first one or two individuals will be female, and the more uppermost ones male.  Slipper limpets are broadcast spawners and suspension-feeders

photograph of a black turban snail Chlorostoma funebralis with attached limpet Lottia asmi
Chlorostoma funebralis bearing limpets Lottia spp. 2X
 
 

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Research study 3

 

Although the slipper limpet Crepidula adunca is found on several histograms showing effect on escape speed from attacking sea stars and righting times in snails Calliostoma ligatum with and without attached slipper limpets Crepidula aduncaspecies of gastropods, in San Juan Island, Washington its only host is Calliostoma ligatum.  The benefit for Crepidula may be protection from predation by sea stars, most notably, in this region, the 6-armed Leptasterias hexactis – a species that Calliostoma normally runs from.  Because Crepidula may increase the mass of its host snail by up to 24%, both escape speed and righting time may be significantly affected (see graph).  The authors note that the ability of Calliostoma to extend its foot over its shell to clean off settling organisms and/or deposit a layer of protective mucus is not effective against slipper limpets. Vermeij et al. 1987 The Nautilus 101: 69.

NOTE other west-coast hosts include the gastropods Chlorostomafunebralis, Amphissa columbiana, and Searlesia dira

NOTE when touched with a tube foot of a sea star Calliostoma rears up, rotates 180o, and crawls away

photograph of black turban snail Chlorostoma funebralis bearing several  slipper limpets Crepidula adunca
Chlorostoma funebralis bearing 3 male Crepidula adunca sitting on a 4th, larger female 1.5X. Photo courtesy Dave Cowles, Walla Walla University,Washington wallawalla.edu
 

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Research study 4

 

histogram showing limpet abundances on beaches in southern California proportions of black turban snails Chlorostoma funebralis bearing slipper limpets Crepidula aduncaOn beaches in southern California 7 different limpet species occupy the shells of Chlorostoma (Tegula) funebralis, but six of these are represented by juveniles only.  Only Lottia asmi are found as adults on Chlorostoma.  The frequency of occurrence on Chlorostoma varies from 12-47% depending upon locality, with an overall mean of 25% for all beaches surveyed (shown in green on the graph). The most abundant of all limpet species on the 5 beaches surveyed is Lottia pelta, representing 58% of the total number of limpets present.

Maximum occupation on an individual Chlorostoma is 7 limpets. Maximum number of different limpet species on a single Chlorostoma is 3.  What are the limpets doing on the snails?  The author suggests that they use Chlorostoma shells as an early food source, but this does not explain why L. asmi continue on as adults.  Also, are limpet-occupied Chlorostoma shells actually cleaner than limpet-free shells?  Brewer 1975 Veliger 17: 307.

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Research study 5
 

graph showing size difference of black turban snails Chlorostoma funebralis at high and low shore levels at Mukkaw Bay, WashingtonIn Mukkaw Bay, Washington Chlorostoma (Tegula) funebralis settle in the higher intertidal region then, after several years, migrate to the lower zone.  During these early years they are in spatial refuge from predation by ochre stars Pisaster ochraceus, but are at risk from other sources of mortality, such as sea gulls, storm waves, and sand burial.  Even though densities in the higher part of the shore may reach 800 . m-2 , the migration is independent of density; hence, implying another cause, perhaps access to more or better food in the lower zone leading to greater growth and fecundity.  This is supported by the fact that smaller individuals grow better higher up on the shore than they would lower down (where growth increments on the shell become progressively smaller).  Once in the low zone the snails maintain a relatively large, more steady, growth. Note the more closely spaced growth lines on the high-level snail in the graph, as compared with the low-level one. Note also the break in the two lines at about 16mm shell diameter, equivalent to about 10-11y in age.  This is also the approximate age of sexual maturity. 

With respect to fecundity, equivalent-sized females in the low zone produce 4 times more energy in eggs per spawning than ones in the high zone. After their migration to the lower zone, then, the once high-level snails find more food and contribute more to reproduction, and these features presumably outweigh the disadvantage of habitat overlap with the predatory Pisaster.   Paine 1969 Ecology 50: 950.

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Research study 6
 

histogram showing numbers of Chlorostoma funebralis eaten by sea stars Pisaster ochraceusA later study in California puts a slightly different interpretation on the relationship of Chlorostoma funebralis with its predator Pisaster ochraceus. First, in field and in laboratory feeding experiments the author shows that Pisaster shows a slight preference for larger-sized Tegula (see graph on Left)

Moreover, although all sizes of Chlorostoma withdraw as deeply as possible into their shells when contacted by Pisaster (past the first third of the body whorl), larger-sized snails are actually more vulnerable to attack.  This can be demonstrated by grinding off the first one-quarter of the body whorl and then exposing these treated snails along with untreated control snails to sea stars for a 7-d period.  The results show once again that the larger size classes of snails are eaten more than smaller size classes (blue-coloured bars in graph on Right) but, additionally, that the larger snails are made significantly more vulnerable to being eaten when the withdrawal response is made less effective by grinding off the shell edges. The author points out that since Chlorostoma matures at about 15-16mm shell length, well below the sea stars’ preferred size of  >21mm, then a portion of the population that is reproductive is at least partially immune from sea-star predation.  This puts a different light on the significance of shore-level size gradients noted in the earlier study.  Markowitz 1980 J Exper Mar Biol Ecol 45: 1.

NOTE  in this 17-d study, sea stars P. ochraceus are presented with a selection of equal numbers of 4 size classes of Chlorostoma

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Research study 7
 

Shells appear to be of limited value in defense of black abalone Haliotis cracherodii at Laguna Beach, California against predation by octopuses. Out of 865 shells collected in total over a 2-yr study period, 158 or 18% are found to be dead.  Of these, 58 or 37% exhibit octopus-inflicted boreholes.  Of the 58, 67% are bored completely; the remainder, incompletely.  The former are significantly smaller than the latter (52 vs. 60mm shell length), photograph of black abalone Haliotis cracherodiisuggesting that thinner shells are more susceptible to being drilled.  Dead shells exhibiting no shell damage are significantly smaller than other damage types, suggesting that octopuses may be prying the small individuals from the substratum without drilling.  The author discusses the statistical implications of using dead shells to infer temporal variation in mortality rates from different factors, but this does not affect the conclusion that octopuses are a major source of mortality of abalones at Laguna Beach.  Interestingly, the abalone are higher on the shore in summer than in other seasons, suggesting to the author that greater foraging activity by octopuses at lower intertidal levels may be driving the population upwards at this time of year.  Tissot 1988 J Exp Mar Biol Ecol 117: 71.

NOTE  55 of the total of 158 dead shells (35%) are broken at the edge

Black abalone Haliotis cracherodii
at the Seattle Aquarium 0.6X

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Research study 8
 

map showing collecting sites in Barkley Sound, British Columbia used for study of predation on abalone Haliotis kamtschatkanaAs adults, northern abalone Haliotis kamtschatkana have many predators, including seastars, crabs, sea otters, wolf-eels, cabezon, and octopuses.  Many of these same predators, as well as some polychaetes and snails, also consume juvenile abalone.  An investigation at the Bamfield Marine Sciences Centre, British Columbia sets out to identify specific predators of juvenile H. kamtschatkana, to assess their relative importance in natural populations, and to determine if the abalone’s susceptibility to these predators changes as they grow larger.  Of 37 possible predators collected in abalone habitats around Bamfield, lab tests show that 14 are capable of eating juvenile abalone of <28mm shell length.  Of these, 3 species of crabs and one species of sea star are classified as “serious” predators of the juveniles.  The authors find that risk of predation for juvenile H. kamtschatica in the laboratory decreases  markedly at shell lengths greater than 13mm, a phenomenon described by them as an “ontogenetic shift”.  The “shift”-size varies for different predators.  Of the 4 “serious” predators of juveniles identified, 2 species of crabs Lophopanopeus bellus and Scyra acutifrons appear not to eat juvenile abalone much greater than about 10-15mm shell length.  However, the third species of crab Cancer productus is able to eat all sizes of juveniles offered.  Sea stars Pycnopodia helianthoides of less than 30cm disc diameter appear not to be able to eat abalone larger than about 28mm shell length, although the largest sizes of this predator can readily eat all sizes of abalone if they can catch them.  The descriptor “ontogenetic shift” is a good one, for it implies more than an abalone just reaching a refuge in size.  As an abalone grows larger, it develops a stronger shell, faster locomotion, and greater tenacity, just to name some of several ontogenetic changes that may occur.   Griffiths & Gosselin 2008 J Exp Mar Biol Ecol 360: 85.

NOTE  the “shift” part of this expression is the attainment by the prey of a size beyond which a certain predator’s success falls off suddenly

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