Predators & defenses
   
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  Invertebrate predators
  Defenses are described for different predators.  The topic of invertebrate predators:octopuses & sea stars is considered in this section, while INVERTEBRATE PREDATORS: SNAILS & CRABS and VERTEBRATE PREDATORS are dealt with in other sections.
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Octopuses

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Research study 1
 

photograph of an octopus Enteroctopus dofleiniSurprisingly, clams represent a favoured prey of octopuses.  A study in British Columbia shows that 5 species of clams account for 60% of 3492 prey remains collected at 117 Enteroctopus dofleini dens. Owing to their habit of living right at the sediment/water interface, cockles Clinocardium nuttallii represent the largest component of the bivalve total (almost 50%).  The second commonest prey at the octopus middens is the littleneck clam Protothaca staminea, representing 16% of the 3492 prey items, and also a comparatively shallow burrower.  These observations suggest that Enteroctopus dofleini forages more on sand/mud than on rock substrata.  Hartwick et al. 1981 Veliger 24: 129.

NOTE more information on octopus diets can be found at OCTOPUSES & RELATIVES: FEEDING & GROWTH: DIETS


Octopuses Enteroctopus dofleini kill bivalve prey by
drilling and injecting toxin, or sometimes by
cracking their shells with their beaks 0.1X

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Sea stars

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Research study 1
 
The most common sea-star predators of clams are ones that regularly occupy sand- and mud-flats, such as Pisaster brevispinus and Pycnopodia helianthoides.  These species are commonly seen positioned over pits that they excavate using their tube feet, likely homes for clams.
photograph of a pink sea star Pisaster brevispinus in the process of eating a clam that it has dug up
Pisaster brevispinus digging up a clam. The sand around the arms represents that dug up during the quest 0.25X
photograph of a pink sea star Pisaster brevispinus with a prey clam Humilaria kennerleyi
Pink star Pisaster brevispinus with a clam Humilaria kennerleyi 0.3X
photograph of a pink sea star Pisaster brevispinus with a prey cockle Clinocardium nuttallii
Pisaster brevispinus with a cockle Clinocardium nuttallii 0.5X
photograph of a sunflower star Pycnopodia helianthoides digging for a clam
Sunflower star Pycnopodia helianthoides digs up a mass of sand in its hunt for a clam 0.25X
photograph of a sea star Orthasterias koehleri with a captured clam Humilaria kennerleyi
Sea star Orthasterias koehleri with a captured clam Humilaria kennerleyi 0.33X
photograph of a sunflower star Pycnopodia helianthoides diffing for a prey clam
Judging by its shape, this Pycnopodia helianthoides has captured the clam it has been digging for 0.3X
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Research study 2
 
Cockles Clinocardium nuttallii have short siphons and live right at the sediment/water interface.  In this vulnerable position various sea star predators can attack them, but escape is usually enabled by use of their strong, thrusting foot.
photo sequence of a juvenile sunflower star just commencing an attack on a cockle Clinocardium nuttallii photo sequence of a juvenile sunflower star just commencing an attack on a cockle Clinocardium nuttallii.  The cockle has extended its muscular foot and is thrusting away from the predator
Within a few seconds of dropping a juvenile sunflower star Pycnopodia helianthoides onto a cockle, the latter begins its first foot thrust (upper Left). The thrust is so powerful that it sends the cockle spinning away (lower Left). A second thrust sends the cockle back towards the sea star (above). So effective is this escape behaviour that it is a rare event for a cockle to be caught by a sea star, even one as fast moving as P. helianthoides
photo sequence of a juvenile sunflower star just commencing an attack on a cockle Clinocardium nuttallii.  The cockle has extended its muscular foot and is thrusting away from the predator
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photograph of a cockle Clinodardium nuttallii preparing to escape from an attacking sunflower star Pycnopodia helianthoides taken from a video

CLICK HERE to see a video of the cockle escaping the juvenile sunflower star.

NOTE video replays automatically
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Research study 3
 

photograph of empty burrows of rock piddocks Penitella penita
Although rock-boring piddocks Penitella penita would seem to be well-protected in their boreholes, pink sea stars Pisaster brevispinus in the Monterey region of California are often seen positioned over the holes with their stomachs everted.  Experiments with 7 species of pholids, including P. penita, inserted into artificial boreholes made in shale, confirm that P. brevispinus will attack and eventually consume all species.  Haderlie 1980 Veliger 22: 400.

 

 

Burrows of rock piddocks, likely Penitella penita, in
shale rocks at Botanical Beach, British Columbia 0.33X

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Research study 4
 

photograph of a jewel-box clam Chama arcanaphotograph of a sea star Pisaster giganteus courtesy NOAA Channel Islands National Marine SanctuaryThe jewel-box clam Chama arcana lives fixed immovably to rocks and other solid objects in low intertidal and subtidal regions, and is potentially susceptible to predation by sea stars (see photo on Left).  The shell of Chama is normally covered by a dense growth of plants and animals (over 40 species of algae, sponges, hydroids, anemones, bryozoans, entoprots, and tunicates).  Studies in Santa Catalina Island, California show that removal of these makes the bivalve more susceptible to attack by predatory sea stars Pisaster giganteus in the laboratory, and substantially increases the mortality of Chama from predatory Pisaster in the field. Because Chama inhabits areas outside the normal foraging ranges of benthic grazers such as sea urchins that would otherwise eat the epibionts, the latter are given refuge from the grazers.  In the view of the author, then, the relationship is a mutualistic one, Chama benefitting through protection from sea-star predation, and the epibionts avoiding being consumed by foraging sea urchins.  The shells of C. arcana are naturally rugose, and this appears to favour settlement and recruitment of the epibionts.  The author notes that the relationship is a facultative one for both “partners”, as either can live on its own. Vance 1978 Ecology 59: 679. Photo of Pisaster giganteus courtesy NOAA Channel Islands National Marine Sanctuary 0.33X

NOTE  in older literature, including this 1978 one, C. arcana is often misidentified as C. pellucida, which is a South American species

NOTE  this is the only common asteroid present at the study site (Bird Rock).  Observation of 299 feeding P. giganteus over a 2.4-yr period show that Chama arcana constitutes 77% of the sea star’s diet.  If we assume the clams being eaten are bearing a normal complement of epibionts, the defense does not seem very effective.  However, data from the author’s field assays in 0.1m2 quadrats, where half the Chama are brushed clean and half left with their epibiont load intact, indicate that sea stars favour the experimentally cleaned clams by a ratio of about 20:1 over control clams

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Research study 5
 

graph showing effect on feeding rates of sea stars Pisaster giganteus and P. ochraceus by the presence of corallimorpharians Corynactis californicaObservations in the California Bight region (specifically: Santa Barbara and Malibu) show that densities of Chama arcana are greater within aggregations of the corallimorpharian Corynactis californica, a polyp that is avoided by most sea stars.  The question arises as to whether the bivalve is gaining protection from sea stars by living in proximity to Corynactis.  This is tested in the laboratory in seawater trays containing a 75% cover of rounded rocks ("Corynactis absent") or a 75% cover of rounded rocks and Corynactis ("Corynactis present").  A sea star is added to each tray and maintained over 4-d periods with a steady supply of mussels (3cm shell length: species not specified, but likely Mytilus galloprovincialis).  photograph of a blood star Henricia leviuscula picking its way through an aggregation of corallimorpharian polyps Corynactis californicaThe cumulative results (see graph) show that significantly more mussels are consumed in the absence of Corynactis than in their presence.  Although the experimental evidence is indirect, the authors conclude fairly that Chama will likely enjoy the same protective benefits in the presence of Corynactis as do the mussels, because of the decreased predatory activity by sea stars. Patton et al. 1991 Bull Mar Sci 48: 623.

NOTE  the authors remark that Pisaster giganteus does not occur commonly in aggregations of Corynactis, and that P. ochraceus avoids them



A blood star Henricia leviuscula appears to step gingerly
around an aggregation of Corynactis californica 0.33

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