title for limpet section of the Odyssey
   
  Defenses & predators
 

Defenses of limpets include attachment strength, shell, escape crawling, camouflage (both visual and chemical), and possibly toxic metabolites. Chief predators are crabs, fishes, and sea stars when the tide is in, and birds and perhaps humans when the tide is out. There is overlap between defenses and predators. For example, attachment strength is useful against predation by both sea stars and birds, and shells provide protection against both crabs and fishes. For this reason, defenses and predators are intermixed in the sections to follow.

NOTE  examination of stomach contents of 13 species of tidepool fishes at Portuguese Bend, California reveals the remains of limpets in only 2 of the fish species, the spotted kelpfish Gibbonsia elegans and dwarf perch Micrometrus minimus.  Only in the latter is the limpet identified to species (Lottia scabra), suggesting that that shell is still present and, if so, indicating that the perch may have sucked the limpet in rather than crushing its shell, discarding the bits, and then eating the flesh.  Mitchell 1953 Am Midl Nat 49: 862.

NOTE  most or all First Nations Peoples on the west coast have had a tradition of eating limpets.  They are easy to harvest and yield a small nugget of flesh that although chewy, is tasty and protein-rich.  The only species currently harvested is the owl limpet Lottia gigantea along the northern coast of Baja California

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  Attachment-strength protection
  Attachment-strength protection is considered in this section, and topics of SHELL PROTECTION, ESCAPE-CRAWLING FROM SEA STARS, PREDATION BY BIRDS, CAMOUFLAGE, and DEFENSIVE CHEMICALS, are considered in other sections. DEFENSES OF KEYHOLE LIMPETS are dealt with separately and include camouflage, mantle response, and (sometimes) aggressive defensive activities of a symbiotic polychaete.
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Research study 1
 

graph showing tenacities and shell strengths of west-coast limpetsWest-coast limpet species are subject to stresses of wave impact, and to predator-induced prying forces of birds and crabs.  Studies on foot tenacity and shell strength in Lottia and related species at the Bamfield Marine Sciences Centre, British Columbia reveal an evolutionary selection for linking of the mechanical performances of these 2 features.  Thus, the greater the foot tenacity, the greater the shell strength (see graph). The author suggests that the close correlation may owe to selection pressure from predators such as crabs and birds that specialise in prying limpets from rocky substrata.  The  main research question in the study is how to tease out the role played in evolution of a single selective pressure, in this case shell strength, against a background of possibly conflicting selective pressures and other adaptive constraints. The author actually compares safety factors in the shells of temperate and tropical limpets (6 species on the west coast of Panama) in the context of the usual predation pressures experienced by each group. The tropical limpet species have a source of predation not experienced by west-coast limpets, that is, shell-crushing fishes that subject the margin of the shell to lateral crushing forces with their jaws.  West-coast limpets are also eaten by fishes, notably sea-perches, but these suck their prey from the substratum. Details of the tropical part of the study are not included here. Lowell 1987 Evolution 41: 638.

NOTE  several species are represented more than once in the graph, based on whether the measurements of tenacity and shell strength are made from the anterior (A), posterior (P), or right-hand side (R). The species are also arrayed in order of ascending magnitude of each force. Note that the same 4 data points are largest in each array

NOTE  the extent of strength of a shell beyond the normal breaking stresses placed on it in everyday life

 
Research study 2
 

photos showing how attachment strength of a limpet's foot is disrupted by formation of leaks courtesy Smith 1991 J Exp Biol 161: 151
Combined with the physical protection of the shell, attachment strength of the foot may be a limpet’s best line of defense against shearing and lifting forces of waves, and attempted removal by predators.  There are 2 methods of attachment.  The first is suction, which provides strong attachment force with no limitation on motility.  In this mode the foot can slide across a smooth surface, yet still remain tightly attached. At the same time, however, the foot is susceptible to “leaks” if it passes over holes or other surface irregularities. The second method of attachment is glue-like adhesion, which relies on stickiness of the foot mucus to bond to the substratum.  Its advantage is greater tenacity and thus greater resistance to shear forces, with no susceptibility to leaks.  Smith 1991 J Exp Biol 161: 151; Smith 1992 J Exp Mar Biol Ecol 160: 205.

 
Research study 3
 


In the suction mode of attachment the foot adheres by decreasing the pressure of the fluid it encloses.  Mucus around the rim of the foot helps to create a good seal.  In air, the air’s mass pushes the foot against the substratum with a force equal to the difference in pressure outside the foot and photograph of limpet Acmaea mitra being attacked by a sea star Orthasterias koehlerithe pressure inside.  In water, ambient water pressure creates the force.  If a sea star, for example, exerts an increasing upward force on the limpet, a minimum inside pressure is reached, known as the cavitation threshold.  When the fluid enclosed by the foot cavitates, it undergoes cohesive failure with sudden expansion of gas bubbles.  At that point the foot releases its attachment.  When the tide comes in and covers an air-exposed limpet with water, ambient pressure and, thus, attachment strength, theoretically increases.  The extent of increase will depend upon the strength of the foot musculature in creating a pressure differential between the fluid inside and that outside.  Once the limpet can no longer counteract the increasing hydrostatic force with increasing depth, then it can no longer attach by suction.  Smith et al. 1993 Biol Bull 184: 338.


Acmaea mitra being attacked by a sea star Orthasterias koehleri.
The limpet appears to be holding its own at this stage of the
attack, but the outcome is inevitable 0.5X

 
Research study 4
 

Limpets use the 2 methods of attachment differently, depending upon whether they are covered by the tide or exposed to air.  Studies Hopkins Marine Station, Pacific Grove, California show that when immersed by the tide, 73% of individuals of 4 limpet species sampled use suction, but when exposed to air 75% use glue-like adhesion.  Because graph showing tenacity of limpets Lottia scabra, L. pelta, L. digitalis, and L. limatula at high and low tidessuction provides less tenacity than glue-like adhesion, tenacities are therefore weaker at high tide than at low tide (0.14 and 0.18 MN . mm-2, respectively; see graph on Left).

The species used in the study, Lottia scabra, L. pelta, L. digitalis, and L. limatula, exhibit similar magnitudes of tenacity when immersed and also when emersed (see histogram on Right). The author notes that the alternating attachment mechanisms are clearly linked to the limpets’ foraging cycles.  Smith 1992 J Exp Mar Biol Ecol 160: 205.

NOTE  tenacities are measured with a 3-pronged grapple attached to a spring scale.  A Newton is the unit of force required to impart an acceleration of 1m/sec/sec to a mass of 1kg.  In terms of gravity, 1N equals the force of Earth’s gravity on an object, like a small apple, weighing 102g.  1MN = 1000N

NOTE  from the point of view of the subject being considered in this section, namely defenses, it also would be interesting to compare tenacities of these limpets in relation to potential risk of predation in their respective habitats, and also in relation to the presence or absence of certain of their known predators, such as sea stars

 
Research study 5
 

photograph of Lottia peltaThe effect of depth on tenacity of 4 species of limpets from San Pedro, California is tested by scientists within a hyperbaric chamber.  A force transducer attached to the limpets with monofilament line and plastic harness allows tenacities to be recorded at sea level and at simulated depths of 10 and 24m.  The limpets are submerged during testing.  Results show that for the 4 species averaged, attachment strength increases from 50 kPa at sea level to 59 kPa at depth (see "means" line in Table).  However, this difference owes largely to a strong and significant effect recorded for Lottia pelta (and also a large but non-significant effect for L. scabra). Neither of the other species shows a significant difference.  Increased depth from 10m (tenacity of 59 kPa for all species combined) to 24m does not increase tenacity significantly (60 kPa; not shown on the table).  The authors of the study note that the limpets’ tenacity seems to be limited by their ability to maintain a seal at their foot margins. 

Of the 4 species used in these tenacity tests, only Lottia pelta is known to live at depths below 10m; the others are generally higher in the intertidal zone and are likely subject to relatively small pressure effects from the tides.  However, in this regard, the authors note that an increase of only 2m depth when the tide is in could increase tenacity by 20 kPa, or about 25% on average for these species.  When under water, the limpets move around to forage and this greatly reduces their tenacity. The tests here are done with the limpets attached to smooth plastic, but the authors note that tenacities would likely be greater on natural, rough stone.  The authors summarise by stating that they have provided experimental confirmation of the effect of depth on suction attachment and that intertidal limpets are capable of taking advantage of this effect to some extent. Smith et al. 1993 Biol Bull 184: 338.

NOTE defined in the study as force of attachment in kPa divided by foot area.  One kPa or kiloPascal is equivalent to 1000Newtons of force . m-2.  One kP is equal to about 1% of the atmospheric pressure near sea level

NOTE it is puzzling why the authors present their data as combined means when 3 out of 4 sets are non-significant. It would seem more productive to focus on what makes the single species, L. pelta, different from the others, perhaps in shell shape, mucus consistency, or foot morphology and, more importantly from the standpoint of defenses from predators being considered in this section, whether greater exposure to predators at depth may have led to selection for greater tenacity in this species. The trend in L. scabra is also worthy of further investigation

 
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