title used in an account of west-coast marine invertebrates entitled A Snail's Odyssey
  Predators & defenses
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  Whelks
  In this section whelks as predators are considered, while studies on FISHES, LIMPETS, SEA STARS, and INSECTS are found in other sections.
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Research study 1
 

simulation of 3 whelks crawlin on the shore Nucella lamellosa, N. canaliculata, and N. ostrina courtesy Linda Schroeder, Northwest Shell A 9yr study in San Juan Islands, Washington shows that while barnacles Balanus glandula settle over the entire vertical range of the shore, they survive only at the top of the shore.  This mainly owes to the predatory activities of 3 species of whelks Nucella spp. that not only eat most or all Balanus, but also other barnacles including Semibalanus cariosus and Chthamalus dallli.  The whelks consume from a lower intertidal position and effectively set the lower distributional limits on all 3 species of barnacles.  The high-zone Balanus exist in spatial refuge while the lower-zone settlers are fodder for the whelks.  The author’s determinations of feeding rates of the whelks in this area are consistent with the loss of barnacles throughout schematic showing distributions of west-coast barnacles and their whelk predatorsthe year.  Connell 1970 Ecol Monogr 40: 49. Photographs courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC and Dave Cowles, Walla Walla University, Washington rosario.wallawalla.edu.

Schematic showing zones of competition between 3 species of west-coast barnacles,
and the effects of predation by 3 species of whelks Nucella in setting lower limits of
distribution of the barnacles. Regular annual settlement of barnacles B. glandula
below its refuge zone at the top of the shore provides a food supply for Nucella spp.
and has created, in an evolutionary sense, a special high-level niche for N. ostrina

 
Research study 2
 

photograph of a cross-sectional view of a barnacle test to show suture lineshistograms showing drill-hole dispositions of whelks Nucells spp. in barnacle plates and suture linesStudies at Friday Harbor Laboratories, Washington on location of drill sites in barnacle prey by 4 species of Nucella1 indicate a selection for suture lines between plates.  These are areas of growth where the test is thinner (see photo of an Atlantic-coast barnacle B. improvisus on the Left). For Balanus glandula, combined drillings by the 4 whelk species (348 boreholes) were 69% at the suture lines and 31% on the plates (see lower histogram on Right). For Semibalanus cariosus the comparable values are 54% and 46% (see upper graph on Right). Interestingly, although suture lines are significantly preferred for B. glandula (but not for S. cariosus), final consumption is invariably via the opercular plates which gape open. likely following injection of a toxin2 by the whelk.  This may explain why most descriptions of whelks eating barnacles have the predator feeding in an opercular-plate location.  In this way, the whelk need only drill a small hole to inject its toxin and then move to the opercular region where no further drilling is needed. The author suggests that the propensity of whelks to select suture lines as primary attack sites may have been a strong selective factor in the evolutionary reduction3 of plate number in balanomorph barnacles from 8 and 6 plates, to the more common present-day number of 4.  Palmer 1982 Paleobiology 8: 31.

NOTE1  N. lamellosa, N. ostrina, N. canaliculata, and N. lima.  For convenience, data for the 4 whelk species are combined here, but there are some significant between-species differences present in the data

NOTE2  a toxin has been shown for a related thaidid Thais haemastoma (southern Atlantic coast of N.A. and areas of the Gulf of Mexico)

NOTE3  although apparently not in either Balanus or Semibalanus genera which, according to the author, have remained constant at 6 plates in the fossil record

 
Research study 3
 

histogram showing settlement of cyprid larvae of barnacles Balanus glandula and Semibalanus cariosus onto treated slate platesIs it possible that  settling marine invertebrate larvae, such as barnacles, avoid areas where there will be a high probability of death from a superior space-inhabiting competitor or a predator?  Few tests of this hypothesis have been done, but the answer seems to be 'yes' for studies done at the Friday Harbor Laboratories, Washington, showing that Balanus glandula larvae settle less on slate plates previously crawled upon by predatory whelks Nucella lamellosa.  The experimental protocol involves suspending slate plates in the sea in late winter to develop a microflora suitable for settlement of barnacle cyprids in the springtime.  The plates are cross-hatched with grooves (1 x 1mm) for further inducement for the larvae to settle.  In early spring, 23 whelks are allowed to crawl around for 2d on the experimental plates (within cages).  Each experimental plate is paired with a no-whelk control plate.  After 2d, the whelks are removed, settled barnacles scraped clean from both experimental and control plates, and the plates placed out on the shore 1.3m above MLLW.  After 4-d exposure to settling cyprids the plates are brought to the laboratory and attached barnacles counted. 

histogram showing recruitment of cyprid larvae of barnacles Balanus glandula and Semibalanus cariosus onto treated slate platesResults show a significant avoidance of the whelk-inhabited plates by larvae of Balanus glandula but not by larvae of Semibalanus cariosus (see histogram upper Right). Recruitment of Balanus is also significantly more on the control plates than on the experimental plates.  Clearly, some residual effect of the presence of the whelks, likely associated with their foot mucus, deters settlement of Balanus.  Mucus itself is not a deterrent because the larvae of Semibalanus readily settle onto the whelk-inhabited plates. 

An avoidance of substratum occupied by whelks is a good strategy for Balanus because they rarely survive in the zone frequented by predatory Nucella spp. and, instead, persist in refuge above this zone.  As to why Semibalanus cariosus larvae respond differently, the authors offer the following explanation.  The species is physically incapable of surviving above the zone frequented by whelks but, because of their thick shell plates and growth to large size, they do reach refuge from predation in the whelk zone.  An avoidance by Semibalanus larvae of substratum occupied by whelks, therefore, might actually be deleterious to their survival.  Johnson & Strathmann 1989 J Exp Mar Biol Ecol 128: 87; see also Grosberg 1981 Nature 290: 700.

NOTE refers to survival of the settled barnacles to juvenile stage, as opposed to settlement, which refers to attachment and metamorphosis of the cyprid larvae

NOTE  other findings presented by the authors, such as avoidance of barnacle settlement on plates coated with mucus from the brown alga Fucus and also on plates coated with foot mucus of limpets Lottia scutum, and stimulus of settlement by mucus from nudibranchs Archidoris montereyensis and Onchidoris bilamellata, the latter a predator of barnacles, suggests that there is more to this story, and further research is justified.  Johnson & Strathmann 1989 J Exp Mar Biol Ecol 128: 87.

 
Research study 4
 

map of California showing range extension of volcano barnacles Tetraclita rubescens from San Francisco north to Cape MendocinoThe volcano barnacle Tetraclita rubescens is usually considered a southern California/Baja California species but, over the past quarter of a century, has been extending its range northwards, perhaps following warming water temperatures (see map on Left.  Their survival may be related to a failure of northern whelk predators Nucella ostrina and N. canaliculata to eat them.  Cafeteria-style feeding experiments at the Bodega Marine Laboratory, California show that whelks Nucella ostrina eat many fewer Tetraclita than other barnacles, including Balanus glandula and Semibalanus cariosus.  Tetraclita is unusual in that it has only 4, instead of 6, parietal plates, meaning that there are less suture lines available for drilling into by the whelk.  Furthermore, the side plates extend up more in Tetraclita than in the other barnacles, hiding the opercular plates – another popular spot for Nucella to drill.  Interestingly, while the northern histograms showing predation by whelks Nucella ostrina and N. canaliculata on species of barnacles Semibalanus cariosus and Tetraclita rubescensspeciesof Nucella (ostrina and canaliculata) rarely attempt to drill into Tetraclita, the southern whelk N. emarginata readily preys on this barnacle in laboratory trials perhaps, as suggested by the authors, reflecting a longer shared evolutionary history between these species.

What if the tops of the plates are filed off, permitting easier access of the whelk to the operculum? This is done with Tetraclita and Semibalanus, with treated individuals being exposed to predation by the northern species of whelks N. canaliculata and N. ostrina. Results show that photograph of volcanto barnacle Tetraclita rubescens and thatched barnacle Semibalanus cariosussignificantly more Semibalanus are eaten than Tetraclita by both species of whelks, as expected, but no significant differences are found for barnacles with the tops of their plates filed off versus intact control individuals (see histograms upper Right). Sanford & Swezey 2008 J Exp Mar Biol Ecol 354 (2): 220.

NOTE for range extensions see Connally & Roughgarden 1998 Calif Fish Game 84: 182

NOTE the researchers include a third treatment in the experiment: a treatment control in which the barnacles are filed on the sides of their tests and not the tops


The thick vacuolated test plates ofTetraclita species throughout the
tropics are usually thought to be adaptive for thermal buffering.
However, based on the results ofthe present study, they may also
provide protection from boring predators such as Nucella. The
aspect of thermal buffering in T. rubescens seems not to have
been investigated
by west-coast researchers

 
Research study 5
 

photographs of 3 morphs of acorn barnacle Chthamalus fissusgraph showing morphologies of oval and narrow apertured morphs of barnacles Chthamalus fissusDoes aperture morphology of acorn barnacles provide protection against predatory whelks?  This idea arises from results of a study on southern California/Baja California barnacles Chthamalus fissus. The species exists in 3 morphs, one with an oval operculum found on rocks around La Jolla, California, one with a narrow, slit-like operculum, and one with an operculum bent to one side, the  last 2 being found south of Ensenada, Baja California (see graph on Right).  The bent morph is by far the least common.  Translocation experiments involving reciprocal exchange of rocks with attached juvenile barnacles suggest that the morphologies owe to plasticity of expression affected by local conditions, and not to genetic differences.  A few months after translocation the oval morph from La Jolla begins to resemble the narrow morph and, by 9mo, at the end of the experiment, the translocated barnacles and native barnacles have indistinguishable narrow apertures. Additionally, a few of the La Jolla transplants (about 8%) develop into the bent morph.  The relevance to barnacle defense is that in a 3d predation study a local whelk Mexacanthina lugubris finds the oval morph easier to attack and eat than the narrow morph, and the bent morph is not eaten at all.  Thus, although aperture morphology may be proximally determined by local environmental conditions, there may be additional survival benefit from predators. Jarrett 2008 J Crust Biol 28 (1): 16.

NOTE  this species was previously limited to Ensenada south, but is now moving north into the San Diego region

NOTE  the author does not test for specific environmental stresses such as waves, desiccation, and temperature on morph survivorship, which would be a good follow-up study if not already done, but does mention that perception of the whelk might act as a proximal cue for the transformation – also with good potential as a follow-up research project

 
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