title for limpet section of the Odyssey
   
  Habitats & ecology
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  Population & community dynamics
  This part of habitats & ecology deals with population & community dynamics, while topics of COMPETITION, HOMING & TERRITORIALITY, SHELL GROWTH (SHAPE) & COLOUR, SEASONAL MOVEMENTS, and LIFE IN THE INTERTIDAL ZONE are presented in other sections.
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Research study 1
 

histograms showing number of limpets of several species on pilings at Santa Barbara, CA before and after manipulations of density of limpetsAre limpet populations regulated in size? This question is addressed in a study involving manipulation of densities of limpets on pilings in 2 locations in Santa Barbara, California.  Six pilings are used in each experiment.  From one pair, the DECREASED-DENSITY pilings, all Lottia are removed and transferred to another pair, the INCREASED-DENSITY pilings.  The third pair of pilings is left as a CONTROL.  After 2.5mo, limpet numbers on the pilings are re-counted, with the following main results: 1) density on the control pilings more than doubles, likely owing to springtime larval settlement (see Right set of bars in histogram), and 2) taking into account the magnitude of this recruitment increase, limpet densities on the other 2 sets of pilings are more or less back to their original numbers.  Their numbers have been regulated.  Stimson & Black 1975 Oecologia 18: 111.

NOTE  the authors define regulation as the return of a population to an equilibrium density

NOTE  4 species are present on the pilings: Lottia digitalis, L. strigatella, L. scabra, and  L. pelta in the ratio 3:3:1:<1. 

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If regulation does, in fact, occur, how is it accomplished? Consider the possibilities listed listed below, then CLICK HERE for explanations.

The limpets migrate from piling to piling in density-dependent responses. 

Settling limpet larvae appraise the density of adults in a habitat and avoid areas where densities are high. 

Settling limpet larvae use other cues to assess the suitability of a habitat. 

Settling limpet larvae and juveniles suffer less mortality on the decreased-density pilings. 

 
Research study 2
 

photograph of limpet Lottia digitalisphotograph of control plot with limpets present 3 months after start of experiment courtesy John Cubit
The best and perhaps the only way to determine the role of limpets or any dominant consumer in the community is to remove them and monitor the effects of the removal over time.  In open-coast areas of Oregon, where Lottia digitalis is the most abundant herbivore in the high intertidal area (1.8-2.8m above MLLW), exclusion of these and other limpets from experimental plots graphically illustrates the effect of these herbivores on the algal community.  Copper paint, over which limpets will not pass, is used to demarcate 2 plots of about 2000cm2 each.  From one of these, the EXPERIMENTAL plot, all limpets are removed.  The other,photograph of experimental exclusion plot with limpets Lottia digitalis prevented from entering by strip of copper paint.  Three months later the algae growth is rich the CONTROL plot, is left untouched.  After 3mo the results are as shown in the photos, with a luxurious growth of algae appearing in the control plot and the experimental plot remaining bare.  Cubit 1984 Ecology 65: 1904.  Photographs of rocks courtesy John Cubit.

NOTE  the paint is an anti-fouling type, but with the addition of copper powder

NOTE  these include diatoms, filamentous red and green algae (Bangia and Urospora), and membranous red (Porphyra) and green (Ulva) algae

 
Research study 3
 


schematic depicting a web of interactivity of herbivorous limpets, barnacles, and algaeIn the Montecito area near Santa Barbara, California, limpets Lottia pelta are the dominant invertebrate herbivores on upper- to mid-intertidal boulders.  Other members of the community are limpets Lottia digitalis and L. scabra, barnacles Balanus glandula, Chthamalus dalli, and C. fissus, chitons Mopalia sp. and Nuttallina sp., macroalgae Ulva sp. and Enteromorpha sp., and microalgae including diatoms and sporelings.  By scraping and burning 20 x 60cm areas on large boulders in January and then monitoring their recolonisation for 5mo, successional events, both direct and indirect, can be determined.  Some plots are subdivided to allow removal of all limpets and chitons, or of all barnacles. 

photograph of intertidal area showing a mixed community of macroalgae, mussels, barnacles, and limpets (L. digitalis) in Barkley Sound, British Columbia, perhaps similar to the one in Santa Barbara.The results show that algal mats, the first macro-colonisers, inhibit the settlement of barnacles 1.  Herbivores remove the algal mats 2, thereby indirectly facilitating the recruitment of barnacles 3.  By inhibiting grazing by limpets 4,  barnacles indirectly provide refuge for macroalgae 5.  In the absence of herbivores, macroalgae grow in dense mats that directly interfere with feeding of barnacles 6The study emphasises the indirect, as well as direct, influences that these molluscan herbivores have on successional events in west-coast intertidal communities.  Van Tamelen 1987 J Exp Mar Biol Ecol 112: 39.


A mixed community of macroalgae, mussels, barnacles, and limpets (L. digitalis)
in Barkley Sound, British Columbia, similar to that described in Santa Barbara.

 
Research study 4
 

Perturbation-type experiments like the one in Research Study 3 bring up questions of community stability.  How resilient is a community to, say, removal of a dominant herbivore?  Or removal fo a dominant carnivore?  On replacement of the missing species will the community recover or be permanently altered? The answers are hypothesised to depend upon the following: 1) if the consumer can still eat its food species (i.e., the food species have not become too large or otherwise protected), then their populations within the community will quickly re-establish to former levels; 2) if the food species grow to a large, inaccessible size or become otherwise protected, then return to “normal” will be delayed until these protected individuals die off naturally; and 3) if, however, these "inaccessible" individuals provide refuge in some way for new recruits from being eaten by the consumer, then the community may never return to “normal” status.

histograms showing percentage cover of algae and barnacles in experimental plots with and without limpetsThese ideas are tested in a study in Boiler Bay, Oregon in which limpets, mainly Lottia digitalis (65%) and L. strigatella1 (35%) are removed along with all algae, barnacles, and other organisms, from small patches2 (plots) on vertical rock faces on the shore.  Other patches, similarly cleared, are left as controls.  The research question is how resiliant is the community, specifically, its algal and barnacle3components, to removal of the consumer species, the limpets? Both short- and long-term exclusions are done, but only results from the former are presented here. Results show that shortly after the plots are cleared, they are quickly and heavily colonised by algae (see Left bar in histogram).  Barnacles also recruit to the plots and some are initially killed by algal overgrowth.  Within 1yr the plots are fairly stable.  At this time, the limpets are allowed to return to the plots and this leads to quick return of algae to pre-clearance levels.  Barnacles, however, persist for about 17 months before dropping to pre-clearance levels.  The author concludes that limpets have different time-related effects on the community members.  Algae are strongly affected but, after removal of the limpets, the new algal community is not a persistent alternative state.  Also, because re-introduction of the herbivores causes an immediate return to the pre-removal state (see #1 above) then the limpet removal does not exceed the “resiliance capacity” of the community.  The case of the barnacles, however, is different, and here the resiliance capacity of the community is exceeded.  In the absence of limpets the barnacles grow to a size where they are in refuge from bulldozing by the limpets on their return (see #2 above), and only later when these resistant individuals die out does the community return to pre-exclusion levels.  Farrell 1988 Oecologia 75: 190.

NOTE1 the taxonomic status of this species is uncertain. It appears to be sometimes described as L. paradigitalis, which itself may be a hybrid between L. pelta and L. digitalis

NOTE2  the patches, 15 x 15cm in size at 2.0-2.8m above MLLW, are first scraped clean of all organisms, then the experimental ones are surrounded by 2-3cm borders of copper-based paint which exclude limpets from re-entering.  At the start of the study, limpet densities in some patches exceed 2000 individuals . m-2 .   After 1yr the paint is chiseled off and within 2-3wk the limpets are back.  The study is interesting in that it includes not only unpainted control plots, but also “paint-control” ones (i.e., with normal densities of limpets enclosed) to monitor effects of the paint itself on the community.  These latter data, although useful to the author in interpreting results in the other control and experimental plots, are not included here

NOTE3  Chthamalus dalli and Balanus glandula

 
Research study 5
 

The interactive effects of limpet herbivory with respect to the presence or absence of barnacles Balanus glandula is examined in a study at Bodega Marine Laboratory, California.  Here, the dominant grazers are limpets Lottia digitalis and L. scabra, and littorines Littorina plena.  In one of several sets of experiments, 2 histograms showing percentage cover of algae with and without limpets, barnacles, and littorines being presenttypes of experimental plots are created: those cleared of all algae and barnacles and those cleared of algae but with the original complement of barnacles left intact. After 5mo, in the plots where limpets only are removed1 with normal barnacle cover, there is little effect on algal recruitment2 (see second bar in from Right in histogram). However, if littorines are also removed, algal cover increases significantly (see Right-hand bar). When limpets are removed from plots where all barnacles have been cleared, algal cover again increases, despite the presence of littorines (see second bar from Left; note that the first bar indicating algal cover in the presence of both limpets and littorines is zero). In summary3, littorines are the more effective grazers where barnacles are abundant, while limpets are more important where barnacles are absent. The results underscore the role played by barnacle cover in the importance of each herbivore in the community.  Geller 1991 J Exp Mar Biol Ecol 150: 1.

NOTE1  the author creates limpet exclosures by application of 3-cm borders of copper paint.  Other exclosures for littorines are created by application of borders of highly adhesive vegetable-gum resin (Tree Tanglefoot) to which wandering snails stick.  This material quickly fouls, but overall, both limpet and littorine abundances are greatly reduced in the experimental plots

NOTE2  in these barnacle-dominated areas the algae consists mainly of diatom mats interspersed with blue-green algae and filamentous green algae

NOTE3  the one set of data shown here represents just a small part of the entire study.  It is not clear how the barnacles exert their effect: perhaps in providing protective cover for littorines and/or restricting movement of limpets? Note also that each diagonal line of text on the histogram refers to a single bar. The first bar, that of "limpets & littorines present", is zero...they eat all the the algae

 
Research study 6
 

In the San Juan Islands, Washington 2 species of crustose coralline algae Lithophyllum impressum and Pseudolithophyllum whidbeyense compete for space on tidepool rocks.  Interestingly, in the upper shore zone L. impressum tends to dominate, while in the lower shore zone P. whidbeyense is dominant.  The switch-over occurs at about the +1m level above MLLW (see graph on Right).

histogram showing percentage of limpets penetrating algal mats to different depths in high and low intertidal tidepoolsWhat causes these differences? Both species are grazed by limpets, primarily Lottia pelta and L. scutum, and it is a difference in frequency of radula scraping and depth of scraping by the limpets that seems to regulate which coralline alga will be dominant.  At high levels the limpets graze often but at low penetration depth (about 20um on average), while at low levels they graze relatively less frequently but at a higher penetration depth (about 30um; see histogram on Left).  The algae differ in their susceptibilities to the grazing based on crust thickness and healing capability. Thus, L. impressum has a thick, multi-layered crust that allows it to survive well in the upper-level tidepools, while P. whidbeyense has a thinner crust and is more susceptible to injury from limpets.  In its weakened state, it succumbs to overgrowth by the more robust L. impressum.   In contrast, at the lower tidal levels where injury to the algae is apparently common, healing in P. whidbeyense is much faster and leads to a thicker crust than in L. impressum, and the former easily overgrows and outcompetes the latter for space.  Steneck et al. 1991 Ecology 72: 938.

NOTE  there are at least 5 genera and 9 species of encrusting coralline algae on the west coast and the taxonomy is in a state of flux.  Species are difficult to distinguish in the field and must be separated by differences in crust thickness, cell size, and reproductive structures.  Mondragon & Mondragon 2003 Seaweeds of the Pacific Coast Sea Challengers, Monterey, California.

 
Research study 7
 

graph comparing commercial landings of owl limpets in different regions of CaliforniaThe effects of climate change on population and community dynamics of marine invertebrates is becoming a popular subject for west-coast researchers.  As an example, scientists at the Bodega Marine Laboratory, California examine possible future synergistic interactions between global warming and commercial fishing pressure on distribution and abundance of owl limpets Lottia gigantea, California sea mussels Mytilus californianus, and black abalone Haliotis cracherodii. In all 3 species, effects of climate change, most notably warming of air and ocean, are likely to force distributions downwards on the shore and into subtidal regions. The authors note that owl limpets, in particular, may be most affected by global warming owing to their relatively greater sensitivity to thermal stress. The shrinking intertidal populations of limpets will more and more become the focus of fishers, and especially vulnerable will be the larger individuals, or females, that represent the major breeding stock.  Through synergistic effects of climatic warming coupled with fishing pressure, then, local populations of Lottia gigantea are likely to be even further reduced or even eliminated. Harley & Rogers-Bennett 2004 Calif Cooperative Oceanic Fisheries Invest Rep 45: 98.

NOTE  selection of these particular species for study is puzzling, as none is associated with a commercially viable fishery.  Owl limpets support a small, artisanal-type fishery mainly in Baja California, but populations in California have been over-fished in all but marine protected areas (see graph) and a commercial fishery has been closed for 2 decades.  A small recreational fisheries exists in Californa. California sea mussels presently support only a modest fishery (mainly for bait) and a fishery for black abalones, along with other California-abalone species, is essentially non-existent and/or closed (the fishery for black abalone was closed in 1993)

 
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