title for limpet section of the Odyssey
  Habitats & ecology

All but one of the 2 dozen or so west-coast species of limpets live intertidally on rocky shores. 

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This part deals with competition, while topics of POPULATION & COMMUNITY DYNAMICS, HOMING & TERRITORIALITY, SHELL GROWTH (SHAPE) & COLOUR, SEASONAL MOVEMENTS, and LIFE IN THE INTERTIDAL ZONE are presented in other sections. Limpets potentially compete for space and food with conspecifics (intraspecific competition) and with other species of limpets and algal-eating invertebrates (interspecific competition). 

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Research study 1

Although many species have overlapping distributions, habitat differences are such that they actually seem to coexist without competition.  For example, in the Monterey Bay region of California 2 closely related species Lottia digitalis and L. scabra live at similar intertidal heights on the same shore and eat the same foods; hence, are potentially competing.  Closer analysis of microhabitats occupied, however, shows that digitalis prefers vertical surfaces while scabra favours flatter, drier1 surfaces.  The first species aggregates to reduce water loss while the second occupies home sites2 that fit the shell-shape of an individual perfectly.   The end result is a separation3 of the microhabitats of the two species, minimising competition.  Haven 1970 Veliger 13: 231.

photograph of Lottia scabra in a shallow tidepool on a rocky beach near Monterey, Californiaphotograph of a cluster of Lottia digitalis on a vertical sandstone wall at Port Renfrew, British ColumbiaNOTE1  sunlight hits a flatter surface at a steeper angle of incidence and is therefore more intense, especially in summer

NOTE2  the shell of the limpet grows precisely in shape to fit irregularities in the rock surface

NOTE3  might there be other, alternative, explanations for the apparent preference of Lottia digitalis for vertical surfaces?  This is considered in a study that re-examines the spatial segregation of L. digitalis and M. scabra from the perspective of predation by black oystercatchers: see DEFENSES & PREDATORS: PREDATION BY BIRDS

Left: a cluster of Lottia digitalis on a vertical sandstone wall at Port Renfrew, British Columbia 1X Right: 3 Lottia scabra in a shallow tidepool on a rocky beach near Monterey, California 0.7X

Research study 2

graph of number of individuals of Lottia digitalis and Lottia scabra in relation to substratum angle at Dillon Beach, CaliforniaThe different preferences of Lottia digitalis and L. scabra for angle of substratum surface occupied is confirmed in a later study at Dillon Beach, California.  Note in the graph that as substratum angle increases, density of Lottia digitalis increases, while density of L. scabra decreases.  There is a significant tendency, moreover, for size of the latter species to decrease slightly as angle of substratum increases (a similar tendency in L. digitalis is not significant).  The study adds size of individual to a growing list of factors relating to desiccation in limpets (temperature, air movement, RH, salinity of bathing fluids, composition and texture of rock surfaces, species behaviour, and so on) and on this basis the author remarks that it is no wonder that the relationship between desiccation resistance and distribution of sympatric limpet species is unclear.  Collins 1976 Veliger 19: 199.

NOTE  in a continuation of this comparative study the author marks individual limpets of both species and finds that as angle of substratum increases, L. digitalis moves around relatively more, while L. scabra moves around relatively less.  Collins 1977 Veliger 20: 43. 

Research study 3

graph of shell lengths of limpets Lottia scabra in high- and low-density plots in Bodega Head, CaliforniaIn Bodega Head, California larval Lottia scabra settle widely throughout its natural intertidal range of about 1.2-2.0m above MLLW. With its strong homing tendencies, individuals tend not to move much up or down the shore after settling.  This propensity to “stay put” helps to create a shore-level size-gradient because higher-level individuals grow faster and become larger than lower-level ones, possibly owing to less intra- and interspecific food-competitors being present in the higher zone. Natural densities are about 5 times greater in the low-zone population than in the high-zone one. Because of the larger size of individuals in the high-zone , total biomass of L. scabra is actually greater there than in the low-zone.  A test of whether intraspecific competition for food may be reducing growth of L. scabra in higher densities involves creating 2 sets of plots in the low-zone region of the shore with different densities of limpets. In one set of plots, the start densities are about 1400 individuals . m-2, equivalent to natural densities at that shore level, while in another set of plots the start densities are about 275 individuals . m-2, equivalent to natural densities at the higher shore level.

Results after 6mo show that growth in the low-density experimental plots is significantly greater than in the high-density ones (see graph). The author describes a type of "intraspecific aggression" in these populations, involving an individual pushing with its shell against another until the “loser” turns away and moves off in a different direction. While this could be just chance encounter of individuals while foraging and not really "aggression", if one individual turns away (as from a good feeding area), the end result would be the same. Sutherland 1970 Ecol Monogr 40: 169. 

Note the plots are unfenced, but the author clears them at regular intervals of other herbivorous invertebrates. The number of replicates for each treatment is not mentioned

Research study 4

histogram of reproductive output of Lottia scabra in Bodega Head, CaliforniaOne would predict that the greater biomass of Lottia scabra in the high-level region would have greater energy output in the form of reproductive products, but this may not be the case.  In a 2yr study at Bodega Marine Laboratory, California where reproductive-output data are collected along with other energetic aspects of the 2 populations (see Research Study 2 above), reproductive output differs from year to year and from zone to zone, but not always in the pattern predicted. Thus, compared with output from the low-zone population, output from the high-zone population is less and tends to be more variableSutherland 1972 Ecology 53: 430.

NOTE  the data shown here differ from what the author presents and should be considered more of a "visual synopsis" of what is presented in the original paper. The author has data for only 1yr for the low-zone population, but assumes, based on the steadiness of other population parameters, that reproductive output would likewise be relatively constant from year to year. The reproductive data are obtained from decreases in live mass of individuals in the populations, and these are assumed to represent spawnings. Such estimates are likely to be much less accurate than direct measures of gonadal indices as is more commonly done in such studies

Research study 5

graph of growth of L. scabra in the presence and absence of L. digitalis over an 8-mo period in Monterey Bay, California, and of Lottia digitalis in the presence and absence of Lottia scabra over an 8-mo periodA study on sympatric and potentially competing Lottia scabra and L. digitalis in Monterey Bay, California involves removing each species in turn from fenced enclosures and monitoring subsequent effects on growth of the species remaining. In the first experiment, which compares growth of L. scabra in the presence and absence of L. digitalis over an 8-mo period, food competition between the species is strongly suggested (see upper graph on Right). One to two months after L. digitalis is removed, algae begin to grow and algal cover increases for a few months.  At this time growth of L. scabra in the absence of L. digitalis is significantly greater than when L. digitalis is present. By the 5th month the algae are eaten and the rocks become as bare as in the control plots. 

In the reciprocal experiment, where growth of L. digitalis is monitored in the presence and absence of L. scabra a trend is apparent, but the data do not differ significantly.  Overall, L. digitalis seems to have a greater effect on L. scabra than the reverse, possibly because of its superior size. The results suggest a direct exploitative competition for food by these 2 limpet species. Haven 1973 Ecology 54: 143.

NOTE  20 x 40cm, with a 4.5cm-high screen-mesh fence

Research study 6

A study on 3 species of limpets inhabiting concrete wharf pilings at Ellwood Pier, Santa Barbara region reveals indications of interspecific competition, presumably of an interference type, between 2 of the species.   The left of the 2 graphs shows the average vertical distributions graphs of interference competition in limpets Lottia digitalis, L. paradigitalis, and L. scabra on pilings at Ellwood Pier, Californiaof Lottia digitalis, L. paradigitalis, and L. scabra on the control sides of 3 pilings over a 1-yr period.  Levels occupied are fairly constant, but with a tendency for some downward movement through the warm months of summer and a return upward movement during winter.  The right of the 2 graphs shows the effect of removing Lottia digitalis on distributions of the other species.  Lottia scabra is largely unaffected, perhaps owing to its occupation of home scars in barnacle clearings, but there is a quick and significant upward response by L. paradigitalis into areas formerly grazed by L. digitalis.  Survival of L. paradigitalis also goes up in the absence of its competitor.  Choat 1977 J Exp Mar Biol Ecol 26: 1.

NOTE the taxonomic status of this species is uncertain. At least one author thinks it may be a hybrid between digitalis and pelta

Research study 7

Many species of limpets overlap in distribution with barnacles and may compete with them for space on the rocks.  The limpets require grazing space while the barnacles require growing space.  At first, barnacles encourage recruitment of limpets by providing 3-dimensional settling habitat, as well as photograph of a limpet Lottia pelta that appears to have scoured its grazing patch free of barnacleprotection from drying and predators.  Later, as grazing space diminishes with increased settlement and growth of the barnacles, the limpets may move to higher regions of the shore or, as has been discovered in some species, they may grow more slowly when among barnacles and remain at a smaller size.  Throughout, however, the limpets aggressively compete for space by bulldozing newly settled barnacle spat from the rocks with their shells and tentacles, and sometimes eating the spat.


A limpet Lottia pelta appears to have scoured its grazing patch free
of barnacles. Most barnacles are gone while others are worn smooth.
A few newly settled spat of Chthamalus dalli appear to be surviving
by virtue of inhabiting depressions and living on the limpet's shell 2X