Research Study 1

Fig. 1.  A cluster of Lottia digitalis on a vertical sandstone wall at Port Renfrew, British Columbia.

Fig. 2.  Three Lottia scabra in a shallow tidepool on a rocky beach near Monterey, California.

Limpets potentially compete for space and food with conspecifics (intraspecific competition) and with other species of limpets and algal-eating invertebrates (interspecific competition).  Although man ylimpet 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 two closely related species Lottia digitalis (Fig. 1) and L. scabra (Fig. 2) live at similar intertidal heights on the same shore and eat the same foods; hence, are potentially competitive.  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.

NOTE¹  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 MOLLUSCA>LIMPETS & RELATIVES>PREDATORS & DEFENSE>PREDATION BY BIRDS

Research Study 2

Fig. 1.  Effect of substratum angle on limpet density

The 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 Fig. 1 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 L. scabra 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.

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

NOTE  this begs the question as to how a limpet assesses substratum angle.  Has this been looked at?

Research Study 3

Fig. 1.  Effect of density on growth of limpets Lottia scabra

In Bodega Head, California, larval Lottia scabra settle widely throughout the natural adult 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. Also, natural densities are about five 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 two 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 Fig. 1). 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.

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 known

Research Study 4

Fig. 1.  Supposed energy output by Lottia scabra at different intertidal levels

One 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 two populations, reproductive output differs from year to year and from zone to zone, but not always in the pattern predicted (Fig. 1). Thus, compared with output from the low-zone population, output from the high-zone population is actually less and tends to be more variable. 

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

Fig. 1.  UPPER GRAPH: effect of presence of Lottia digitalis on growth of L. scabra          LOWER GRAPH: effect of presence of Lottia scabra on growth of L. digitalis

A 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 8mo period, food competition between the species is strongly suggested (Fig. 1). 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 fifth 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 (Fig. 2).  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 two limpet species.

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

Research Study 6

A study on three species of limpets inhabiting concrete wharf pilings at Ellwood Pier, Santa Barbara region reveals indications of interspecific competition, presumably of an interference type, between two of the species.   The leftmost of the two graphs shows the average vertical distributions of Lottia digitalis, L. paradigitalis, and L. scabra on the control sides of three pilings over a 1yr 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 two 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. 

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

On rocky wave-swept shores in central California the limpet species¹ Lottia scabra and L. austrodigitalis occupy similar upper-shore habitats and appear to feed on similar microphytobenthos² organisms.  Are they competing for food? This challenging question is tested by researchers at 6 sites in Monterey Bay by comparing three treatments: ungrazed, grazed by L. scabra, and grazed by L. austrodigitalis.  In fact, as might be expected from the microscopic jumble of the food source and lack of any apparent fine-scale sorting mechanism in a limpet's gut, the species do not partition the food resource and, hence, do not appear to engage in food-based competitive exclusion.  Somehow, though, perhaps through taste selection, both species do graze their foods such that more nutritious³ components are eaten in preference to others, resulting in lower quality foods being left behind.  Although temperature seems not to be strongly involved, the authors unconvincingly couch their results in a context of global warmng.

NOTE¹  these consist of mixed blue-green algae, diatoms, and macroalgal spores, germlings, and juvenile plants, as well as numerous invertebrate larvae

NOTE²  the two species are morphologically similar and the authors caution that identifications in a few instances may have been confused

NOTE³  these less-nutritious foods surprisingly have a higher proportion of nitrogen to carbon