Limpets on Atlantic Shores: Nahant

Predator-prey relationships are usually assumed to be negative interactions, with predation limiting prey distribution and abundance (Littler and Littler 1995).  Some predator-prey relationships involve host-plant specialization and incur benefits to the prey (Hay et al., 1989; Littler and Littler 1995). This is evident in many terrestrial species: grazers maintain grassland biomass (McNauhghton 1979), omnivorous mammals in North America aid in seed dispersal (Willson 1993). Selective pressures that give rise to these relationships are not understood (Hay et al., 1989).  Marine specialists may be easier to understand due to their rarity and occurrence only intense selective regimes (Hay et al., 1989), yet few studies have investigated marine mutualisms (Vermeij 1983). 

Marine herbivores typically have a pelagic larval stage with little ability to actively select its landing habitat.  Generalist herbivores able to colonize on multiple substrates would have a competitive advantage over specialists.  Plant-host specialization is therefore unlikely to occur except in rare situations with extreme selective pressure on later life history stages (Hay et al., 1989).  Marine examples follow a pattern described by Steneck (1992): small herbivores with low mobility, low dietary needs, and high predation risk tend benefit from associating with slow-growing, predictable macroalgae.

Robert Steneck (1982) did the first experiments to conclusively determine an association between the corralline algae Clathomorphum circumscriptum (Figure 1) and the limpet Tectura testudinalis.  T. testudinalis was found not only in higher abundances on C. circumscriptum, but also to preferentially forage on it.  Most juvenile limpets were found on C. circumscriptum, suggesting preferential settlement there by limpet larvae.  The radula of T. testudinalis is morphologically specialized to ‘bite’ through calcareous epithallial cells, such as those on C. circumscriptum.  Gut and feces content analyses determined that C. circumscriptum is the primary prey of T. testudinalis (Steneck 1982).  Steneck asserted that morphological characteristics of the coralline algae, such as multilayered epithallus, reduced calcification, starch in epithallial cells, and lost epithallial sloughing, indicated co-evolution between C. circumscriptum and T. testudinalis, though Pueschel and Miller (1996) argue that these traits are comparable throughout coralline species.  T. testudinalis only directly associates with encrusting algae during foraging; it preferentially seeks other substrates for resting though it does not exhibit homing behavior (Lord 2008).  Limpets spend little time moving diurnally, the bulk of which occurs in the first half hour of daylight when limpet movement accelerates until it finds a vertical surface resting site without encrusting algae (Lord 2008). 

These studies focus on the foraging activities of T. testudinalis and how they relate to its algal association.  Its diurnal, resting preferences are indirectly investigated.  Steneck (1982) reasoned that T. testudinalis benefits from resting on C. circumscriptum because it is smooth and planar, unlike most coralline species.  This allows the limpet to bind flush against the substrate, decreasing the effects of abiotic factors and predation.  Lord (2008) found that limpets did not associate with encrusting algae diurnally when given the options of C. circumscriptum, H. rubra, rock, and a tin pan in a laboratory setting.  Vertical surface may be the principal factor in resting site selection for this non-homing limpet, but substrate is likely still important.  This study seeks to determine what substrate(s) T. testudinalis associates with for a home site diurnally while emersed in a field site on the Massachusetts coast.  Distance from C. circumscriptum is also considered.