Research Study 1

Black oyster-catcher Haematopus bachmani  in Barkley Sound, British Columbia

An early study on predation of limpets Lottia spp. and Acmaea mitra by black oyster-catchers Haematopus bachmani in Sitka Sound, Alaska provides details on methods of capture and feeding.  Limpets are rarely cracked; rather, the bird delivers a quick, sharp blow to the shell’s edge with its beak.  If the limpet is not immediately knocked free of its attachment, the bird firmly and intently pushes its prey from side to side and, when the shell is lifted sufficiently, forces its bill under the shell and levers it up.  The limpet’s soft parts are chipped free, swallowed in one piece, and the shell flicked away.  The author notes that only 4sec is required from first sighting to shell disposal for a small limpet.

NOTE  other prey in the area include mussels (2 species Mytilus trossulus and M. californianus comprising about 35% of the oyster-catcher’s diet), chitons Katharina tunicata (5%),goose barnacles Pollicipes polymerus (15%) and worms Nereis sp. (1%); see table of data

Research Study 2

Diets of black oystercatchers Haematopus bachmani  in Barkley Sound, British Columbia consist of 40-44% limpets, with Lottia digitalis, L. pelta, L. persona, and L. scutum predominating.  The birds appear to select their prey by size rather than by contrasting colour against the substratum.

Research Study 3

Other studies by the same author in Barkley Sound, British Columbia on optimal foraging of black oystercatchers Haematopus bachmani show that while limpets predominate in the diet of adults, mussels Mytilus spp. figure more importantly in the diet of the nestlings. This may be because large-sized prey such as mussels yield optimal return for time and energy spent by the adults in carrying food to the nestlings.  When the chick is closer to the feeding area, however, the adults switch to smaller prey items as food for their young. 

NOTE  based on 1250 feeding observations on adults and 517 on chicks

NOTE  mainly Lottia persona, L. pelta, and L. digitalis

Research Study 4

A study on predation of limpets by black oystercatchers Haematopus bachmani in areas around Cape Arago, Oregon emphasises the large effects that these and other shore birds can have on limpet populations.  During one 7mo period from Sept-Mar oystercatchers killed over 3000 individuals of 8 species of limpets, the majority being represented by Lottia scutum (41%), L. digitalis (34%), and L. pelta (21%).  Is there a preference exhibited by the birds for a certain limpet species?  This is tested by comparing relative prey availability with relative numbers of prey eaten (known in this study for three species for the months of Aug, Oct, and Mar, as shown in the accompanying table). Based on these data, which the author cautions should not be considered definitive, oystercatchers select less numbers of L. persona and L. pelta than are represented in their habitat.  Whether this is because they are too large to eat, or some other reason, is not known.  The author notes specifically that L. pelta reaches a refuge in size, but does not make the same observation for the other large species L. persona that is also eaten less than predicted by its availability in the habitat. 

NOTE  estimated as numbers encountered by a human observer per hour of search time

Research Study 5

An oystercatcher Haematopus bachmani surveys Barkley Sound, British Columbia for something tasty to eat

How important are oystercatchers, as just one type of shellfish-eating seabird, as agents of limpet depredation on the Oregon coast?  Observations indicate that an oystercatcher can eat one limpet per minute, or an estimated minimum of 100 per day.  Over a 7mo feeding season, then, a single bird could potentially eat 21,000 limpets – a significant potential source of mortality from a single predator type.

Research Study 6

Relative abundances of limpets Lottia pelta, L. scabra, L. digitalis, L. limatula, and L. gigantea in St. Nicholas Island, California

Further evidence of the limpet-killing efficacy of black oystercatchers Haematopus bachmani comes from a study in St. Nicolas Island, California.  Here, a wintering group of 3-6 birds visiting a 10 x 20m sandstone bench area twice in a 27h period are observed to eat a large number of limpets. The schematic on the Left lists the prey species and shows their vertical distributions on the shore and relative abundances 24h before the two predation events.  

The table on the Right shows the relative abundances of all limpets before the predation events and the relative abundances in the diet of the oystercatchers based on shell remains after their two visits.  In total, the birds eat 479 limpets, 380 of these in a period of less than 3h.  Of five species of limpets on the shore, two are eaten in proportions significantly different from their frequency of occurrence on the bench. These are Lottia digitalis, which is clearly preferred as prey by the birds and L. scabra, which is clearly avoided.  A third species Lottia pelta is the second-most favoured species to be eaten, but is consumed in almost direct proportion to its availability in the habitat.  Two other species, Lottia limatula and L. gigantea, are eaten in numbers that are too low for conclusions to be drawn.  The authors attribute the preference for the small-sized L. digitalis to its high position on the shore (the birds have longer time to feed on it) and to its propensity to aggregate during low-tide periods (“find one, find many”).  As for avoidance of the most abundance species L. scabra, the authors point out the difficulty the birds have in prying individuals of this species from their home scars.

Research Study 7

Gull Larus occidentalis

Three species of birds preying on Lottia digitalis in open-coast areas of Oregon appear to partition the limpets based on size (see data on Right).  Thus, turnstones Arenaria melanocephalus favour small-sized limpets (<10mm shell length), gulls Larus occidentalis/glaucescens eat mid-sized limpets (5-15mm), and black oystercatchers Haematopus bachmani prefer large-sized ones (>10mm).  An oystercatcher’s superior ability to dislodge limpets with a blow of its beak to the edge of the shell may explain its preference for larger limpets (i.e., more meat per kill).  The flesh is then eaten from the shell.  In comparison, turnstones and gulls are able to dislodge most easily the small limpets, which they then swallow whole.  Limpets comprise about 30% of the stomach contents of turnstones in this area.

Research Study 8

Limpets on sandstone

Limpet species segregated with respect to oystercatcher accessibility

Early studies on two sympatric limpet species, Lottia digitalis and L. scabra, in the Monterey Bay area of California suggest that competition for food leads to spatial separation of the two species, with the former occupying vertical surfaces and the latter, horizontal surfaces.  But, is there more to it than this?  Later investigation of predation by black oystercatchers Haematopus bachmani on populations of these two limpet species in the same area show that L. digitalis, especially large individuals (> 1cm), are eaten preferentially, even in areas where L. scabra is more abundant.  On horizontal surfaces, especially sandstone, the birds readily eat L. digitalis, but find L. scabra harder to peck free because of the greater tenacity provided by their home scars (see figure above Left).

On vertical surfaces, L. digitalis tends to predominate, but only above the pecking reach of the oystercatchers (see schematic).  Within pecking reach on vertical surfaces, only non-preferred smaller-sized L. digitalis, and L. scabra in their home scars, survive. The two species are segregated with respect to oystercatcher accessibility, with L. digitalis primarily occupying vertical surfaces out of reach of oystercatchers.  Lottia scabra is free to inhabit horizontal surfaces owing to better tenacity in home scars, but is discouraged from occupying vertical surfaces because of competition with the more abundant, larger-sized L. digitalis.

NOTE  this earlier work by Haven can be found at MOLLUSCA > LIMPETS & RELATIVES > HABITATS > COMPETITION

NOTE  data shown are for an area characterised by intermediate wave exposure at a tidal height (at the bird's feet) of 2m above MLLW, just as the top of the red-alga Endocladia zone. The authors provide data for birds pecking at higher and lower intertidal heights

Research Study 9

Oystercatcher-killed Lottia scutum. Note the pecking damage to the shells

In view of the large impact of oystercatchers on limpet survival, the authors of the foregoing Research Study pose the question: do Lottia digitalis when small migrate upwards on vertical surfaces to escape predation by birds?  This may explain some aspects of seasonal migration by the species.  The authors also note that oystercatcher predation must exert strong selective pressure on L. scabra, favouring homing of young limpets by eliminating those that don't by the time they reach a size acceptable to the birds.

NOTE a counter-argument to this idea, however, is that if oystercatchers are such a dominant source of mortality for the limpets, then strong selective pressure would be exerted on them to move down the shore, thus decreasing their susceptibility to the birds, which can only hunt during low tides or in the swash zone

Research Study 10

Unusual colour & shape of seaweed form of Lottia pelta

Typical rock form of Lottia pelta

Shells of Lottia pelta often bear a record, in colour and texture, of previous habitats occupied. For example, occupation of seaweeds may lead to colours and patterns quite different from those on a rock habitat (see accompanying photos). Moreover, if an individual lives for a time on one habitat, then moves to another, it may show transitional markings from the two habitats.  One disadvantage to L. pelta in having heterogenous markings on its shell like this is that they attract the attention of predatory birds such as oystercatchers Hematopus bachmanii.  In one collection of 237 L. pelta from different rock habitats (flat rock, benches, rock faces) on the central California coast, 7% are transitional types, that is, reflecting in shell shape and colour the history of at least two different habitats occupied.  In comparison, a collection of 1970 dead L. pelta shells from oystercatcher nests in two different locations discloses a 20% incidence of transitional morphs.  Thus, limpets that have changed substrata are eaten about three times greater frequency than would be expected based upon their frequency of occurrence in adjacent rocky intertidal areas.  The authors propose that the discontinuities in shell pattern make these limpets more obvious to visual predators. 

NOTE  this topic is considered in more detail for Lottia pelta elsewhere in this section on MOLLUSCA > LIMPETS & RELATIVES > HABITATS > SHELL GROWTH (SHAPE) & COLOUR

Research Study 11

Courtesy the authors

A 2yr study of 34 breeding pairs of black oystercatchers Haematopus bachmani on 21 islands in the Gulf Islands, British Columbia reveals that limpets are the preferred prey for provisioning of chicks.  Of 1620 identified prey items delivered to the chicks during the study, 70% are limpets Lottia, generally less than 20mm in shell length.  Other prey species are chitons (12%) and barnacles (13%), with shore crabs, polychaetes, isopods, cockles, and mussels making up a miscellaneous remainder.  Some interesting statistics are that each parent of a single 35d-old chick must provide one limpet every 8min during the low-tide cycle.  For two chicks, this increases to one limpet every 4min and, for three chicks, one every 2.6min.  Perhaps for this reason it is rare for an oystercatcher pair to rear three chicks successfully.

Research Study 12

Effects of birds on limpets and community dynamics of the rocky shore

Direct effects of predation and interference competition have been well studied in marine intertidal ecosystems, including ones containing limpets.  However, indirect effects, other than exploitative competition, have been less well studied.  Let’s consider the interactive effects of predation by gulls and oystercatchers on community dynamics of intertidal invertebrates at Tatoosh Island, Washington.  The study focuses on three sympatric limpet species Lottia pelta, L. digitalis, and L. paradigitalis¹. In the Tatoosh Island area, L. pelta lives both on rocks and on mussels, the latter being their preferred habitat. The following relationships are described by the author and represented in the accompanying chart:

1. the birds eat Lottia pelta and L. digitalis, but not L. paradigitalis possibly because it is too small
2. removal of the two limpet species allows more algae to grow
3. more algal food for L. paradigitalis leads to increase² in their population numbers
4. mussels and goose barnacles compete³ for space on the rock
5. by also consuming goose barnacles, the gulls free up more space for mussels
6. as mussel numbers increase, preferred space for L. pelta on their shells also increases.  Thus, although L. pelta are eaten by the birds, there is no net change in their population numbers over time
7. changes in densities of mussels and goose barnacles, by affecting the availability of rock surface, affect density of algae
8. by preying on goose barnacles, the birds indirectly free up more space on the rocks for L. digitalis

In other geographical areas where L. digitalis preferentially occupies goose barnacles, the interactions can be even more complex. The research provides insight into the indirect effects of predators on competitive interactions, and how these effects may ramify through space and time.

NOTE¹  the author terms this species strigatella, but recent molecular-genetics evidence provided by researchers at the University of California, Berkeley points to it being paradigitalis. Lottia strigatella is now restricted in distribution to an area from San Diego around the coast of Baja California to the northern part of the Gulf of California, where it is sympatric with a new species L. argrantesta along the southwest coast of the Gulf. The name strigatella in this account is changed to paradigitalis. Simison & Lindberg 2003 The Veliger 46 (1): 1

NOTE²  this is termed “competitive release”, as the smaller limpet is freed from exploitative competition with the larger species

NOTE³  in the study this is termed “interference competition” but, as space is in short supply, it could also be considered “exploitative competition”