Population & community ecology, & genetics
  This section is divided into two parts: abalones Haliotis, and trochids Calliostoma & other snails
  black dot
  Abalones: Haliotis
  black dot
Research study 1
 

map of sites used in genetics study of black abaloneBlack abalones Haliotis cracherodii are limited in their distribution to rocky shores of central and southern California, and the question arises as to their degree of population differentiation.  This is investigated by researchers at the Scripps Institution of Oceanography, La Jolla using specimens from 7 sites between the counties of Santa Cruz and Santa Barbara (300km distance), and assessing allelic frequencies at 3 loci.  The authors report significant differentiation among sites for all 3 loci, a level 3-fold greater than that observed in red abalone H. rufescens.  The explanation may relate to a short, summer breeding season that may limit its larval dispersal. In comparison, red abalone photograph of black abalone Haliotis cracherodiispawn year-round and their larvae experience a much wider range of oceanographic conditions.  Hamm & Burton 2000 J Exp Mar Biol Ecol 254: 235.

NOTE  the polymorphic enzyme-encoding GP1, AAT-1, and PGM


Black abalone Haliotis cracherodii 0.75X

  black dot
Research study 2
 

Following the closure of the British Columbia fisheries for Haliotis kamtschakana in the early 1990s, there has been much interest in its future status. The initial concern of fisheries scientists was that recruitment was low or non-existent.  One recent study in the Broken Group Islands provides comparative data on population densities and some interesting information on community interrelationships.  First, densities of H. kamtschatkana are low in the Broken Group Islands (0.1 individuals . m-2) as compared with those on the north coast (≈2 . m-2) and Haida Gwaii (28 . m-2).  However, juveniles (<45mm shell length) represent 42% of the population, which is apparently high and a sign of good recruitment.  As noted, however, the population is still in low numbers.  Interestingly, 7% of juvenile abalones are found under the spine canopies of adult red urchins Strongylocentrotus franciscanus. Second, there is an inverse relationship between abalone size and abundance of red urchins.  This agrees with comparable data on red abalone H. rufescens in California, which suggest that abalone and red urchins compete spatially, either exploitatively or by interference.  Oddly, however, the B.C. data also show that abalone and sea urchin densities are positively correlated.  This suggests to the authors that the abalone are benefiting is some way from association with red urchins (e.g., in their sheltering of juvenile abalone), but it could be related to the presence of both herbivores in areas of plentiful kelp food.  Finally, abalone abundance in the Broken Group Islands generally is negatively correlated with the abundance of predators, including octopuses, sea stars, and decapod crustaceans.  Tomascik & Holmes 2003 J Shellf Res 22: 831.

NOTE  this may seem a lot, but the authors note that it represents only 6 individuals out of the total sample.  Since juvenile urchins also shelter under the adults’ spine canopies, there could also be interspecific competition for this space


An abalone Haliotis kamtschatkana and red urchins Strongylocentrotus franciscanus feed on kelp (the abalone is in the centre of the photograph, facing into the dark piece of kelp) 0.15X
  black dot
  Trochids: Calliostoma & other snails
  black dot
Research study 1
 

photograph of trochid snails Chlorostoma funebralis in a tidepoolOf all west-coast trochids, black turban snails Chlorostoma funebralis have the widest diversity of habitat preferences.  Is there any correlation of genotype with specific habitats preferred?  A study on on this topic using populations of C. funebralis at 2 west-coast sites (Cape Arago, Oregon and Mukkaw Bay, Washington) involves tagging and moving snails to a different intertidal habitat.  Habitat in the study is defined as the height of a snail in relation to the waterline of a permanent tidepool, measured before and after tagging and translocation. Translocation involves placing a tagged snail at the waterline of the tidepool, waiting 48h, recording its new position, relocating it again at the waterline, and re-recording its position after at least 2 subsequent 48-h intervals. Several such measurements for an individual provide a mean intertidal-height value that is equated by the author to habitat preference.  The snails are then sized, sexed, and analysed for enzyme polymorphisms. Results show that at one site (Cape Arago) size and LAP genotype (but not PGI) are significantly associated with intertidal height, with larger individuals being located at lower intertidal levels.  Thus, at this site, a snail’s size and the presence or absence of the LAP allele are significant predictors of mean intertidal height. Similarly, at Mukkaw Bay both size and PGI genotype (but not LAP) are associated with intertidal height, again with larger individuals being located lower down the shore. An inverse relationship between size and intertidal height has been previously reported for C. funebralis (see Research Study 5 in PHYSICAL DEFENSES) , and may result from a scarcity of low-intertidal predators that would otherwise eat larger-sized individuals. Byers 1982 Behav Genetics 13: 65; see also Byers & Mitton 1981 Mar Biol 65: 149 for further details on the field experiments involved in this study.

NOTE  the author actually calculates a standard-deviation statistic for each mean for each individual, and later uses these in multiple-regression and correlation analyses. This may not be appropriate because the individual values used to calculate the means are themselves not independent.  Sex, for example, is considered by the author to be significantly associated with intertidal height at the Cape Arago site, but because the correlation is based on the statistic standard deviation of height for a given snail, it may not be valid

NOTE  11 enzymes are resolved using starch-gel electrophoresis.  Only the 2 enzymes with the most highly polymorphic loci are used in the comparison. These are leucine aminopeptidase, LAP, with 5 alleles, and phosphoglucose isomerase, PGI, with 6 alleles. The author argues convincingly that the observed enzyme phenotypes result from allelic variation at structural gene loci; hence, are appropriate to use in the analyses

 

black dot
Research study 2

 

Studies in Bodega Bay, California show that a shore-level size gradient exists for Chlorostoma funebralis, with shell length increasing in a downward direction.  Light is implicated as a factor in this distribution.  The authors reciprocally translocate snails from low and high zones. Specifically, they gather 52 large snails from the low zone (mean length: 2.0cm), split them into 2 groups of 26, replace one group of 26 in the low zone, and move the other group to the high zone. The same procedure is done with small snails (1.3cm) from the high zone. After 1d they measure the sizes of marked snails from each zone and find that the size gradient has been re-established. The results underscore the importance to Chlorostoma of maintaining its shore-level size gradient, but the meaning for it is unclear.  Physiological responses to stress would actually favour larger individuals being higher on the shore than smaller ones.  Susceptibility of small snails to more intense predation in the lower region is a possibility, but the authors test this and find that larger snails actually move upwards more than smaller ones in response to the presence of Pisaster ochraceus.  Food is more abundant in the lower zones, but small individuals would benefit just as much, if not more from this.  Perhaps food competition is involved and this may be a subject for further research.  Doering & Phillips 1983 J Exp Mar Biol Ecol 67: 159.

NOTE the same pattern appears to exist for Chlorostoma funebralis in Pacific Grove, California. Wara & Wright 1964 Veliger 6(Suppl.): 30.

NOTE  it is unclear why the authors “recycle” half of each of the high and low groups.  Perhaps the intent is to determine what effect handling and relocation have on subsequent movement. Presumably, these individuals are excluded from the calculations of post-translocation averages

  black dot
Research study 3
 

schematic showing disposition of experimental kelp plants in a study on competition between 3 species of trochid snailsA detailed study of 3 species of trochid snails Chlorostoma brunnea, C. montereyi, and Promartynia pulligo in kelp forests around the Hopkins Marine Station, Pacific Grove, California provides information on competitive interactions between them.  Distributions of the 3 species are only partially overlapping, with brunnea occurring on algae-covered rocks in the low intertidal/shallow subtidal region, pulligo living in deeper water (7-12m), and montereyi inhabiting similar depths as pulligo, but in much fewer numbers.  The investigation is mainly to determine if competition plays a role in bathymetric segregation of the species. 

The design of this experiment is somewhat complex, but basically involves creating 3 types of experimental kelp habitats, one a pair of  “source” plants at 7.5m depth where batches of test snails are released, and 2 other “spill-over” pairs of kelp habitats where the snails can go if conditions are too crowded on the “source” plants (see schematic).  One of these habitats is a pair of “canopy” plants attached by length of rope to the sea bottom and floating at the surface in normal canopy fashion; the other, a pair of “topped” plants with their canopy parts removed, but with normal attachment by holdfasts to the sea bottom.  Note in the drawing that the canopies of the “source” plants and “canopy” plants are contiguous, allowing snails to crawl from one to the other. The experiment starts with 125 snails of each of brunnea and pulligo hung in open baskets at different heights on each of the “source” plants (similar to their natural densities).  Note that snails moving away from the “source” plants can either go via the canopies to less crowded conditions on the “canopy” plants, or along the sea bottom to equally less crowded conditions on the “topped” plants.  After 15d the results show little or no evidence of competition for space between the 2 species.  On this basis and that of complementary lab experiments, the author concludes that competition is unlikely to be responsible for the bathymetric segregation of the 3 species.  Watanabe 1984 Ecology 65: 920. Photographs courtesy Gary McDonald, Long Marine Laboratory, Santa Cruz

NOTE  the snails are marked with numbers to enable movements of individuals to be followed over 15d of study.  Predatory sea stars are removed from around the kelp plants.  A third “source” plant is employed with 250 pulligo released onto it, but their role in the eperiment is not made clear.  The experiment is replicated

NOTE  this paper is packed with information and what is given here is just a brief overview summary

  black dot
Research study 4
 

graphs showing intra- and interspecific effects on growth of 2 species of trochid snails Chlorostoma eiseni and C. aureotincta in Santa Catalina Island, Californiagraph showing densities of 2 trochid species in microhabitats occupied in common by them in Santa Catalina Island, CaliforniaTwo species of trochid snails Chlorostoma aureotincta and C. eiseni inhabit similar shallow subtidal reef areas around Santa Catalina Island, California and eat the same kinds of microalgae growing on rocks.  A 6-mo field study on their potential competitive interactions, both inter- and intraspecific, shows that each species depresses the individual growth of the other to a similar degree (see graph on Left), but survivorship is only slightly affected even at highest snail densities. Interestingly, the intraspecific effects of high densities are different for each species.  Thus, note in the graph that high densities of C. aureotincta depress the growth of conspecifics relatively more than high densities of C. eiseni depress the growth of fellow C. eiseni. Long-range estimates suggest that there is little fluctuation in overall population densities of the 2 species over 6yr, suggesting that they do, in fact, coexist. 

However, a closer examination of their microdistributions (0.1m2 quadrats) reveals that they co-occur less often than expected by chance, suggesting that some sort of competition is occurring. The author notes that the issue of coexistence of competitors is not specifically addressed in this study but see Research Studies 6 & 7 to follow.  Schmitt 1985 Ecology 66: 950.

NOTE  the classification of this species is uncertain

NOTE  as most of the study involves caging snails at different densities, and measuring growth and so on, the author assesses the possible effects of the cages themselves, such as possible constraints on foraging, reduced light, altered current flow, and so on, in a separate experiment.  This involves tagging 50 individuals of each species and allowing them to forage freely outside of the cages.  After 6mo, recapture of 20 or so of each species shows no significant differences in growth between free-ranging and caged individuals

  black dot
Research study 5
 

photograph of top shell Pomaulax undosaThe 2 species of trochid snails Chlorostoma aureotincta and C. eiseni at Santa Catalina Island, California share a common set of invertebrate predators, most notably lobsters Panulirus interruptus, octopuses Octopus bimaculatus, and whelks Kelletia kelletii, with top shells Pomaulax undosum and sessile bivalves Chama arcana.  The distributions of the bivalves and snails are essentially separate, but with some overlap. Throughout the study areas, density of predators graph showing predator/prey relationships of gastropods at Santa Catalina Island, Californiatends to be positively correlated with density of gastropods.  If bivalves are added experimentally to areas with snails, the predators congregate and mortality of snails increases (see graph on Left).

Similarly, in areas where bivalves and snails coexist naturally, mortality of bivalves is especially high owing to increased numbers of predators. Thus, each group of prey is negatively affected by the presence of the other, because each alternative prey increases the local density of photograph of sessile bivalve Chama arcanapredators.  Such a doubly negative indirect interaction between prey, mediated by a common predator, is known as “apparent competition”.  The author remarks that this is the first experimental demonstration of its existence, and notes that the combination of shared predation and apparent competition may be the mechanisms by which habitat segregation is maintained by the prey groups.  Schmitt 1987 Ecology 68: 1887.

NOTE formerly Astraea, then Lithopoma, now Pomaulax



Sessile bivalve Chama arcana 

 

  black dot
Research study 6
 

In a later paper the author notes that the snails Chlorostoma aureotincta and C. eiseni in Santa Catalina Island, California live in the same rocky, shallow areas in about the same abudances, and eat the same benthic-diatom and filamentous-algae foods, and asks the question: do they compete for food?  Behavioral experiments show that they do, but the competition is exploitative rather than interference, and can be both intra- and interspecific.  For example, individuals of either species are less likely to feed on a recently grazed patch when the previous forager is C. aureotincta.  If food is generally scarce, however, C. eiseni is more likely than C. aureotincta to feed on a previously grazed patch.  The cue to previous grazing seems to be mucus coating on an algal patch.  If algae are plentiful, C. aureotincta grows faster than C. eiseni, but if algae are scarce, then the reverse is true.  Grazing habits of the 2 species differ.  Chorostoma aureotincta grazes rapidly and harvests twice the area per unit time than C. eiseni, a behaviour termed area-extensive grazing. In comparison, the latter species spends longer in a given area and harvests more alga per unit area grazed, a behaviour termed area-intensive grazing.  The first strategy yields a greater food intake when microalgae are abundant, while the alternate strategy yields more food when microalgae are scarce.  The snails are able to coexist through these different grazing strategies. Schmitt 1996 Ecology 77: 408.

  black dot
Research study 7
 

photograph of several turbinid snails Pomaulax undosum showing growth formAlso in Santa Catalina Island, body size of turbinid shells Pomaulax undosum decreases with increasing population density. Since population density decreases with greater depth, there is a significant size gradient with depth, deeper individuals being larger. The authors investigate attachment strengths, competition, and predation as possible contributing factors. With respect to the first of these, laboratory experiments show,that while only 50% of a size range of test animals are dislodged in current speeds of 4m . sec-1, 100% are dislodged at 8m . sec-1 (see graph lower Left). Moreover, larger individuals with greater absolute forces acting on them, are the first to go.  Thus, greater hydrodynamic forces in the shallower subtidal zones may force individuals deeper, although data provided by the authors show that such forces would actually be minimal at depths graph showing growth of turbinid snails Pomaulax undosumexceeding 2-3m. 

Caging experiments in laboratory and field show that intraspecific competition may play a major role in reducing body sizes, especially for small individuals in the most densely populated shallow areas. Finally, field tethering experiments indicate graph showing relationship between body size and dislodgement in different current speeds for turbinid snails Pomaulax undosumsignificantly less mortality in the shallowest depths, but otherwise with no clear pattern.  Overall, the authors conclude that a combination of physical and biological processes controls both population density and sizes of P. undosum over a depth gradient.  With respect to food, other than commenting on the “sub-optimal” quality of calcareous algae in the shallow subtidal areas, the authors do not discuss possible differences in food quality at the other depths in explaining the size gradient. Alfaro & Carpenter 1999 J Exp Mar Biol Ecol 240: 259.

NOTE formerly Astraea undosa/Lithopoma undosum

NOTE  direct measurements of currents show that velocities of 3.5m . sec-1 are reached in shallow water, dropping by almost an order of magnitude in the deeper part of Pomaulax’s distribution

  black dot
Research study 8
 

map showing collecting sites in study of size-distributions in latitudinally separated populations of Chlorostoma funebralisA recent study by researchers at the Oregon Instititute of Marine Biology, Charlston reveals that habitat features such as shape of coastline and latitude have significant effects on age-structure of Chlorostoma funebralis  populations.  Thus, collections from 22 sites ranging from Oregon to Baja California show that in wave-protected northern sites populations are comprised of larger proportions of pre-reproductive individuals, while in wave-exposed northern sites they are made up of mostly large, reproductive individuals.  In contrast, southern sites in California and Baja California are made up predominantly of juveniles, regardless of coastal topography.  As a result of these differences, egg production is 2 orders of magnitude greater in northern wave-exposed populations than in southern populations.  Size differences in the northern sites likely owe to greater movement of larvae offshore and to poor recruitment at the wave-exposed sites, although the authors are unclear as to why this should be. Cooper & Shanks 2011 Mar Ecol Prog Ser 424: 133.

NOTE  note on the map that the 4 “southern” populations of the 22 sampled are actually in northern California

  black dot
  RETURN TO TOP