Robert W. Elner

Inference in Ecology: The Sea Urchin Phenomenon in the Northwestern Atlantic

We have reviewed the considerable body of research into the sea urchin phenomenon responsible for the alternation between macroalgal beds and coralline barrens in the northwestern Atlantic. In doing so, we have identified problems with both the scientific approach and the interpretation of results. Over a period of approximately 20 years, explanations for the phenomenon invoked four separate scenarios, which changed mainly as a consequence of extraneous events rather than experimental testing. Our specific concerns are that results contrary to the keystone-predator paradigm for the American lobster were circumvented, system components of the various scenarios became accepted without testing, and modifications of some components appeared arbitrary. Our review illustrates dilemmas that, we suggest, have hindered ecological progress in general. We argue for a more rigorous experimental approach, based on sound natural-history observations and strong inference. Moreover, we believe that the scientific community needs to be cautious about allowing paradigms to become established without adequate scrutiny.

Interactions between two marine snails, Littorina rudis Maton and Littorina nigrolineata Gray, a predator, Carcinus maenas (L.), and a parasite, Microphallus similis Jägerskiold

Abstract In the laboratory, the shore crab Carcinus maenas (L.) preferred the smallest size classes of Littorina rudis Maton and Littorina nigrolineata Gray available, but preyed more heavily on L. rudis. L. rudis has a greater prey value (energy yielded per unit handling time) for crabs than L. nigrolineata which has a thicker shell. The energy content of both snails is a function of their size, but if attack success rate is taken into account with handling time, smaller snails become more valuable than larger snails. Comparisons with Carcinus predation on mussels, Mytilus edulis L., and dogwhelks, Nucella lapillus L., substantiate the hypothesis that foraging behaviour is a consequence of prey dispersion patterns. Although crab predation may account for intra-shore differences in shell thickness and aperture size between Littorina rudis and L. nigrolineata, inter-shore differences cannot be entirely attributed to crab predation, at least in L. rudis. It is suggested that differences between L. nigrolineata and L. rudis in tolerance to physical factors and vulnerability to crab predation allow them to exploit different shore levels. In addition, interactions between the two snails are perhaps affected by their different susceptibilities to trematode. Microphallus similis Jagerskiold, infection, which in turn may be related to their prey value to shore crabs.

BIOFILM GRAZING IN A HIGHER VERTEBRATE: THE WESTERN SANDPIPER, CALIDRIS MAURI

We show that a higher vertebrate can graze surficial intertidal biofilm, previously only considered a food source for rasping invertebrates and a few specialized fish. Using evidence from video recordings, stomach contents, and stable isotopes, we describe for the first time the grazing behavior of Western Sandpipers (Calidris mauri) and estimate that biofilm accounts for 45-59% of their total diet or 50% of their daily energy budget. Our finding of shorebirds as herbivores extends the trophic range of shorebirds to primary consumers and potential competitors with grazing invertebrates. Also, given individual grazing rates estimated at seven times body mass per day and flock sizes into the tens of thousands, biofilm-feeding shorebirds could have major impacts on sediment dynamics. We stress the importance of the physical and biological processes maintaining biofilm to shorebird and intertidal conservation.

Multiple reproductive strategies in snow crab, Chionoecetes opilio: Physiological pathways and behavioral plasticity

Abstract We review the three major reproductive pathways displayed by snow crab, Chionoecetes opilio . Females can copulate in either a soft- or hard-shelled condition and also fertilize eggs with sperm stored in their spermathecae. Spermatophore partitioning and preservation mechanisms in the spermatheca plus postulated spermatophore removal abilities in males play central roles in shaping mating behavior. However, fitness consequences of the various strategies are enigmatic. Fecundity is influenced by both spermatophore source and female characteristics; also, egg production estimates are confounded by high loss rates and uncertainties surrounding brooding period. Males challenge the concept of “maturity” as they copulate both as morphometrically immature (MI) with small chelae and morphometrically mature (MM) with large chelae individuals. Reproductive opportunities for each male morph vary according to long-term population cycles influencing inter-male competition levels. Thus, during phases when MM males are abundant, reproductive opportunities for MI males are probably low; however, the window opens when MM males become scarce. We explore possible mechanisms that control whether males with small chelae either continue to molt and remain MI or attain full sexual maturity and, hence, probable terminal molt-status.

Reproductive Processes Revealed by Spermatophore Dehiscence Experiments and by Histology, Ultrastructure, and Histochemistry of the Female Reproductive System in the Snow Crab Chionoecetes Opilio (O. Fabricius)

ABSTRACT Histological, histochemical, ultrastructural, and microbiological techniques were used to elucidate the structure and function of the female reproductive system in the snow crab Chionoecetes opilio (0. Fabricius). The anatomy of the ovary and oviduct conform to that of other brachyuran species. The spermatheca comprises a dorsal (glandular) and a ventral (chitin-lined) region, with no intervening anatomical separation. Ultrastructural and histochemical bases are presented for the dual role of the spermatheca in secretion and sperm storage. Proteinaceous polysaccharides are the dominant holocrine secretion of the glandular epithelium. A dense, morphologically homogeneous bacterial population was observed in the spermathecae of all of 25 crabs examined. A discrete layer of nonspermatophore-containing acid mucopolysaccharides appears to be transferred to the vagina and ventral spermatheca region by the male after insemination. A functional partitioning of spermatophores seems to occur at insemination; some dehisce immediately (probably due to mechanical forces), while those resisting initial dehiscence are stored in the spermatheca. In vitro experiments show that storage in the spermatheca greatly increases the tendency of spermatophores to dehisce when exposed to sea water. Functional correlates to these data are explored, with emphasis on roles and interactions of secretions, bacteria, spermatophore storage, and dehiscence, and mechanisms of ensuring lastmale precedence under conditions of sperm competition.

Variable and complex food web structures revealed by exploring missing trophic links between birds and biofilm

Food webs are comprised of a network of trophic interactions and are essential to elucidating ecosystem processes and functions. However, the presence of unknown, but critical networks hampers understanding of complex and dynamic food webs in nature. Here, we empirically demonstrate a missing link, both critical and variable, by revealing that direct predator-prey relationships between shorebirds and biofilm are widespread and mediated by multiple ecological and evolutionary determinants. Food source mixing models and energy budget estimates indicate that the strength of the missing linkage is dependent on predator traits (body mass and foraging action rate) and the environment that determines food density. Morphological analyses, showing that smaller bodied species possess more developed feeding apparatus to consume biofilm, suggest that the linkage is also phylogenetically dependent and affords a compelling re-interpretation of niche differentiation. We contend that exploring missing links is a necessity for revealing true network structure and dynamics.

Foraging behaviour of an excavating predator, the red rock crab (Cancer productus Randall) on soft-shell clam (Mya arenaria L.)

We investigated the influence of soft-shell clam (Mya arenaria) burial depth on red rock crab (Cancer productus) foraging behaviour. In laboratory tanks, crabs searched for and excavated clams buried at depths of 5, 10, and 15 cm. Handling time of prey was partitioned into excavation time, breaking time, and eating time. Prey excavation formed 23% of mean total handling time (mean 8.2 min total 35.7 min−1) at 5 cm prey depth, 62% at 10 cm depth (mean 49.3 min total 79.5 min−1), and 73% at 15 cm depth (mean 93.5 min total 128.2 min−1). Mean excavation time for clams increased at a rate of 8.4 min cm−1 of depth; clam profitability (energy intake per unit handling time (J s−1)) decreased with burial depth. Predation rates decreased with prey burial depth. Often crabs gave up attempts to excavate a clam, and abandonment frequency and excavation time before abandonment increased significantly (p<0.05) with burial depth. Our findings confirm earlier studies which indicate that burial depth affords an important refuge from crab predation, significantly increasing prey handling time and decreasing the profitability of individual prey.

Gonopods of the Majid Crab Chionoecetes Opilio (O. Fabricius)

ABSTRACT To further elucidate the reproductive processes of the snow crab Chionoecetes opilio (O. Fabricius) the first and second gonopods of adult specimens were examined using scanning electron microscopy and histology. Surface features and setal types are described for both gonopods. The gonopods are highly modified compared to other brachyuran crabs and lower crustaceans. Suggestions for setal function are presented, based on size, orientation, location, fine structure, and cuticular insertion. Tubulation of the endopod is observed in both the first and second gonopods. Rosette glands are abundant in the basal half of the first gonopod, but are absent in the second gonopod. A duct network from the rosette glands terminates in cuticular pores that are restricted to the ejaculatory canal, indicating that the rosette glands function in copulation, probably contributing to the seminal fluids. A mechanism for transfer of seminal fluids is proposed, based on the relative sizes of the gonopods and the morphology of the second gonopod.

Sediment in Stomach Contents of Western Sandpipers and Dunlin Provide Evidence of Biofilm Feeding

Abstract. Western Sandpipers and Dunlin are capable of grazing biofilm. As there has been no assessment of this dietary constituent in stomach contents, the stomachs of 89 Western Sandpipers and 56 Dunlin collected during breeding migration through the Fraser River delta, British Columbia, Canada, were examined. Invertebrates, traditionally regarded as the principal prey, comprised a minor fraction of mean stomach volumes (Dunlin: <25%; Western Sandpiper <10%). Three phyla accounted for most of these invertebrates: molluscs, annelids and arthropods. In comparison, sediment (a mixture of sediment particles, broken and unbroken diatoms plus organic detritus) comprised the major component of stomach volumes (Dunlin: >40%; Western Sandpipers: >75%). Although the mean volume of sediment was significantly greater in Western Sandpipers than Dunlin, there was no effect of sex for either species. Stomach sediment volume appears a convenient index of biofilm intake and sediment loads indicative of biofilm grazing have been reported in stomach contents from other shorebird species. Re-examination of shorebird diets appears necessary given that conditions promoting biofilm are not necessarily conducive for invertebrate prey.

Influence of HabItat VarIatIon, nest-sIte selectIon, and Parental beHaVIor on breedIng success of ruddy-caPPed nIgHtIngale tHrusHes (Catharus frantzii) In cHIaPas, MexIco

Abstract We examined the influence of ecological and behavioral factors on breeding success of Ruddy-capped Nightingale Thrushes (Catharus frantzii) in contiguous primary- and secondary-forest habitats during the 2000–2003 breeding seasons in the Central Highlands of Chiapas, Mexico. Breeding density was higher in primary, undisturbed forest than in secondary forest with less understory vegetation. Nest-site selection was related to nest-concealment attributes (visibility, canopy cover, and shrub density) at the nest-site and patch levels. Nest-site selection was stronger in secondary forest, which indicates that preferred nest-site attributes were more limited in that habitat. Overall success per breeding attempt varied annually from 20% to 35%, with a higher mean annual success in primary forest (42%) than in secondary forest (19%). Daily nest survival rates were higher in primary than in secondary forest and varied with nest cover, lateral visibility, and density of ground epiphytes. The number of parental visits to nests was higher in primary than in secondary forest and lower for successful than for failed nests during incubation but did not vary with habitat or fate during the nestling stage. Female nest-attentiveness was higher in primary forest than in secondary forest throughout the nesting attempt and was higher for successful nests during the nestling stage only. Thus, Ruddy-capped Nightingale Thrushes showed habitat-specific breeding performance, with the primary-forest habitats (cloud forest, riparian) supporting higher densities and nesting success than secondary, disturbed habitats.