Selective settlement of the barnacle Semibalanus balanoides (L.) facilitates its growth and reproduction on mussel beds in the Wadden Sea

On the unstable sedimentary tidal flats of the Wadden Sea, a suitable attachment substrate for sessile organisms is generally lacking. Epibenthic mussel beds (Mytilus edulis L.) provide the only and strongly limited settlement sites available for the barnacle, Semibalanus balanoides (L.). Field investigations showed that barnacles were non-randomly distributed within a mussel bed. They preferentially occurred near the siphonal apertures of living mussels but rarely grew on dead mussels or shell fragments. Field experiments revealed that this was due to selective settlement of barnacle cyprid larvae. Growth of barnacles was significantly higher upon living mussels than on empty mussel shells. Moreover, a higher reproductive output was obtained by individuals on living mussels which produced twice as many nauplii larvae than barnacles attached to empty shells. This study shows that selective settlement of S. balanoides cyprid larvae on living mussels is adaptive with respect to individual fitness. Received in revised form: 15 January 2001 Electronic Publication     

Assessment of a trap for measuring larval supply of intertidal barnacles on wave-swept, semi-exposed shores

Larval supply and settlement of Semibalanus balanoides (L.) on a semi-exposed wave-swept shore in E Scotland were assessed by means of a new larval trap design. Two midshore sites were utilised throughout the peak of the 2001 settlement season (May 11–June 3). Larval supply was quantified by 176 ml baffled cylinder traps filled with a 4-M killing solution of urea in seawater. Larval trap washout of urea typically ranged from 25% per day under minimal wave action to a maximum of 39% under heavy wave action during an onshore gale. The maximum capture for a single trap was 334 cyprids over one tide. Significant and consistent positional effects, both on urea retention and larval capture, were found for replicate traps separated by 9 cm. These indicated fine-scale variations in hydrodynamic flow and larval supply over the substratum which will have implications for the quantification of spatial heterogeneity of larval input to the benthos. The retention of urea and larval capture efficiency generally were not compromised by traps being serviced daily, as opposed to tidally, except perhaps for captures at very low larval densities. Larval supply measured on a tidal basis was strongly correlated between sites (r=0.975), but supply at Site C was 5× that at Site T: the reduction at Site T could not be explained by its slightly shorter immersion time and hence ‘availability’ of cyprids alone. At Site C settlement was quantified for 5×5 cm quadrats of natural substratum cleared daily. Settlement on these clearances (subject to possible grazing by limpets) was quantified only from May 20–June 3, after the peak of larval supply on May 17, and the variation in larval supply explained 65% of the variance in settlement. The larval supply/settlement relationship at Site T was quantified using grooved acrylic panels. A high correlation coefficient (0.961) was obtained for the daily supply/settlement relationship for these panels over the period May 15–June 3, which included the peak of supply. Larval supply varied up to 15-fold within a site on consecutive tides and up to 100-fold between sites on the same tide. Although further improvements to both the larval trap and the settlement panels can be made, this larval trap does appear capable of providing high-resolution data on tidal or daily larval supply over a wide range of wave conditions and larval concentrations.     

Discrimination at settlement in barnacles: Laboratory and field experiments on settlement behaviour in response to settlement‐inducing protein complexes

A major driving force to mechanistic studies of barnacle gregarious settlement is to contribute to an understanding of observed patterns of settlement in nature. In particular, how cyprids perceive adult conspecifics and how they discriminate between conspecific and allospecific barnacles are questions which have taxed researchers for nearly 50 years. The putative, active component of adult barnacles to which the cyprids respond has long been known to be a glycoprotein, referred to here as the settlement‐inducing protein complex (SIPC). The present study examines the discriminatory abilities of laboratory‐reared Balanus amphitrite and wild Semibalanus balanoides cyprids at settlement. Using a recently developed nitrocellulose membrane‐choice settlement assay, laboratory studies revealed that both species settled at a significantly higher rate on regions of membrane on which crude conspecific SIPC had been adsorbed compared to untreated regions. Moreover, when offered a choice between conspecific and allospecific SIPC, a trend to greater settlement on the conspecific regions was observed. The membrane assay was also evaluated in field trials using real‐time video footage of cyprid searching behaviour. Of 211 S. balanoides cyprids recorded during exploratory behaviour, only one settled. Exploratory behaviour was, however, clearly associated with regions of the membrane treated with either conspecific or allospecific (B. Amphitrite) SIPC compared to untreated regions. These results are generally in accord with previous reports on the discriminatory abilities of barnacle cyprids and suggest that the membrane assay may be usefully applied to field studies of settlement behaviour.     

The conditional ancestral selection graph with strong balancing selection

Using a heuristic separation-of-time-scales argument, we describe the behavior of the conditional ancestral selection graph with very strong balancing selection between a pair of alleles. In the limit as the strength of selection tends to infinity, we find that the ancestral process converges to a neutral structured coalescent, with two subpopulations representing the two alleles and mutation playing the role of migration. This agrees with a previous result of Kaplan et al., obtained using a different approach. We present the results of computer simulations to support our heuristic mathematical results. We also present a more rigorous demonstration that the neutral conditional ancestral process converges to the Kingman coalescent in the limit as the mutation rate tends to infinity.     

Inference on the strength of balancing selection for epistatically interacting loci

Existing inference methods for estimating the strength of balancing selection in multi-locus genotypes rely on the assumption that there are no epistatic interactions between loci. Complex systems in which balancing selection is prevalent, such as sets of human immune system genes, are known to contain components that interact epistatically. Therefore, current methods may not produce reliable inference on the strength of selection at these loci. In this paper, we address this problem by presenting statistical methods that can account for epistatic interactions in making inference about balancing selection. A theoretical result due to Fearnhead (2006) is used to build a multi-locus Wright-Fisher model of balancing selection, allowing for epistatic interactions among loci. Antagonistic and synergistic types of interactions are examined. The joint posterior distribution of the selection and mutation parameters is sampled by Markov chain Monte Carlo methods, and the plausibility of models is assessed via Bayes factors. As a component of the inference process, an algorithm to generate multi-locus allele frequencies under balancing selection models with epistasis is also presented. Recent evidence on interactions among a set of human immune system genes is introduced as a motivating biological system for the epistatic model, and data on these genes are used to demonstrate the methods.