Isotopic niche provides an insight into the ecology of a symbiont during its geographic expansion

Abstract

The study of the recent colonization of a symbiont and its interaction with host communities in new locations is an opportunity to understand how they interact. The use of isotopic ratios in trophic ecology can provide measurements of a species' isotopic niche, as well as knowledge about how the isotopic niches between symbiont and host species overlap. Stable isotope measurements were used to assess the sources of carbon assimilated by the host species (the bivalves Mytilus galloprovincialis and Scrobicularia plana) and their associated symbiont pea crab Afropinnotheres monodi, which occurs within these bivalves' mantle cavities. The mixing model estimates suggest that all of them assimilate carbon from similar sources, particularly from pseudofaeces and particulate organic matter in this symbiotic system based on filter feeding. The symbiotic species occupy comparable trophic levels and its association seems to be commensal or parasitic depending on the duration of such association. The pea crab A. monodi reflects a sex-specific diet, where males are more generalist than the soft females because the latter's habitat is restricted to the host bivalve. The high isotopic overlap between soft females and M. galloprovincialis may reflect a good commensal relationship with the host.

Keywords: bivalves; commensal; kleptoparasitism; parasitism; stable isotope; trophic position.

Figure 1.

Map of the Iberian Peninsula and the 3 locations sampled of the Gulf of Cadiz: (1) San Pedro estuary (Cádiz) for the collection of clams (S. plana); (2) Cádiz Bay, and (3) Carreras estuary Huelva for the collection of mussels (M. galloprovincialis).

Figure 2.

Position of A. monodi (small circles inside ellipses) in dual isotope space relative to its potential prey items (mean ± SD = dashed lines) at 3 locations of the Gulf of Cadiz: (1) San Pedro estuary (Cadiz), (2) Cadiz Bay, and (3) Carreras estuary (Huelva). The trophic discrimination factors for A. monodi muscle were calculated using muscle-specific diet tissue discrimination factors from Dubois et al. (2007a). Location was considered as a random factor. Prey types (Pseudofaeces, Microalgae, , Macroalgae and the bivalves M. galloprovincialis and S. plana) are depicted by line drawings. For sample sizes, see Table 1.

Figure 3.

Density distributions and associated boxplots of modeled outputs from Bayesian stable isotope mixing models estimating the proportion of contributions of likely types of diet (Pseudofaeces, Microalgae, POM, Macroalgae, and the bivalves M. galloprovincialis and S. plana) of the pea crab A. monodi at 3 locations of the Gulf of Cadiz (Location 1, San Pedro estuary, Cádiz, Location 2 Cádiz Bay, and Location 3 Carreras estuary, Huelva).

Figure 4.

Bayesian posterior TPs for the female and male pea crabs A. monodi, the filter-feeding species Balanus sp. and the oyster M. gigas (pooled sample) and different tissues of the host mussel M. galloprovincialis as calculated from 2 sources Bayesian model at the location of Carreras estuary (Huelva). Overlap of the 95% credible intervals indicates similarity between groups. Gray boxes represent 50% (dark shading), 75% (medium shading), and 95% (light shading) credible intervals, with black dots designating the median.

Figure 5.

(A) Independent calculated ellipses containing 95% of the isotopic data for the male and female of the pea crab A. monodi and the host bivalves S. plana (Location 1) and M. galloprovincialis (Locations 2 and 3) from the Cadiz Bay. (B) Posterior distributions of isotopic niche size (BSEA) for the females and male pea crab A. monodi and the host bivalves M. galloprovincialis and S. plana at 3 locations of the Gulf of Cadiz: (1) San Pedro estuary (Cádiz); (2) Cádiz Bay; and (3) Carreras estuary (Huelva). Gray boxes represent 50% (dark shading), 75% (medium shading), and 95% (light shading) credible intervals, with red crosses designating the model values and black dots the median.

Figure 6.

Density distributions of the overlap proportion of BSEAs and associated boxplots between the niche area of the host bivalves and the male and female pea crab A. monodi obtained from posterior distributions, at 3 locations of the Gulf of Cadiz: (1) San Pedro estuary (Cádiz); (2) Cádiz Bay; and (3) Carreras estuary (Huelva). Overlap was calculated as the percentage of shared area of each individual ellipse (male or female of the pea crab) with each relevant other ellipse (the host bivalves).