The dark side of soft tissues: Unexpected inorganic carbonate in the invasive slipper limpet Crepidula fornicata and its implications for stable isotope interpretations

RATIONALE

Stable isotopic analysis is extensively used in trophic ecology. Inorganic carbonates, usually originating from shell fragments, are routinely removed from samples using an acid treatment because they affect δ¹³ C values. However, acid treatment can also change δ¹⁵ N values. For some taxa such as molluscs, the general assumption is that acid pre-treatment is not necessary since their shell is easily dissected from soft tissues and represents the only source of inorganic carbonates. However, other sources of non-dietary carbon (e.g., intracellular inorganic carbon) enriched in¹³ C thus get overlooked.

METHODS

Soft tissues (foot) of the invasive gastropod Crepidula fornicata of different size classes were analysed for their δ¹³ C and δ¹⁵ N values with and without acid pre-treatment using isotope ratio mass spectrometry. In toto microscopic investigations coupled with acid treatment, scanning electron microscopy and energy-dispersive spectroscopy were used to highlight the presence of inorganic carbonate. A correction model was derived and applied to existing stable isotope data for C. fornicata. We used both seasonal variations in δ¹³ C signatures and mixing model outputs to assess the error in δ¹³ C values.

RESULTS

Acid pre-treatment had a significant effect on the stable isotope compositions of C. fornicata foot tissue, especially on δ¹³ C values: isotopic differences increased with size, up to 3‰ for large females. No effect was detected for small (below ~20 mm) and motile males. In toto microscopic analysis revealed the presence of small spherules of inorganic carbonate, hence explaining the differences in δ¹³ C values. Mixing model outputs and seasonal variation of δ¹³ C values showed that untreated samples can lead to large misinterpretations about diet proportions and degree of trophic niche overlap, respectively.

CONCLUSIONS

Spherules of inorganic carbonate in C. fornicata soft tissues are likely to be linked with the motility of this species and their mucus production. We recommend assessing the presence of inorganic carbonate in soft tissue of sessile gastropods.

Multiple effects of a Gracilaria vermiculophylla invasion on estuarine mudflat functioning and diversity

The invasive Japanese seaweed Gracilaria vermiculophylla has become established over the past several years in numerous European estuaries, from Portugal to Norway. In the Faou estuary (48.295°N-4.179°W, Brittany, France), it forms a dense population at the mud's surface. The effects of G. vermiculophylla on metabolism, diversity, and the food web were studied. Community gross primary production (GPP) and respiration (CR) during emersion, chlorophyll-a content, macrofaunal and meiofaunal diversity and abundance, and stable isotopes (δ¹³C and δ¹⁵N) of representative macrofaunal species and main food sources were measured at low tide in winter, spring, summer 2014, and winter 2015. Results show significant seasonal variation in GPP and CR. Moreover, GPP was significantly higher in areas where G. vermiculophylla was present than in the control area (bare mud). However, this high GPP appeared to be linked to the increase in biomass in primary producers, with their efficiency (primary productivity, i.e. assimilation number) remaining relatively stable compared with the control area. Significant variation in abundance of meiofauna and macrofauna was also detected and new epifaunal species were collected, mainly in Gracilaria-colonized areas. Isotopic food-web Bayesian mixing models strongly suggested that G. vermiculophylla plays a major role in the diet of some dominant species. Mechanisms interacting with the functioning and diversity of the mudflat are discussed. Finally, the invasive seaweed G. vermiculophylla affected the mudflat ecosystem in three ways: as a new primary producer (increase in metabolism), as a habitat-forming species (changes in diversity and abundance of macrofauna and meiofauna), and as a new abundant food source, likely through the detrital pathway.