Trophic interactions between two neustonic organisms: insights from Bayesian stable isotope data analysis tools

The by-the-wind sailor Velella velella (Linnaeus, 1758) and its predator, the violet snail Janthina globosa (Swainson, 1822) are both floating neustonic organisms. Despite their global oceanic distribution and widespread blooms of V. velella in recent years, many gaps remain in our understanding about prey/predator interactions between these two taxa. Using stable isotope ratios of carbon and nitrogen, we aimed to study the trophic relationship between V. velella and J. globosa and investigate diet variation of V. velella and J. globosa in relation to individuals’ size. Bayesian approaches were used to calculate isotopic niche metrics and the contribution of V. velella to the J. globosa diet. Our data showed that the isotopic niche of V. velella differed markedly from that of J. globosa. It was larger and did not overlap that of the J. globosa, indicating a more variable diet but at a lower trophic level than J. globosa. The isotopic niche of V. velella also varied according to the size class of the individual. Small individuals showed a larger isotopic niche than larger animals and low overlap with those of the larger individuals. J. globosa displayed very low isotopic variability and very small isotopic niches. In contrast, there were no isotopic composition nor isotopic niche differences between J. globosa of any size. This very low isotopic variability suggested that J. globosa is a specialist predator, feeding, at least in this aggregation, principally on V. velella. Moreover, outputs of a stable isotope mixing model revealed preferential feeding on medium to large ( 500 mm2) V. velella colonies. While our isotopic data showed the trophic relationship between V. velella and J. globosa, many questions remain about the ecology of these two organisms, demonstrating the need for more fundamental studies about neustonic ecosystems.

Trophic relationship between the invasive parasitic copepod Mytilicola orientalis and its native blue mussel (Mytilus edulis) host

Invasive parasites can spill over to new hosts in invaded ecosystems with often unpredictable trophic relationships in the newly arising parasite-host interactions. In European seas, the intestinal copepod Mytilicola orientalis was co-introduced with Pacific oysters (Magallana gigas) and spilled over to native blue mussels (Mytilus edulis), with negative impacts on the condition of infected mussels. However, whether the parasite feeds on host tissue and/or stomach contents is yet unknown. To answer this question, we performed a stable isotope analysis in which we included mussel host tissue and the primary food sources of the mussels, microphytobenthos (MPB) and particulate organic matter (POM). The copepods were slightly enriched in δ15N (mean Δ15N ± s.d.; 1·22 ± 0·58‰) and δ13C (Δ13C 0·25 ± 0·32‰) with respect to their host. Stable isotope mixing models using a range of trophic fractionation factors indicated that host tissue was the main food resource with consistent additional contributions of MPB and POM. These results suggest that the trophic relationship of the invasive copepod with its mussel host is parasitic as well as commensalistic. Stable isotope studies such as this one may be a useful tool to unravel trophic relationships in new parasite-host associations in the course of invasions.

Review of Lepidoptera with trophic relationships to Picea abies (L.) in the conditions of Czechia

Trophic relationships of Lepidoptera (Insecta) occurring in the territory of Czechia to the Norway spruce (Picea abies L.) was evaluated on the basis of the excerption and critical evaluation of literature. Each species was classified into the following categories – spruce as the host plant, regular development on spruce, narrow trophic relationship, indirect relationship and episodical occurrence. The particular taxa were also characterized according to their distribution and the form of larval life was specified. The development on spruce was documented in 96 species of Lepidoptera, which represented less than 3% of taxa belonging to this group and being reported from Czechia. Of that, spruce was a common host plant for 67 species, 23 species were polyphagous and might develop on spruce, and 6 species belonged to soil species damaging spruce roots, mainly in forest nurseries. Among the species of Lepidoptera, which regularly develop on spruce in the Czech conditions, 55 species were classified. As narrow specialists with special trophic relationship to spruce, 33 taxa could be considered. There were 15 spruce species with forestry importance, which were able to outbreak their populations regularly or irregularly. Among spruce species it was possible to classify 16 taxa as rare. The provided information on Lepidoptera with trophic relationship to spruce is applicable also for other Central European areas. Besides the species with importance for forest pest management, also rare taxa, which can become endangered by climate change or by forest management, were indicated.