Trophic links and the parasites that exploit them: the case of New Zealand's endemic rough skate

Abstract

Understanding how natural systems are structured and function is central to ecological theory. Although easily overlooked, parasites are ubiquitous and fundamental components of natural systems. Among their various roles, parasites strongly influence the flow of energy between and within food webs. Within marine food webs, elasmobranchs (sharks, skates and rays) are both important predators and hosts to tapeworm parasites. Feeding links are potential transmission routes for tapeworms to exploit, and resolving these pathways provides insight into the ecological role of predators and their parasites within ecosystems. Over 1000 tapeworms are known to parasitise elasmobranchs, although few life cycles are resolved. This thesis furthers our understanding of parasite trophic transmission through investigating the feeding ecology and parasitic links of a relatively understudied elasmobranch species, the New Zealand’s rough skate, Zearaja nasuta. Skates were obtained off the east coast of New Zealand. Their stomachs and intestines were analysed to determine their diet, their parasites, and the parasites of their prey. A fragment of the 28S gene was amplified from each different tapeworm morphotype recovered from either skates or their prey. Phylogenetic relationships were inferred from molecular data using Bayesian inference. Rough skates in this area between the Summer and Autumn months were found to be specialised predators, occupying a unique role in the benthic realm. An ontogenetic shift in diet was found whereby larger, more mature individuals consume significantly fewer but larger prey items. The application of genetic techniques allowed identification of larval and adult parasites infecting the prey species of skates and the skates themselves. Rough skates hosted at least seven species of tapeworms from four tapeworm orders. In three cases, trophic transmission was resolved between the skate and its prey items, i.e. a genetic match was found between larval tapeworms in prey and adult worms in the skate. Several parasites infecting prey did not seem to infect the skate, suggesting other definitive hosts may be involved in their life cycles. This study also uncovered the first case of an adult trypanorhynch tapeworm parasitising rough skates. These findings contribute to this under-researched area as well as providing insights into predator ecology, importance of intermediate and definitive hosts in regard to feeding links, and how food web ecology and parasitology can inform each other.