The influence of geographical location, host maturity and sex on intestinal helminth communities of the double-crested cormorant Phalacrocorax auritus from the eastern United States

EXPERIMENTAL ELUCIDATION OF THE LIFE CYCLE OF DREPANOCEPHALUS SPATHANS (DIGENEA: ECHINOSTOMATIDAE) WITH NOTES ON THE MORPHOLOGICAL PLASTICITY OF D. SPATHANS IN THE UNITED STATES

The echinostomatid Drepanocephalus spathans (syn. Drepanocephalus auritus) parasitizes the double-crested cormorant Phalacrocorax auritus. In North America, the marsh rams-horn snail Planorbella trivolvis and ghost rams-horn snail Biomphalaria havanensis serve as snail intermediate hosts, both of which inhabit catfish aquaculture ponds in the southeastern United States. Studies have demonstrated D. spathans exposure can be lethal to juvenile channel catfish Ictalurus punctatus. Two studies were undertaken to elucidate the life cycle of D. spathans to establish a developmental time line. In both studies, D. spathans cercariae collected from naturally infected P. trivolvis individuals were used to infect channel catfish fingerlings, which were then fed to double-crested cormorants (DCCOs) that had been pharmaceutically dewormed. In study 1, laboratory-reared P. trivolvis and B. havanensis individuals were placed in aviary ponds with experimentally infected DCCO and examined bi-weekly for release of cercariae. Trematode eggs were observed in the feces of exposed birds 3 days post-infection. Birds were sacrificed 18 days post-exposure (dpe), and gravid adults morphologically and molecularly consistent with D. spathans were recovered. Snails from the aviary pond were observed shedding D. spathans cercariae 18-54 dpe. In study 2, trematode eggs were observed in the feces of exposed DCCOs beginning 8 dpe. Once eggs were observed, birds were allowed to defecate into clean tanks containing naïve laboratory-reared P. trivolvis individuals. Additionally, eggs from experimental DCCO feces were recovered by sedimentation and placed in an aquarium housing laboratory-reared P. trivolvis individuals. Birds in study 2 were sacrificed after 60 days, and gravid D. spathans specimens were recovered. Snails from the experimental DCCO tanks shed D. spathans cercariae 89-97 dpe. Lastly, trematode eggs were isolated and observed for the hatching of miracidia, which emerged on average after 16 days at ambient temperatures. No D. spathans adults were observed in control birds fed non-parasitized fish. This is the first experimental confirmation of the D. spathans life cycle, resolving previously unknown developmental time lines. In addition, the effects of fixation on adult trematode morphology were assessed, clarifying reports of pronounced morphological plasticity for D. spathans.

Using species distribution models to define nesting habitat of the eastern metapopulation of double-crested cormorants

When organisms with similar phenotypes have conflicting management and conservation initiatives, approaches are needed to differentiate among subpopulations or discrete groups. For example, the eastern metapopulation of the double-crested cormorant (Phalacrocorax auritus) has a migratory phenotype that is culled because they are viewed as a threat to commercial and natural resources, whereas resident birds are targeted for conservation. Understanding the distinct breeding habitats of resident versus migratory cormorants would aid in identification and management decisions. Here, we use species distribution models (SDM: Maxent) of cormorant nesting habitat to examine the eastern P. auritus metapopulation and the predicted breeding sites of its phenotypes. We then estimate the phenotypic identity of breeding colonies of cormorants where management plans are being developed. We transferred SDMs trained on data from resident bird colonies in Florida and migratory bird colonies in Minnesota to South Carolina in an effort to identify the phenotype of breeding cormorants there based on the local landscape characteristics. Nesting habitat characteristics of cormorant colonies in South Carolina more closely resembled those of the Florida phenotype than those of birds of the Minnesota phenotype. The presence of the resident phenotype in summer suggests that migratory and resident cormorants will co-occur in South Carolina in winter. Thus, there is an opportunity for separate management strategies for the two phenotypes in that state. We found differences in nesting habitat characteristics that could be used to refine management strategies and reduce human conflicts with abundant winter migrants and, at the same time, conserve less common colonies of resident cormorants. The models we use here show potential for advancing the study of geographically overlapping phenotypes with differing conservation and management priorities.