Introduction: The endoparasite Dendrogaster argentinensis infects the intertidal brooder sea star Anasterias antarctica. This sea-star species is in the highest trophic level in the Beagle Channel. Objective: To study the effects of parasitism by D. argentinensis on the fitness and reproduction of A. antarctica. Methods: Adults from the brooder sea-star were collected from the rocky intertidal of Ensenada Zaratiegui bay (54°51’ S & 68°29’ W), Argentina. Eight seasonal samplings were performed (four seasons in two years) in the upper and low intertidal. During dissection, parasites were removed, and all organs were extracted and weighed separately. Results: Dendrogaster argentinensis prevalence was the highest for the region (20.4 %). Parasitized individuals were more frequent in the low intertidal in all seasons, with a higher difference in summer, where it is likely that the higher temperatures and strong winds could make the upper intertidal more challenging for a parasitized individual. Five parasitized individuals were castrated. Generally, the gonadal (GI) and somatic (pyloric caeca, PCI; stomach, SI; body wall, WI) indexes were lower in parasitized than non-parasitized individuals. Conclusions: Parasitism by D. argentinensis negatively affects A. antarctica condition. It affects reproduction because it reduces the GI, and can also produce castration. The parasite competes for the sea-stars’ energetic resources, also decreasing the individual’s capacity for feeding (reduced stomach) and growth (reduced body wall).

The Beagle Channel, located south of Tierra del Fuego (~54° S), is South America’s southernmost intracoastal ecosystem. Here, we applied environmental DNA metabarcoding using a partial sequence of the 12sRNA mitochondrial gene to evaluate the seasonal and spatial dynamics of marine vertebrate communities along the longitudinal gradient of the Beagle Channel. Water samples were collected across seasons and sites spanning the central and eastern sectors of the channel. Vertebrate DNA was amplified and analyzed against a custom reference database enriched with locally generated sequences. A total of 43 vertebrate taxa were detected, including 25 fish, 13 mammals, and 5 seabirds. Vertebrate assemblages were spatially structured along the longitudinal gradient of the Beagle Channel, with the central sector characterized by higher relative abundances of Fuegian sprat (Sprattus fuegensis), Southern elephant seal (Mirounga leonina), South American fur seal (Arctocephalus australis), and domestic species, whereas the eastern sector showed higher relative abundances of fish species including silversides (Odontesthes spp.), Magellanic rockcod (Paranotothenia magellanica), and striped rockcod (Trematomus hansoni), as well as mammals including southern sea lion (Otaria flavescens), North American beaver (Castor canadensis), and native grass mice (Abrothrix spp.). This spatial differentiation was associated with contrasting environmental conditions. No significant seasonal effect was detected. This study provides the first environmental DNA-based baseline of marine vertebrates on the Beagle Channel and validates this approach for ecosystem-scale biodiversity monitoring in subantarctic coastal environments.

Changes in natural and artificial light presumably have dramatic influences on ecological communities, yet measuring these effects can be challenging due to the difficulty in distinguishing ecological signals from background variability. Here, we used environmental DNA (eDNA) to test for light‐associated changes in a nearshore marine community. Over 8 days at Friday Harbor Laboratories, Washington, USA, we sampled seawater at midday and midnight with and without an artificial light treatment, analyzing a total of 84 samples (3 L triplicate samples) using COI and 12S markers on an Oxford Nanopore MinION platform. We detected 229 eukaryotic (COI) and 77 vertebrate (12S) taxa across 11 trophic groups, dominated by primary producers (89 species), benthic invertebrates, and fishes. Metabarcoding captured community restructuring within 30 min, such that we could observe meaningful effects of artificial light treatments at night (particularly among spawning polychaetes), as well as more fundamental community shifts in day vs. night (e.g., with primary producers, gelatinous zooplankton, and filter feeders more common during the day). These techniques offer a means of near‐real‐time assessment of wholescale changes in ecological communities, and our results here illustrate species‐ and group‐specific effects of light levels that are likely important in structuring nearshore marine communities worldwide.