Capsule Three quarters of tracked Northern Gannets (Morus bassanus) at Grassholm gathered in rafts around the colony, concentrated within a recently designated at-sea Special Protection Area (SPA), but rafting was not correlated with foraging effort. Aims To investigate the incidence, distribution and foraging implications of Northern Gannet rafting behaviour in waters adjacent to a large colony. Methods Using bird-borne global positioning system (GPS) loggers we reconstructed at-sea movement and used a speed filter to identify rafting behaviour within 10 km of the colony. We mapped the spatial distribution of rafting events from 160 breeding individuals over 5 years, and investigated the relationship between foraging effort (trip duration and total distance travelled) and the presence/absence of rafting. Results On average, 74% of tracked birds engaged in rafting. Of the 381 foraging trips analysed, rafting was recorded on 237 (62%). Birds were more likely to raft on outbound (224 trips, 59%), than inbound journeys (38 trips, 10%). Presence/absence of rafting did not correlate significantly with foraging trip distance or duration nor with duration of nest attendance. The majority of rafting was concentrated in a 2-km radius around the colony within a recently designated seaward SPA extension. Birds showed low individual repeatability in rafting, although there was lower variation within, than among, individuals. Conclusion Our results show that rafting is important for breeding gannets on Grassholm, and a recently designated at-sea SPA encapsulates the core distribution of rafting. Rafting did not appear to be correlated with foraging behaviour. Given the dearth of literature on rafting and the wealth of GPS tracking data for seabirds, we suggest that similar research be conducted elsewhere to further elucidate the ecological and applied significance of this behaviour.

Parasites play significant roles in the function of ecosystems and can make up a large proportion of overall biomass. Yet, fundamental aspects of their ecology are often understudied relative to other organisms. Gnathiid isopods are the primary ectoparasites of fishes in coral reef ecosystems. While some studies have investigated their host-detection capabilities, the means by which they seek hosts are largely unknown. Gnathiids are benthic and live on the bottom, and all collection efforts involving live hosts have thus far involved traps set at or near the reef substrate. We investigated the distance gnathiids will travel vertically in the water column to attach to a fish host. The majority of gnathiids collected were at or less than 1 m above the reef substrate, and gnathiids were collected in serially diminishing numbers at 2 and 3 m above it. No gnathiids were collected from any fish hosts set more than 3.5 m above the reef substrate. Results suggest that gnathiids will actively seek host fishes in situ and will travel further from their benthic habitat than previously known. By swimming into the water column, gnathiids can exploit larger, highly-mobile fishes, which can serve as a major source of dispersal.

Organisms with a parasitic lifestyle comprise a high proportion of biodiversity in aquatic and terrestrial environments. However, there is considerable variation in the ways in which they acquire nutrients. Hematophagy is a common consumption strategy utilized by some terrestrial, aquatic, and marine organisms whereby the parasite removes and digests blood from a host. Gnathiid isopods are marine hematophagous parasites that live in benthic substrates from the intertidal to the abyss. Although ecologically similar to ticks and mosquitoes, they feed only during each of 3 juvenile stages and adults do not feed. They have long been considered as generalist fish parasites and to date, there have been no reports of their successfully feeding on invertebrates. Based on observations of gnathiids attached to soft-bodied invertebrates collected from light traps, we conducted a laboratory experiment in which we collected and individually housed various common Caribbean invertebrates and placed them in containers with gnathiids to see if the gnathiids would feed on them. All fed gnathiids were subsequently removed from containers and given the opportunity to metamorphose to the next developmental stage. In total, 10 out of the 260 gnathiids that were presented with 1 of 4 species of potential invertebrate hosts had fed by the next morning. Specifically, 9 of a possible 120 gnathiids fed on lettuce sea slugs (Elysia crispata), and 1 of a possible 20 fed on a bearded fireworm (Hermodice carunculata). Eight of these 10 fed gnathiids metamorphosed to the next stage (5 to adult male, 2 to adult female, and 1 to third-stage juvenile). Even though feeding rates on invertebrates were considerably lower than observed for laboratory studies on fishes, this study provides the first documented case of gnathiids' feeding on and metamorphosing from invertebrate meals. These findings suggest that when fish hosts are not readily available, gnathiids could switch to soft-bodied invertebrates. They further provide insights into the evolution of feeding on fluids from live hosts in members of this family.