Microplastic ingestion by pelagic gooseneck barnacles (Lepas spp.) and other insights on their biology and ecology in the South Pacific Ocean

Pelagic gooseneck barnacles from genus Lepas are the most common rafting organism in the open ocean. Attaching to floating objects, they are a foundational part of the rafting ecosystem. Lepas barnacles are thought to be the most capable of ingesting microplastics of any rafting life. Ten large tsunami detection buoys (DART), and two smaller buoys, around New Zealand and in the wider Southwest Pacific Ocean were sampled for Lepas specimens. These samples were analysed for their species and morphological characteristics and dissected to find microplastics. This study revealed that Lepas barnacles do ingest microplastics in the study area, with 1.6% of barnacles having microplastics in their digestive system. Sites which were at latitudes modelled in a previous study to have the highest microplastic density also had the most barnacles with ingested microplastic. Inedible objects such as pumice were also found in the barnacles digestive systems. The lack of evidence for any negative effects of microplastics on pelagic gooseneck barnacles suggests they are not a viable bioindicator for the effects of microplastic pollution. However, these species are still useful for understanding the relative bioavailability of microplastics if dissected to compare to other studies from around the world.This study found that Lepas species in the study area have very clear latitudinal distributions. These distributions are likely a result of temperature preferences and ocean dynamics. The capitulum and peduncle lengths of Lepas specimens decreased towards the equator, with significant differences in peduncle size between species. The peduncle lengths of the barnacles were longer at sites with higher densities of Lepas spp., but also were significantly longer on the float of the buoys near the surface than 2 metres deeper on the transducers, perhaps due to increased competition for space. A form of optimal foraging may also be utilised by L. anatifera which often settles in less optimum locations away from its competitor L. testudinata. There was a very strong linear increase in the proportion of spotted barnacles with decreasing latitude towards the equator. The translucency of a barnacles peduncle was strongly associated with its position on the buoy, suggesting that sun-exposure causes the darkening of Lepas peduncles. The highest minimum growth rate for a Lepas barnacle peduncle was estimated as 1.1 mm per day. Overall, this study has begun to reveal that Lepas barnacles show many interesting and unexplained patterns which future work will explore further.