# Antarctic Krill with parasites grow slower than uninfected peers

## TL;DR

Antarctic krill infected with certain parasites grow more slowly, which could significantly impact their population and ecosystem dynamics.

## Contribution

This study identifies specific parasites affecting krill growth and quantifies their ecological impact for the first time.

## Key findings

- 15 distinct parasite types were found in Antarctic krill samples.
- Apicomplexa and epibiotic ciliate infections were linked to reduced krill growth rates.
- Parasites may reduce krill biomass as much as vertebrate predators annually.

## Abstract

Antarctic krill, Euphausia superba (Dana i Am J Sci Arts Ser 2(9):129–133, 1850), are both central ecosystem components in the Southern Ocean, and the target of a growing commercial fishery. Understanding the trophic interactions which shape krill population dynamics is essential to sustainably managing human impacts on this key species. While the roles of krill as grazers of phytoplankton and as prey for vertebrate predators are relatively well understood, very little is known about interactions with their smallest predators– the parasites. We investigated the assemblage of parasites present in E. superba, and the impacts of parasites on krill somatic growth. We found 15 distinct parasite types across a sample of 100 krill, including seven types of Gregarine Apicomplexa, two types of internally infecting ciliates, three types of epibiotic ciliates, fungi, syndiniales, and a Parorchites zederi cestode worm. Apicomplexa and epibiotic ciliate infections were linked with lower growth rates, with these two parasites explaining 10% and 24% of the observed variation in krill growth, respectively. Although much uncertainty remains, scaling these results to the population level suggests parasites may be responsible for as much reduction in krill biomass annually as vertebrate predators, indicating the importance of considering these trophic links in food web modelling and ecosystem-based management.

The online version contains supplementary material available at 10.1007/s00227-025-04673-w.