A hug of hunger: epibiosis of an introduced macroalga on an economically valuable mussel

Abstract

Epibiosis, defined as the interaction where one organism lives on another, can result in a range of implications. Within coastal ecosystems, mussel beds function as important surfaces for the attachment of diverse epibiont species. Consequently, epibiosis on mussels can hinder water flow and oxygen availability or increase the risk of dislodgement due to added weight. Moreover, epibiosis on commercially valuable mussels can have detrimental effects on fishing yield. We evaluated the effects of the introduced macroalga Undaria pinnatifida as an epibiont on the commercial mussel Aulacomya atra in San Matías Gulf, Argentina (40°57’S, 65°06’W). We compared individual characteristics (e. g. size, weight, condition) of mussels with or without epibiosis of U. pinnatifida (n= 111 of each condition) retrieved from a commercial bed during austral summer 2023 and 2024. Additionally, in October 2023 we evaluated the effect of epibiosis on the clearance rate (CR) through laboratory experiments using mussels with or without epibiosis (n=15 of each condition). Individuals with epibiosis were lighter, smaller, and exhibited significantly lower condition index and CR compared to those without epibiosis. Our results suggest that the presence of the macroalga influences mussels feeding, affecting their size and weight, which may have cascading consequences on the dynamics of A. atra populations. Moreover, since size and weight are crucial issues to the economic quality and value of commercial mussels, U. pinnatifida may affect the domestic market and profitability of the fishery.