Summary: Effects of Endolithic Parasitism on Invasive and
Indigenous Mussels in a Variable Physical Environment
Gerardo Ivan Zardi1¤.
, Katy Rebecca Nicastro1¤.*
, Christopher David McQuaid1
, Marcos Gektidis2
1 Department of Zoology & Entomology, Rhodes University, Grahamstown, South Africa, 2 Institute of Paleontology, Friedrich Alexander University, Erlangen, Germany
Biotic stress may operate in concert with physical environmental conditions to limit or facilitate invasion processes while
altering competitive interactions between invaders and native species. Here, we examine how endolithic parasitism of an
invasive and an indigenous mussel species acts in synergy with abiotic conditions of the habitat. Our results show that the
invasive Mytilus galloprovincialis is more infested than the native Perna perna and this difference is probably due to the
greater thickness of the protective outer-layer of the shell of the indigenous species. Higher abrasion due to waves on the
open coast could account for dissimilarities in degree of infestation between bays and the more wave-exposed open coast.
Also micro-scale variations of light affected the level of endolithic parasitism, which was more intense at non-shaded sites.
The higher levels of endolithic parasitism in Mytilus mirrored greater mortality rates attributed to parasitism in this species.
Condition index, attachment strength and shell strength of both species were negatively affected by the parasites
suggesting an energy trade-off between the need to repair the damaged shell and the other physiological parameters. We
suggest that, because it has a lower attachment strength and a thinner shell, the invasiveness of M. galloprovincialis will be
limited at sun and wave exposed locations where endolithic activity, shell scouring and risk of dislodgement are high.