Predator‐induced morphological defences in two invasive dreissenid mussels: implications for species replacement

Abstract

Summary Induced morphological defences as a response to chemical cues from predators are common in aquatic systems. We evaluated predator‐induced defences and the costs of these responses measured as growth rate reductions in two invasive mussel species. One mussel species (quagga mussel, Dreissena rostriformis bugensis) often dominates the other (zebra mussel, Dreissena polymorpha) in lakes where they co‐occur. We exposed both species to cues from three important North American predators: round goby (Neogobius melanostomus), which is native in the ancestral range of the mussels, and pumpkinseed sunfish (Lepomis gibbosus) and rusty crayfish (Orconectes rusticus) that are native in North America. Both mussel species responded to risk cues by increasing shell thickness and decreasing growth rates. The response to round goby was smaller than the response to the two new predators. The strength of the induced response was negatively correlated with growth in both mussel species and both the strength of the induced defence and the decrease in growth were larger for zebra mussels than for quagga mussels. Survival was higher in zebra mussels than in quagga mussels, but there was no significant difference in survival among different risk cue treatments. The lower response and correlated faster growth of quagga mussels than zebra mussels in response to risk cues may be an additional reason for the replacement of zebra mussels by quagga mussels in North America, at least as long as native predators are not a major source of mortality. If true, we predict that both mussel species will co‐occur after the exotic round goby becomes a dominant mussel predator in North America.

Ocean acidification and temperature increase impact mussel shell shape and thickness: problematic for protection?

“…There is an indication of a flattening of the shell with increasing pCO 2 confirmed by the significant difference in the third component analysis of the mussel shell meshes. Differences in shell morphology due to predator cues have been reported as a defensive mechanism including production of a more rotund shell (Br€ onmark et al 2011;Naddafi and Rudstam 2014). A more rotund shell, rounder and fatter in shape, reduces predator efficiency (Br€ onmark et al 2011).…”

“…The measurements were taken using a digital electronic caliper and the dry shell weight measured using a Mettler AT100 mass balance. STI is considered a good measure of shell thickness in the blue mussel with lower measurement error compared to direct measurement (Freeman and Byers 2006;Naddafi and Rudstam 2014). A high STI indicates a thicker shell, and a low STI indicates a thinner shell.…”

“…Under ocean acidification, changes to growth could impact the protective function of the shell. Morphological changes such as increasing shell thickness and production of a more rotund shell shape have been used by organisms as a defensive mechanism to combat predators (Br€ onmark et al 2011;Naddafi and Rudstam 2014). Here, we investigate how long-term (9 months) ocean acidification (550, 750, 1000 latm pCO 2 ) and increasing temperatures (750 and 1000 latm pCO 2 + 2°C) impact the shape of the M. edulis shell in comparison with the mussel shell growth and shell thickness.…”

Long-term trends in Lake Ontario's benthic macroinvertebrate community from 1994–2008

“…However, Zebra Mussels do persist in the lake on rocky, nearshore substrates that cannot be sampled with Ponar grabs (Karatayev et al, 2013). Possible reasons for the greater success of Quagga Mussels include a higher assimilation efficiency (Baldwin et al, 2002), a lower respiration rate (Roe and MacIsaac, 1997), higher filtration rates during the warmer months (Diggins, 2001) and a faster growth rate in the presence of predators (Naddafi and Rudstam, 2014a). D. r. bugensis also possess longer siphons and byssal threads than D. polymorpha, which enables them to thrive in soft Table 5.…”

Towards a mechanistic understanding of individual‐level functional responses: Invasive crayfish as model organisms

“…Thus, Dreissena mussels can reach a size refuge, and mostly small individuals will be impacted by crayfish predation. Additionally, Dreissena mussels adapt their behaviour and morphology to predation by reducing feeding rate and increasing shell thickness at the cost of lower growth rates (Naddafi & Rudstam, , ).…”