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

Abstract

Ocean acidification threatens organisms that produce calcium carbonate shells by potentially generating an under‐saturated carbonate environment. Resultant reduced calcification and growth, and subsequent dissolution of exoskeletons, would raise concerns over the ability of the shell to provide protection for the marine organism under ocean acidification and increased temperatures. We examined the impact of combined ocean acidification and temperature increase on shell formation of the economically important edible mussel Mytilus edulis. Shell growth and thickness along with a shell thickness index and shape analysis were determined. The ability of M. edulis to produce a functional protective shell after 9 months of experimental culture under ocean acidification and increasing temperatures (380, 550, 750, 1000 μatm pCO 2, and 750, 1000 μatm pCO 2 + 2°C) was assessed. Mussel shells grown under ocean acidification conditions displayed significant reductions in shell aragonite thickness, shell thickness index, and changes to shell shape (750, 1000 μatm pCO 2) compared to those shells grown under ambient conditions (380 μatm pCO 2). Ocean acidification resulted in rounder, flatter mussel shells with thinner aragonite layers likely to be more vulnerable to fracture under changing environments and predation. The changes in shape presented here could present a compensatory mechanism to enhance protection against predators and changing environments under ocean acidification when mussels are unable to grow thicker shells. Here, we present the first assessment of mussel shell shape to determine implications for functional protection under ocean acidification.

A 120‐year record of resilience to environmental change in brachiopods

“…Higher temperatures (~18 to 20°C) caused more rounded shells to be produced, whereas lower temperatures (~6 to 8°C) caused more laterally flattened shells. Increased p CO 2 conditions (750 μatm and 1,000 μatm) resulted in rounder and flatter Mytilus edulis shells which also had a thinner aragonite layer compared to ambient conditions (380 μatm) (Fitzer et al., ). This new shell shape was explained as a compensatory mechanism to enhance protection from predators and changing environments due to the inability of this species to produce thicker shells under increased ocean acidity.…”

A mineralogical record of ocean change: Decadal and centennial patterns in the California mussel

“…Other, aragonite‐containing Mytilus species have shown a diversity of responses to changes in seawater carbonate chemistry, including decreased size and shell thickness during larval stages, decreases in shell density, shell dissolution, and weakened byssal threads (Fitzer et al., , , ; Gaylord et al., ; O'Donnell, George, & Carrington, ; Thomsen et al., ). Mytilus californianus shell mineralogy should be more resistant to ocean acidification compared with other bivalves because it is the only mussel to lay down calcite instead of aragonite in its inner shell layer (Dodd, ), which is exposed to ambient seawater when mussels are feeding at high tide.…”

“…Figure S5 shows a full transect of Shell HC09006. These scans and those from archival 1970s shells ( Figure 2) (Fitzer et al, 2014(Fitzer et al, , 2015(Fitzer et al, , 2016Gaylord et al, 2011;O'Donnell, George, & Carrington, 2013;Thomsen et al, 2010). Figure S6 for additional images and to Figure S2 for an illustration of crystal orientation.…”

“…Calcification can be affected by multiple environmental parameters, including pCO 2 (Fitzer, Cusack, Phoenix, & Kamenos, ; Gazeau et al., ; Kroeker, Kordas, Crim, & Singh, ; Pfister et al., ), temperature (Fitzer et al., ), wave exposure (Fox & Coe, ), food availability (Coe, ; Hahn et al., ; Thomsen & Melzner, ; Waldbusser et al., ), predation pressure (Vermeij, ), and their interacting effects (Kroeker, Kordas, & Harley, ; Melzner et al., ). These calcification responses include changes in growth rate and overall size (Thomsen & Melzner, ), changes in shell morphology (Fitzer et al., ; Pfister et al., ), and mineralogical plasticity (Nash et al., ; Pauly, Kamenos, Donohue, & LeDrew, ). Mineralogical plasticity has been proposed as a potentially beneficial adaptive mechanism (Fitzer et al., ; Leung, Russell, & Connell, ) that allows calcified organisms to continue growth under acidified conditions.…”

“…Mineralogical plasticity has been proposed as a potentially beneficial adaptive mechanism (Fitzer et al, 2014;Leung, Russell, & Connell, 2017) that allows calcified organisms to continue growth under acidified conditions. However, ecological and physiological trade-offs exist between mineralogy, morphology, and mineral strength that can lead to ecological repercussions of environmentally induced skeletal changes (McCoy & Pfister, 2014;McCoy & Ragazzola, 2014), and changes in mussel shell shape or thickness may be detrimental to their survival in natural conditions (Fitzer et al, 2015;Gooding, Harley, & Tang, 2009;Pfister et al, 2016). Laboratory studies have shown mineralogical changes to occur over relatively short timescales (<6 months) in both adults and juveniles (Fitzer et al, 2014(Fitzer et al, , 2016Melzner et al, 2011) and to be especially important to larval and other early developmental stages (Stumpp, Wren, Melzner, Thorndyke, & Dupont, 2011;Thomsen, Haynert, Wegner, & Melzner, 2015;Waldbusser et al, 2013).…”