Effect of reduced pH on physiology and shell integrity of juvenile Haliotis iris (pāua) from New Zealand

The New Zealand pāua or black footed abalone, Haliotis iris, is one of many mollusc species at potential risk from ocean acidification and warming. To investigate possible impacts, juvenile pāua (~24 mm shell length) were grown for 4 months in seawater pH/pCO2 conditions projected for 2100. End of century seawater projections (pHT 7.66/pCO2 ~1,000 μatm) were contrasted with local ambient conditions (pHT 8.00/pCO2 ~400 μatm) at two typical temperatures (13 and 15 °C). We used a combination of methods (morphometric, scanning electron microscopy, X-ray diffraction) to investigate effects on juvenile survival and growth, as well as shell mineralogy and integrity. Lowered pH did not affect survival, growth rate or condition, but animals grew significantly faster at the higher temperature. Juvenile pāua were able to biomineralise their inner nacreous aragonite layer and their outer prismatic calcite layer under end-of-century pH conditions, at both temperatures, and carbonate composition was not affected. There was some thickening of the nacre layer in the newly deposited shell with reduced pH and also at the higher temperature. Most obvious was post-depositional alteration of the shell under lowered pH: the prismatic calcite layer was thinner, and there was greater etching of the external shell surface; this dissolution was greater at the higher temperature. These results demonstrate the importance of even a small (2 °C) difference in temperature on growth and shell characteristics, and on modifying the effects at lowered pH. Projected CO2-related changes may affect shell quality of this iconic New Zealand mollusc through etching (dissolution) and thinning, with potential implications for resilience to physical stresses such as predation and wave action.

Valve microstructure and phylomineralogy of New Zealand chitons

The microstructure and mineralogy of chiton valves has been largely ignored in the literature and only described in 29 species to date. Eight species: Acanthochitona zelandica, Notoplax violacea (Family Acanthochitonidae, Suborder Acanthochitonina, Order Chitonida), Chiton glaucus, Onithochiton neglectus, Sypharochiton spelliserpentis, Sypharochiton sinclairi (Family Chitonidae, Suborder, Chitonina, Order Chitonida), Ischnochiton maorianus (Family Ischnochitonidae, Suborder Chitonina, Order Chitonida), and Leptochiton inquinatus (Family Leptochitonidae, Suborder Lepidopleurina, Order Lepidopleurida) were collected from the Otago Peninsula, South Island, New Zealand. The valves of these chitons were analysed with X-ray diffractometry, Raman spectrometry, and Scanning Electron Micrography (SEM) to determine their mineralogy and microstructure. Both the XRD and Raman data show that the valves consisted solely of aragonite. The observed microstructures of the valves were complex, typically composed of four to seven sublayers, and varied among species. The dorsal layer, the tegmentum, of each species was granular and the ventral layer, the articulamentum, was predominately composed of a spherulitic sublayer, a crossed lamellar sublayer, and an acicular sublayer. The chitonids Sypharochiton pelliserpentis and S. sinclairi had the most complex microstructure layering with three crossed lamellar, two spherulitic sublayers, and a ventral acicular sublayer while the acanthochitonids Acanthochitona zelandica and Notoplax violacea as well as the ischnochitonid Ischnochiton maorianus had the simplest structure with one spherulitic, one crossed lamellar sublayer, and a ventral acicular sublayer. Terminal valves were less complex than intermediate valves and tended to be dominated by the crossed lamellar structure. The leptochitonid Leptochiton inquinatus generated a unique crossed lamellar sublayer different from the other analysed chitonids. Acanthochitona zelandica is the only analysed chitonid that utilizes two different crossed lamellar structures. Clearly, many of these properties do not reflect the currently recognized polyplacophoran phylogeny.