A conspicuous new deep-sea oyster,
Neopycnodonte zibrowii sp. n., is described from the Azores Archipelago, where it thrives in 420 to 4500 m water depth in high densities concealed underneath overhangs. The new species reaches a relatively large size, which may exceed 20 cm, and is characterised by a very unusual hinge line morphology, straight without a bulge of the resilium. It is compared to the extant Indo-Pacic
Empressostrea kostini Huber and Lorenz, 2007 and to the cosmopolitan
Neopycnodonte
cochlear (Poli, 1791), which has a broadly sympatric distribution at shallower depths in the Azores and Bay of Biscay. Radiocarbon dating reveals that individuals reach an impressive lifespan of one to more than ve centuries, placing them among the longestlived molluscs known to date. They often grow on top of each other, forming stacks that resemble dish piles—an effective measure to optimise shell stability with minimal biomineralisation effort, but with the drawback of increased bioerosion ultimately leading to detachment. Three microstructure types are developed in
N. zibrowii: (1) the cross-foliated, calcitic, dorsal to central endostracum and aragonitic ligostracum, (2) the porous vesicular structure of the calcitic ventral endostracum, and (3) the simple prismatic aragonitic myostracum. Foliated and vesicular shell portions show sub-millimetre-scale rst-order increments delineated by conchiolin-rich growth breaks (interpreted as reproductive cyclicity), and less distinct second-order increments (interpreted as annual
in nature). This pattern is clearly reected by the elemental composition with the primary growth breaks lacking Ca and Sr but including Mg and S as organic matrix constituents. The second-order increments within the calcite are mirrored by moderately co-varying Mg/Ca and S/Ca uctuations at stable Sr concentrations, reecting varying proportions of organic matrix. Dorsal and central endostracum transects reveal a low inter-valve, but considerable inter-specimen variability with high Mg/Ca molar ratios and uctuations (22.5717.6mmol/mol), low Sr/Ca values (0.270.1mmol/mol), and a typical to high S/Ca content (6.972.2mmol/mol), when compared to other calcitic bivalves.
Unlike short-lived, shallow-water oysters,
N. zibrowii thrives under very stable environmental conditions. Minimal temperature uctuations and stable open marine salinity provide an optimal basis to recognise biological fractionation processes. Strong Mg/Ca uctuations indicate a physiological control related to metabolism and biomineralisation, prohibiting the use of this ratio as a temperature or ocean chemistry proxy. Low Sr/Ca ratios indicate rather constant and low long-term accretion rates, while short-scale uctuations may be attributed to short-term variations in growth rate and Mg incorporation. Oxygen isotopes yield a considerable spread of 1.8% with a mean
of 2.070.3% d18O V-PDB, and low correlation between different contemporaneous parts of the shell and between specimens. These values surprisingly exceed expected equilibrium conditions, calculated from in situ temperature data (annual mean 12.370.3 1C) and seawater isotopic composition (0.570.1% d18O SMOW), by 0.5% on average. Such positively shifting vital effects, previously reported for limpets and barnacles, are often overlooked in high-temperature and high-amplitude settings and may be more common than is currently believed. Carbon isotopes range from 0.2 to 3.5% d13C V-PDB (mean 1.870.7%) and show an ontogenetic decrease, but may incorporate an environmental signal in adult portions, indicated by a strong correspondence of peaks between specimens. This signal is likely driven by a complex interplay of different factors, such as primary production, current-based food supply and metabolism.
