Cephalopods are champion mineralisers. Nautilids produce robust external shells and internal mineralised tissues; Spirula makes an internal chambered spiral; sepiids produce flat “cuttlebone;” some squids and octopus produce beaks and statoliths. Most cephalopod carbonate is aragonite, but one octopus is an exception: the female Argonauta secretes a fragile calcitic spiral egg-case. Three argonaut cases were collected in NSW, Australia. Four replicate pieces from each were immersed in seawater of varying pH: 8.1 (ambient), 7.8, 7.6, 7.4, 7.1 and 6.7. Weight loss was measured after 14 days. Dissolution rate increased with decreasing pH, with less than 1% loss in 14 days at pH 7.8, 5% loss at pH 7.4, and 20% loss at pH 6.5. Carbonate from all treatments was analysed using x-ray diffractometry, showing no significant changes in mineralogy as shells dissolved. The pelagic life-habit of these cephalopods makes them particularly vulnerable to ocean acidification. Unlike an internal skeleton, which can be protected from seawater while still needed, the Argonauta egg case is exposed to sea water from inception. These egg cases, unprotected by mucous or epithelium, with high surface-area and low volume, and presumably without the capacity to adjust to a less soluble carbonate mineral, are exceptionally vulnerable to dissolution as ocean pH decreases.
Smith A. M., Wolfe K. & Byrne M., 2012. Argonauta at risk: dissolution and carbonate mineralogy of egg cases. Proceedings of the 12th International Coral Reef Symposium, Cairns, Australia, 9-13 July 2012. 8D Effects of ocean acidification. 5 pp. Article.