Laboratory-reared embryos of the Pharaoh cuttlefish (Sepia pharaonis) developed poorly, formed smaller and paler ink sacs, and showed sharply altered expression of melanin-synthesis genes when their seawater was acidified, according to a new study published in Frontiers in Physiology. The research, conducted at Lingnan Normal University in Zhanjiang, China, exposed cuttlefish eggs to four pH levels — 8.0 as a control, plus 7.7, 7.3 and 6.5 — and tracked their growth, survival, organ formation and gene activity day by day. The results point to the embryonic stage as a particularly sensitive window in the life of a cephalopod, an animal whose adults are known to buffer external pH changes reasonably well.
The experimental design was straightforward but demanding. Live egg masses were collected from coastal waters off Zhanjiang in April 2021 and pre-cultured for three days at 28 degrees Celsius and a salinity of 23 parts per thousand to synchronize developmental stages. Eggs from 15 different females were mixed and evenly distributed across four treatment systems, each with three replicate tanks, so that every group contained offspring from all parental sources. Acidified seawater was produced by equilibrating filtered water with a high-purity carbon dioxide and air mixture, with pH adjusted solely by regulating gas flow rather than adding chemical acids. Water chemistry was monitored daily, and the carbonate system parameters were calculated with CO2SYS software. The treatments spanned a wide chemical range: mean pCO2 rose from about 415 microatmospheres in the control to roughly 1,350 at pH 7.7, 3,150 at pH 7.3 and 17,500 at pH 6.5, while the aragonite saturation state fell from 3.75 to 0.15 across the same gradient.
The developmental consequences followed the severity of acidification. On day one, embryos in all treatments looked alike, corresponding roughly to stage 17 of cuttlefish development, with a large transparent yolk sac and a well-defined mantle. By day four, divergence appeared: control embryos and those at pH 7.7 and 7.3 progressed normally, showing mantle expression, yolk absorption and the appearance of chromatophores, while embryos at pH 6.5 showed clear retardation, no visible chromatophores and a persistently large yolk sac. By day seven the gap had widened further, and by day nine the pH 6.5 group had suffered complete mortality, with yolk sac degeneration and tissue deterioration. Control embryos hatched by day 11. Embryos at pH 7.3 remained alive until that point but were markedly developmentally delayed and suffered increased mortality, while the pH 7.7 group showed pronounced developmental delay and failed to hatch, with data collection ending on day nine.
Bioengineer.org, 29 October 2026. Press release.
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