Acantharians plumb the depths to aid reproduction

Posted By Ian Randall on 9 March 2010

The reproductive cysts of acantharians sink seasonally from surface levels of the ocean to deeper waters, oceanographers at the University of Southampton believe. These oft-overlooked single-celled radiolaria, which have mineral skeletons, are believed to behave this way to exploit an annual increase in food.

The acantharian, Stauracantha quadrifurca. Image credit - Ernst Haeckel

Acantharians spend their adult lives in the top 300 metres of the oceans – at these depths, sunlight is able to reach the symbiotic algae contained within each, enabling photosynthesis. To reproduce, cysts are dropped down the water column to release gametes – the resulting juveniles then ascend to the surface while they mature. Despite their relative anonymity, these creatures are found in abundance across most of the planet’s oceans.

“Although acantharians are known to contribute to organic matter transport at shallower depths, we were amazed to discover a high flux of their spore-like reproductive cysts in the deep ocean,” said PhD student Patrick Martin, from Southampton’s School of Ocean and Earth Science.

Cysts were discovered in sediment traps left 2000 metres down in the Atlantic’s Iceland Basin in 2006 – acantharians form skeletal material from a crystalline form of strontium sulphate (celesite); being the densest of all biominerals, the larger celesite cysts (greater than 1 millimetre in length) are capable of sinking to great depths before dissolving.

Acantharian cysts were found to fall at the highest rates in the months of April and May at these latitudes – it is believed that this is a pattern which can be seen each year. It is thought this increase in flux is timed to coincide with the spring phytoplankton bloom – when these dominant algae sink to the sea floor, it is thought they might find juvenile acantharians waiting.

Martin adds: “We speculate that this is part of a reproductive strategy allowing juveniles to feed off the remains of phytoplankton, ‘phytodetritus’, that rapidly sinks to the seafloor following the spring bloom.”


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