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While swimming one morning across Coogee Bay in New South Wales, Australia, we came across thousands of these strange creatures (see photo) floating at depths down to about 2 metres. They were hard but also flexible, with water inside and a small hole at one end. Their length varied from about 3 to 30 centimetres and their walls were between 2 and 5 millimetres thick. Their skin was marked with many small protrusions, the size of which varied from one creature to another. Unlike jellyfish they appeared to be completely harmless. No one I鈥檝e spoken to from the area has ever come across anything like this. So what were they and why were they there?
鈥� The tubular object is a . These are colonial tunicates, related to sea squirts, . Each tube, or tunic, is a leathery gelatinous matrix that contains a number of individual tunicates, or zooids. One end of the tube is blocked and the other open but guarded by a controllable diaphragm. The inner surface of the tube is very smooth. In contrast, the tube鈥檚 outer surface is rough because each zooid projects out of it to feed.
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鈥淓ach pyrosoma is made up of numbers of individuals buried in a common, tubular, gelatinous matrix鈥�
Each zooid is a filter feeder, pumping seawater by ciliary action from outside the tube and passing it through a pharyngeal branchial 鈥渂asket鈥�, or gill, where planktonic food items and oxygen are extracted. The seawater is then expelled to the interior of the tube, before leaving via the tube鈥檚 open end. When all of the zooids in the tube are pumping water into the interior the pyrosoma moves by jet propulsion. Pyrosomas vary a great deal in length, from a few millimetres to 30 metres. They undertake vertical migrations, tending to be at the surface at night and at depth during the day.
The name pyrosoma translates as 鈥渇iery body鈥� and the colonies show intense, sustained bioluminescence when stimulated mechanically or by light. Each zooid has two light organs that contain luminescent bacteria. A lit-up colony can be seen from up to 100 metres away in clear waters in the middle of the night.
Bioluminescent light from one colony will stimulate another to flash. Light output is always preceded by the cessation of ciliary pumping, so the lightshow is generally believed to warn of poor food supply or of the presence of predators.
Pyrosomas are entirely harmless and occur in swarms in productive areas of the world鈥檚 oceans. As with jellyfish, swarms sometimes drift into coastal shallow water. They have another common feature with jellyfish: pyrosomas are 94 per cent water so represent a low-value diet. However, their best-known predator, the (Dermochelys coriacea), still eats enormous quantities, probably hunting them at night.
John Davenport, Professor of Zoology, University College Cork, Ireland
鈥� I had put this question to the back of my mind until I started researching gelatinous zooplankton for a lecture I am preparing. This generated a eureka moment.
I think the creature in the photo is the thaliacean Pyrosoma atlanticum. At first glance it looks like a sea cucumber, or , but the fact that many specimens were found in the water column, and your hint that the body is gelatinous (hard but flexible) gave me doubts.
Despite having a similar gelatinous constitution, thaliaceans are not directly related to jellyfish, which explains why they do not contain cnidocytes 鈥� the stinging cells that provide the nasty sting experienced from some jellyfish species.
P. atlanticum is a colonial species made up of zooids gelled together in a gelatinous tunic, which gives them a 鈥渂umpy鈥� appearance. They can be pink, as shown, and have one hole at the end of the tube. They are often found in swarms, as the questioner describes. These swarms sink rapidly when dead and have been found to accumulate in patches on the deep-sea floor creating an important source of fresh organic carbon for deep-sea animals.
Tania FitzGeorge-Balfour, Queen Mary University, London