based on the picture alone, i guessed sea cucumber (holothuroidea) a relative of the sea urchins, when i read on i saw you said they were floating above the sea bed, something i have never read about before, i thought that all sea cucumbers were benthic detritovores (bottomfeeders). i looked it up and seemingly you do get pelagic (above the sea floor) species, i couldn't find a picture of the pelagic species but the following link is a pretty good match for your picture. did they use their famous defensive technique when you handled them? they squirt out large amounts of sticky white fluid from their rear ends, looks quite amusing...

http://www.fortunecookiechronicles.com/wp-content/uploads/2007/12/seacucumber.jpg

I agree that that they look like Holothurians, but I have never seen that species before. If anyone knows them and knows anything about their habits and biology, please let us kinow.  For example, do they mature into benthic forms?

No doubt in this pelagic phase they feed on plankton, though probably largely not the same species that they would eat once they settle (if they really do; I am only guessing), and I would guess that this pelagic phase is just a dispersal phase before they settle down to feed and breed in earnest.

I can't wait for an expert!

Cheers,

Jon

As I was reading your description of the mystery item, I could not help but be reminded of the description of a simple sea sponge I read in my high school biology class workbook many years ago. It has only one orifice at the top through which food drifts in and the remains drift back out. The other end is usually attached to a rock or some other item under water. Possibly a recent storm was enough to dislodge them. Or this might even be a certain stage in there life cycle when they set themselves loose to drift, their only means of locomotion when nearby edible material is hard to come by.  

Dear Philippe, I am very excited at finally having the chance (and knowledge!) to answer a ‘Last Word’ question!

I think the creature you have described is the thaliacean Pyrosoma atlanticum. I agree that it does look like a sea cucumber (holothurian) at first glance, 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. I had put your question to the back of my mind until I started researching gelatinous zooplankton for a lecture I am preparing, which generated a eureka moment!

Despite having a similar gelatinous constitution, thaliaceans are not directly related to jellyfish, which explains why they do not contain cnidocytes (stinging cells) that create the nasty sting experienced with some jellyfish species. The thaliacean Pyrosoma atlanticum is a colonial species made up of zooids gelled together in a gelatinous tunic, which gives them a ‘bumpy’ appearance. They can be pink, as seen in the picture and have on hole at the end of the ‘tube’. They filter feed on phytoplankton by drawing water through each individual zooid from the outside of the tube to the inside. Pyrosoma atlanticum is a warm-water species and often found in swarms, as you describe. 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

The tubular object is a pyrosoma. These are colonial tunicates, related to sea squirts, salps and doliolids. 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's outer surface is rough because each zooid projects out of it to feed.

Each zooid is a filter feeder, pumping seawater by ciliary action from outside the tube and passing it through a pharyngeal branchial "basket", 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's 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 "fiery body" 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's 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 leatherback turtle (Dermochelys coriacea), still eats enormous quantities, probably hunting them at night.

John Davenport, Professor of Zoology, University College Cork, Ireland