Chorda filum: Interesting sealife from the beach clean

September 23rd, 2019

During our beach clean on Sunday (22nd September) litterpickers found a number of rather strange cord-like objects in the strand line. The objects were generally translucent white, and resembled silicone rubber beading, of the type you might use to seal around the bath or kitchen sink. I brought a sample back home for closer investigation; it was 2.5mm in diameter, and held between tweezers, could be readily stretched by 20 of its resting length (a test section extended readily from 10 to 12cm), and recover apparently undamaged – so, rubber?

The mystery was quickly solved under the microscope, where the cellular structure of the material was evident. The samples were of Chorda filum (I call it sea-whip when I see it diving – for obvious reasons, see the photo below- but I think its common name is actually ‘sea lace’). I couldn’t find any reports of the histology of Chorda filum on the web, so I present a quick report into what might be the rubberiest plant on the planet below the photo!

Chorda filum or sea lace photographed off the south end of Gigha by Barry Kaye June 2018.
Above: Chorda filum living in shallow water of the south coast of Gigha: The fronds can be extended by 20% of their resting length – is this the rubberiest plant on the planet? (Photo BK)

Histology of Chorda filum

Generally seaweeds have very simple internal structures. Microscopy might reveal a gelatinous/slimy outer layer secreted by an organised skin or dermis, but there is rarely much internal structure to speak of. Seaweeds don’t need to transport water and salts from roots to leaves, as they are continually bathed in seawater, they can rely on diffusion for most transport requirements, so generally they lack the complex vasculature we see in higher plants.

Chorda filum, however, shows a very clever internal architecture; a central lumen stretches up the centre of the entire filament that constitutes the plant’s body. The lumen is surrounded by four or five layers of large box-like cells. These cells are at an angle to the axis of the filament – so they coil like a spring down the plant. This almost certainly contributes to the plants amazing elasticity, though I would not be surprised if there were not further mechanisms at the molecular level.

A thick transverse section through the stem to show the pitch of the box cells.

The box-like cells showed no internal structure in the sample I had from the strand line, but in places there was evidence of a further layer of cone-like cells attached by their apex to the outside of the tube of box-cells. These cells had clear chloroplasts in the wider end, suggesting that photosynthetic activity had been an important role in these cells while the plant was alive. I confess that I don’t understand why these cells are only attached at their apices, but this again might be to allow movement required as the plant is stretched and relaxes as each wave passes over it.

Longitudinal section showing the oddly shaped surface cells with narrow attachments to the main plant, and chloroplasts packed into the wider outermost part of the cell.

In conclusion; many seaweeds live in extreme environments. Chorda filum seems to have evolved a particularly interesting way of coping with the mechanical stresses of wave motion, and this may be one of the factors that permit it to colonise seabeds that lack good points of anchorage.

By Barry Kaye

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