A seaweed, iSpot and a microscope

At the beginning of September we were down in Devon. Unlike the rest of the country we had quite nice weather, and one day it was so good we went to the beach. Start Point is the most south-easterly promontory sticking out of the Devon coast and is miles from anywhere, but has interesting geology (mica schist and hornblende schist[1]) and an amazing collection of lichens, plants and much else.

Though part of our visit was spent clambering up the rocks and watching seals, eventually we skirted around the point and headed to the beach at Mattiscombe. It is a gorgeous little sandy cove, and when the sun shines it is completely delightful. Even better, it was low tide, so I couldn’t resist the chance to potter around looking in the pools.

I didn’t have tons of time, and invertebrates were few and far between, though it was useful to see a dead Velvet Swimming Crab (Necora puber) so I could have a close look at it and its distinctive rear legs without it attacking me.

Picture of Velvet Swimming Crab (Necora puber)

Velvet Swimming Crab (Necora puber)

Other than that I mostly just looked at the seaweeds. This still feels like a difficult group, though I’ve begun to make some progress with a few books and some focussed help from the marine specialists on iSpot. I’m still working through some of the specimens I brought back, but this one particularly caught my eye as I’d never seen anything like it before.

Picture of a Codium

A Codium, but which?

Flicking through the book (the great Seaweeds of Britain and Ireland by Francis Bunker et al.) it was obvious that it was a Codium, and that they have a western distribution, which is why I’d not seen any on the Sussex coast. However, it was also clear that there are several species that are macroscopically very similar, so I realised my chances of completely identifying it were slim.

Nevertheless, the suggestion on iSpot that I need to look at it microscopically goaded me to finally get round to buying the microscope that I’ve been promising myself, and to which several members of my family contributed for my birthday over a year ago.

So, a couple of weeks ago I took delivery of a lovely Brunel SP28, cut the tip off my rehydrated Codium, squashed it under a slide and had a look. Inside, the apices of the filaments (utricles) are a key feature in determining the species. The more common species C. fragile has little drawn-out points on the utricles, but mine were a different shape completely.

Picture of Codium utricles

Codium utricles

Using the green seaweed book[2], Derek on iSpot suggested that it looks most like the native C. tomentosum, though the book suggests that hair scars should be common on the utricles, and I’ve not been able to see any on my sample.

Furthermore, the position of the hair scars appears to be important in differentiating between C. tomentosum and C. vermilara. So, what other characters are there? The size of the holdfast is larger in tomentosum (> 2 cm) and considerably smaller in the other species. However, I didn’t check it! Finally, the other feature in the key notes that the utricles of tomentosum are broad-domed, whereas those of vermilara are flat-topped, though judging from the photographs and drawings in the book it is a bit subtle. Nevertheless, the utricles in my sample are pretty well domed, so I’d suggest that this is probably Codium tomentosum.

Map of the distribution of Codium tomentosum

Map of the distribution of Codium tomentosum

[1] Popham, Chris and Terry Cox. “The Start Complex around Prawle Point in South Devon.” Open University Geological Society, 23 June 2013. Web. 27 September 2015. <http://ougs.org/trip_resumes/article.php?id=283&&branchcode=swe>

[2] Brodie, Juliet, Christine A. Maggs and David M. John, eds Green Seaweeds of Britain and Ireland. British Phycological Society, 2007.

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