1 October 2009

Botrylloides and other tunicates I could not remember...

Colonies of Botrylloides diegensis with hundreds of skeleton shrimps (Caprella spec.)

Having a blog and my last post was 3 march 2009... It's about bloody time for an update!

This post is about tunicates, also called sea squirts or ascidians. As strange as it may seem, these animals are chordates: in their short larval state they have a dorsal nerve cord that disappears when they settle on substrate. For more information about their anatomy, ecology etc. see the book and website reference at the end of this post.

At the age of 16, I bought 'The Hamlyn Guide to the Seashore and Shallow Seas of Britain and Europe' (Campbell and Nicholls, 1976). By then it was the first good illustrated and comprehensive fieldguide. I still recognise a lot of animals and seaweeds just from the illustrations of this book; they are etched in my brain. But now at the age of 49 I have problems remembering new species, because they are not in the Hamlyn Guide. And there are a lot of new species invading our Dutch waters.

As you can imagine, my copy is dirty, stained and swollen of all the times I browsed it.

So I forced myself to give a lecture about tunicates for our local 'Strandwerkgroep Waterweg Noord'. Now I do recognize a lot of the ascidians! And I hope Marco Faasse (a very enthusiastic friend who devotes - I think - all his leisure time to marine biology) will no longer be annoyed by my ignorance...

On my blog follows a summary of the more common Dutch ascidians. I start with a few colonial species: Botryllus and Botrylloides.

golden star tunicate (NL: gesterde geleikorst)

All kinds of colour and form of Botryllus growing on a turned stone at Neeltje Jans, North Sea. The golden star tunicate is a species I recognize easily, because... it is described and depicted in the Hamlyn Guide :-)

This ascidian is one of my favourites because of its colour and the star like arrangement of the zooids (the individual embedded animals).

Botryllus growing on sea-lettuce (Ulva spec.).

BOTRYLLOIDES VIOLACEUS (NL: gewone slingerzakpijp)

This colonial ascidian is a more recent invader. It is easy to distinguish from Botrylloides diegensis (see the next species) because it is always in one colour. On the web I found an English name: orange sheath tunicate. A confusing name as you will see in the next photographs.

B. violaceus growing on another tunicate: Styela clava.

The pink and orange ascidians are B. violaceus, the skin coloured one is Didemnum vexillum.

chain sea squirt (NL: tweekleurige slingerzakpijp)

It looks a lot like B. violaceus but fortunately it is always two coloured. Fortunately? It is for a lazy marine biologist like me: I don't have to collect and dissect them under a microscope.

Like lava form an erupting volcano.

Some invaders I really love!

Photographs taken at Wemeldinge (Oosterschelde), Neeltje Jans (North Sea) and 't Koepeltje (Grevelingen).

Book reference:
  • R.H. Millar, 1970
  • P.J. Hayward and J.S. Ryland, 1995
  • A.C. Campbell and J. Nicholls, 1976
  • Cryptosula: photographs of marine flora and fauna by Marco Faasse
  • Anemoon: photographs and descriptions of Dutch marine flora and fauna

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