Sometimes as you walk along the beach you come across seashells with a kind of hair-like growth attached. These tangled clumps are actually the remaining hard-parts of colonies of a type of organism known as a Hydroid – from the class Hydrozoa. Hydrozoa belong to the larger grouping or phylum of marine invertebrate animals called Cnidaria – which includes the jellyfish and comb-jellies. Unlike these more familiar free-floating animals, Hydroids are mostly sessile – they stay put where they are attached. Another name for Hydroids is Sea Firs.
In this post you can see an example of Hydroids growing like some sort of plant on an empty mussel shell. The fine and delicate ‘stalks’ of these Hydroids are composed of an outer or exoskeleton, made up of a branching tube of chitinous material termed the perisarc. The tube of tissue inside the perisarc is called the coenosarc. Most Hydroids have a basal portion or hydrorhiza which can be mat-like (stolonal) or a fibrous mass of tubes. The colonies also have an upright stem which is usually branched.
The main stem, or hydrocaulus, may divide into side branches called hydrocladia. When these are a single tube, as shown in the photographs in this post, the colony is termed monosiphonic or simple. Along the length of the stem are small cup-like structures – the hydrothecae – that protect the soft tentacled feeding polyps or hydranths of the Hydroid animal. The polyps are a continuation of the coenosarc, and their guts are continuous with it.
Different species of Hydroid are distinguished from each other by such details of the colony structure as the overall size, the way they attach to a substrate, whether the stems are a single or multiple tubes, and the shape of the perisarcal cups or hydrotheca. The identification to species involves many other features and, if you would like to find out more about this, a good reference book is:
Hayward P. J. & Ryland J.S. (1995) Handbook of the Marine fauna of North-West Europe, Oxford University Press, ISBN 0 19 854055 8, pp 70 – 112.
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