Hydroids on Seashells

Hydroids on Seashells 1 - Branching growth of plant-like Hydroid colonial organisms attached to a mussel shell. P1260107aBlog1

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.

Hydroids on Seashells 2 -  The upright stems or hydrocauli of a hydroid attached by the basal stolon to a mussel shell. P1260129aBlog2

Hydroids on Seashells 3 - The upright stems or hydrocauli of a hydroid with side branches or hydrocladia - both showing perisarcal cups or hydrothecae bearing the feeding polyps or hydranths. P1260138aBlog3

Hydroids on Seashells 4 - close-up of stems of a monosiphonic Hydroid colony, showing smaller hydrothacae on opposite sides of the hydrocaulus and hydrocladia - also showing larger empty gonothecae that enclose the medusae-producing blastostyles. P1260117aBlog4

Hydroids on Seashells 5 - Holding a mussel shell on the beach which has a large colony of Hydroids attached like a tuft of hair. P1160854aBlog5

Hydroids on Seashells 6 - A mussel shell on the beach which has a large colony of Hydroids attached like a matted tuft of coarse hair. P1160856aBlog6

Hydroids on Seashells 7 - Close-up detail of Hydroids growing on a mussel shell. P1160849aBlog7


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2 thoughts on “Hydroids on Seashells

  1. Very timely blog for me, as I found some a couple of days ago on a piece of kelp, at Gyllyvase beach in Falmouth – exactly like your 2nd and 3rd photos. I also found some odd pink, very thin, waving arms, could they be from an annelid worm or a burrowing brittle star – is there a way I could send you a photo?

  2. Yes, you can send me a photo if you like and I’ll do my best. You could e-mail the image to me at winderjssc@aol.com and I will see if I can recognise what type of seashore creature was waving its arms around. It sounds a bit like a marine worm. Funnily enough, I saw something answering the same description on Rhossili beach a couple of months ago. First time I’d seen anything like it.

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