A load of old Balanus

We're all familiar with barnacles, at least in broad terms - we see them on rocks, ships, whales etc, they turn up in A-level college biology projects, and if we slip on them, they are sharp and they hurt. However, how often do we really look at them?

A typical view of a barnacle-encrusted rocky shore (Start Point, south Devon, England)

They are of course crustaceans (of the class Cirripedia, order Thoracica) and come in three main forms, the suborders Lepadomorpha (the stalked 'goose' barnacles), Verruomorpha (like 'typical' barnacles but with only two opercular plates - I'll get on to that later), and Balanomorpha (the 'typical' barnacles'). I'm only going to focus on the latter, though I can't let the opportunity pass by without mentioning the existence of the order Rhizocephala. These are evolutionarily derived relatives of the barnacles and parasitise decapod crustaceans (such as crabs), their free-swimming juveniles settling on their hosts and developing into an 'interna' of root-like growths and an 'externa' which is a sac of reproductive parts. They lack as body as such, and their name means 'root head', and as with many internal parasites, they redirect their hosts' physiology and behaviour to their own ends. If I find any, I'll definitely write about them, but until then, here's a fine little 4-minute video introducing their life cycle through the medium of hand-drawn cut-out animation.

Anyhow, back to barnacles as we know them... The rocky shore above was well encrusted with barnacles of several species. There were many of the small limpet-shaped ones (such as Chthamalus stellatus in the family Chthamalidae), but what grabbed my attention were some larger, less flattened specimens typical of the the family Balanidae.

A barnacle of the family Balanidae.

As you can see, this barnacle is fairly tall (rather than being a low flat cone). Although the joins are rather obscured in this mature specimen, there are six plates forming the outer wall. It isn't clear from the photo, but the base forms a calcareous layer on the underlying rock (rather than a membranous layer) and hence this is in the genus Balanus. Identification past this point is a little tricky because the colours are so variable and corrode to a greyish mishmash in old specimens, but the obscure sutures, sharp 'beak' (tergum) and ribbed outer surface suggest either B. perforatus or B. balanus.

The scutum (seen here as a 'keel' beneath the sharply beaked tergum) is slightly saw-edged, and the smaller barnacles (which I assume are the same species) below the large one have yellowish rims with brown banding below/around them. Also, the opening in the large specimen isn't especially small. These features combine to suggest that this is B. balanus - a common species although south Devon is at the very SW extent of its range as it is absent from Cornwall, but is a place where this species has been regularly recorded. A closer look should also help clarify a couple of the features mentioned here.

The operculum, tergum and scutum of Balanus balanus

Aside from the smaller barnacles that are using it as a substrate, the beaked tergum is clearly visible as is the rough, serrated edge of the scutum. Both these structures are paired (you can see the join at the top of the tergum), forming four internal plates (collectively the 'operculum') that can move and open to allow the feeding structures (feathery 'cirri') to waft in the water and catch food particles.


So, a closer-than-usual look at a familiar organism - and a relatively rare (for me) foray into marine and littoral/intertidal habitats. Maybe I'll have to do more on this as it was the habitat that first grabbed my attention in terms of the invertebrate fauna...