Saddle Oysters and more ocean wanderers at Stoer

March 12th 2020

Saddle Oysters and more ocean wanderers at Stoer

The beach clean at Stoer Bay (NC9328) on 24th January 2020 yielded a skipful of plastic and other waste weighing over 200 kg (see the AFC Facebook page for that day).  It also produced more ocean wanderers (see story posted on January 25th).

Jane Mallalieu was to the first to spot some more examples of the rough goose barnacle Lepas pectinata.  I later picked up parts of a large black plastic flowerpot, which bore a variety of organisms (photo 1).  There were two kinds of goose barnacles, both the rough and the larger common goose barnacle Lepas anatifera.

The plastic had also been acting as a floating base for at least three other creatures.

There were the occasional calcareous tubes of keelworms (Pomatoceros sp.), one of many bristleworms (polychaete annelids) found in the sea.

More conspicuous were clusters of tiny, almost translucent, shells of juvenile saddle oysters These are bivalve molluscs with a shallow domed upper shell which is anchored to the substrate by byssus threads through a hole in the flattened lower shell.  I am grateful to David Haines for the stacked close-up photos of these rather unusual creatures (photos 2 and 3).

Some four species of saddle oysters occur around British coasts, adhering to rocks, the stems of kelp and floating objects; the adults can attain a diameter of 6cm and are quite frequently found along the tide-line (photo 4).  They are best distinguished by the number and arrangement of the internal muscle scars, but the Stoer specimens were too young for these to be visible. They presumably develop from planktonic larvae, but I have not yet found an account of their life-cycle.

Snaking across the surface of the flowerpot were the remains of a third organism, a colonial hydroid (hydrozoan coelenterate), probably Obelia geniculata (photo 5).  This ‘forms delicate white colonies which rise up from linear creeping stolons which cover the surface like a spider’s web’.  The upright zigzag stems bear bell-shaped polyps on short branches, which generate, by budding, free-swimming medusae (like tiny jellyfishes), which in their turn produce planktonic larvae. These disperse and then settle down to produce the static polyps; a complicated, but effective life cycle.

Ian M. Evans

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