By-the-wind Sailors (Velella velella Linnaeus) have been washing ashore at Rhossili Beach over the past week. The sandy strandline has been dotted with dozens of the dark blue jellyfish-like creatures with their characteristic translucent sails. They are not actually jellyfish but pelagic (free-floating) colonial hydroids. [See also the earlier posts about this marine invertebrate in Sailing by Rhossili Bay and Bright Blue Blobs on the Beach.]
Beach stones with holes in them excite the curiosity of most people. How did the holes get into the rock? There is no single answer but in many cases the holes in pebbles and beach stones have been made by various seashore creatures including several types of bivalve molluscs, marine worms, and sponges. The same creatures can also make holes in thick old seashells. There are several earlier posts on Jessica’s Nature Blog describing how the holes are made, by piddocks, for example Pholas dactylus, sponges such as Cliona celata, and polychaete worms like Polydora ciliata and Polydora hoplura. Frequently, there is evidence for more than one type of organism occupying the same stone.
One of the bivalve molluscs that creates holes in stones and thick oyster shells is the Wrinkled Rock Borer Hiatella arctica (Linnaeus). Like the piddock, this species can actively excavate a burrow in soft stone for shelter and protection although unlike the piddock it can attach itself by byssus threads to the outside of solid objects or in cracks and crevices. However, once embedded in the stone it can no longer exit the burrow but obtains all it needs for sustaining life via the tunnel connecting it to the outside world. Wrinkled Rock Borers are smaller than piddocks, measuring no more than 3.8 cms in length when mature. The valves of the shell are thick and robust with distinct furrows, and the leading edges exposed to view in the burrow are rough and straight edged (truncate). Tebble (1966) says that it is not possible to distinguish between the different species of Hiatella in British waters but the descriptions apply to all the species ever recorded here. Hiatella arctica is common around the British Isles from the lower regions of the shore to considerable depths…… It has a wide geographical distribution in the northern hemisphere from the Arctic south through the Atlantic, Mediterranean and Pacific, but the particular limits of its occurrence are not known. It is almost impossible to remove empty Hiatella shells from the excavated holes without breaking them.
Hiatella holes in rock can be secondarily occupied by a similar but smaller bivalved mollusc called Irus irus (Linnaeus). This grows to about 2.5 cms in length. The protruding frill-like concentric ridges on the shell can be very distorted in shape if the shell is occupying a burrow that is too small to allow normal growth. I am not able to discount the possibility that it is Irus shells occupying Hiatella burrows in some of the beach stones illustrated here. Irus (also known as Notirus irus) occurs from low in the littoral zone to a few fathoms.
Tebble, N (1966) British Bivalve Shells: A Handbook for Identification, published for the Royal Scottish Museum by HMSO, Second Edition 1976, [Hiatella p172-173 & Plate 7h; Notirus p124-125 & Plate 7g].
Hunter, W. R. (1949), The Structure and Behaviour of ‘Hiatella gallicana@ (lamarrck) and ‘H. arctica’ (L.), with special reference to the Boring Habit. Proc. Roy. Soc. Edin. B, 63 III (19): 271-289, 12 figs.
A small sand-covered Pennant’s Swimming Crab, about 2 cm across, emerging from its burrow in the wet sediments at low tide on Rhossili Beach one June.
For more details of this little seashore creature, Latin name Portumnus latipes (Pennant), see the earlier post.
On the beach at Fermoyle on the Dingle Peninsula in Ireland, the sea sifts and sorts the sand grains into different weights and colours to arrange in sinuous ripple patterns across the shore. The red particles from nearby outcrops of Devonian rock make contrasting curved lines along the receding water’s edge. At low tide level, the wet sand has been punctured hundreds of thousands of times by the beaks of shore birds both large and small. I have never noticed that before. There must be something about the texture of the sediment there that preserves the shape of the bills when they are withdrawn from the spot during feeding activities. It is plain that this particular place must have a very rich in-fauna of small seashore creatures like crustaceans, worms, and maybe molluscs.
Following the line of the limestone cliffs towards Kitchen Corner as the tide receded, the tide pools and beach were littered with dozens of living Rayed Trough Shells (Mactra stultorum Linnaeus) as they popped up to the surface of the sand. I don’t quite know why they chose to do this but it afforded an opportunity to see the living animal as opposed to the dead ones and empty shells that wash up more frequently on Rhossili Beach.
Two pale fleshy tubes joined together were extended between the two hinged shell valves. One inhalent siphon for sucking water with suspended nutrients inwards, and one exhalent siphon for dispelling de-oxygenated water with bodily waste products. I was afraid that these bivalved molluscs would die while gaping and exposed to the air, so I picked up a few and put them in the water of the pools but they were not very lively and did not re-bury themselves. I was surprised that no-one else seemed to notice them. Even the dog that I saw appeared more interested in splashing in the pools than snacking on the free harvest.
You don’t exactly have to keep your nose to the ground to see them but you do have to be a keen observer to notice all the different tracks and trails left on the soft wet sediments of the beach at low tide. Larger marks left by people and vehicles are the first ones you see. Bird footprints are every where. The birds are feeding on all sorts of invertebrate seashore creatures like worms, small crustacea and molluscs – all of which leave holes, burrows and furrows as they move in and out of the sand and across the surface. Some of the pictures shown here simply aim to give the general context for the area of Whiteford Sands that I was walking across. If you look closely the other images, you will see not only the ripples in the sand but also the intricate network of traces left by the virtually invisible organisms that inhabit this ecosystem. The larger furrows in photos 1, 12 and 13 are made by the common winkle (Littorina littorea Linnaeus). I cannot name each animal that is responsible for each of the other types of trace. However, I am sure that there will be some specialists out there who could, especially those researchers concerned with the interpretation of trace fossils (the ichnologists).
Click images to view full size.
At low tide many thousands of common winkles or periwinkles (Littorina littorea Linnaeus) seek shelter from dessication and predation by clustering together in the few hiding places available on the beach. At Whiteford Sands these niches include the overhung bases of larger stones, crevices in ancient timbers from the rapidly emerging submerged forest, and nooks and crannies in the recently exposed ancient peat. Alternating layers of peat and clay, overlain by rocks from glacial till, provide algae-covered surfaces on which gastropods can feed, and islands of low tide refuge in the vast expanses of sand on this sea shore.