Lug worm casts and blow holes were widespread over the low-tide sand at Pembroke Bay on the Channel Island of Guernsey when I visited in early October. They had a more scattered distribution and the casts were not so fine as those I have seen on other parts of the island at Rocquaine Bay and Cobo Bay. Two species of Arenicolidae have been recorded for Guernsey and I wonder if I have been looking at the burrows and traces of the two different types. Here on the beach at Pembroke Bay I think they could well be Arenicola marina (Linnaeus) whereas those I had photographed else where could be Arenicola ecaudata Johnston which prefers the rich mud between stones or in rock crevices at low water. Both types of cast are shown in the gallery below. Click to enlarge the images and see the descriptions.
Follow in my footsteps with a virtual walk along beautiful Rocquaine Bay on the west coast of the Channel Island of Guernsey. It is protected by a long sea defence wall which has employed different construction techniques along its length; mostly using local stone but also with along stretch of reinforced concrete (probably originating from German occupation World War II fortifications). The beach is both rocky and sandy with some pebble patches. Seaweeds of every colour abound. Huge limpets with white shells cluster on the bright orange-spattered L’Eree granite bedrock while outcrops of monochrome microgranodiorite occur on the upper shore near Fort Grey. Marine worm casts cover the softer muddy sands. Streams flow across the shore, their clear shallow water reflecting sunlight from the ripple crests and creating shadow patterns. A small stone jetty looks marooned among the rocks and a multi-coloured carpet of weed. Small boats bobbing in the turquoise water, rusty buoys and chains half-buried in seaweed, and algae-encrusted mooring ropes add to the evidence for fishing and leisure boating activities.
Click on the first picture to view the images in the gallery in the sequence that they were taken during the walk.
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.