Marine stonemasons

If I mention the word “worms”, most people think of earthworms and many view them with distaste. There are several species, all looking rather similar, and we don’t find them attractive because they are slimy and often live in decomposing organic matter. However, it is not unknown for infants to try eating them, so our dislike is something we learned from adults or older children.

We are less familiar with marine worms, and there are many different forms. The white calcareous tubes you see on rocks, or the small spiral tubes found on seaweeds like wracks, are secreted by worms and there is every likelihood that the worm is still resident when you see their ”home” at low tide. Marine worms of many species live in tubes and, in addition to those made from secreted calcium salts, these may be poorly consolidated – as with the lugworms beloved of sea anglers – or constructed of grains cemented together to prevent abrasion by moving sand grains. However, these tubes offer little protection against predation by wading birds in shallow water.

The mason worm (Lanice) is widely distributed and extends from the inter-tidal down to 1900 m [1] and may occur in very high densities (up to 20,000 individuals per square metre [2]). Lanice makes tubes that extend above the surface of the sandy mud in which the worms live and each tube has extensions at its tip. Where present as dense reefs, Lanice tubes promote sedimentation of fine mineral and organic particles and these sediments increase biodiversity [3]; the worms being referred to as “ecosystem engineers”. Close examination shows the tube and extensions to be made of sand grains and shell fragments cemented together by a secretion of the worm - thus the term mason worm (see above in a wonderful image captured by Jim Greenfield).

Lanice is in the group of worms known as terebellids and they feed when the worm and its tube are covered by water, so feeding can best be observed when worms are transplanted to an aquarium tank. The tentacles at the front of the body (see above) are extensible and very mobile and, if we  look at them under a microscope, we see many hundreds of thousands of beating hairs (cilia) over their surface and also a covering of mucus, produced from cells within the tissues of the tentacles [4]. Algae and detritus become attached to the tentacles when they are spread on to the substratum and the cilia then carry the mucus-bound “packages” to the mouth where they are ingested. Waste products are removed by the currents of water that the worm generates by moving its body within the tube.

Lanice also feeds by spreading the tentacles over the “fan” of extensions constructed at the top of its tube, collecting particles from the currents that result from wave action. We know that the particles carried in suspension contain micro-aggregates formed by bubbles created when waves break [5] and these, too, form part of the food for the worms, together with anything else that becomes swept up.

How do the worms locate their habitat? The answer is that it is largely a matter of chance. After reproduction (there are separate male and female worms [6]), larvae become planktonic and are carried around in the water column by currents and by their own swimming by means of the ciliated bands on their body (a second use of cilia for larvae as they also use these organelles to gather food). The large majority of planktonic larvae are eaten, or fail to reach a suitable substratum, but, when they do, each larva swims down and begins to transform into a small worm and begin their work as “masons”.

Far from feeling distaste at the sight of these worms, I marvel at their biology and how they evolved their form and habits. The sense of wonder is one of the pleasures of Natural History and the never-ending fascination of looking at living creatures.

[1] R. M. S. Alves, C. Van Colen, M. Vincx, J. Vanaverbeke, B. De Smet, J.-M. Guarini, M. Rabaut and T.J. Bouma (2017) A case study on the growth of Lanice conchilega (Pallas, 1766) aggregations and their ecosystem engineering impact on sedimentary processes. Journal of Experimental Marine Biology and Ecology 489: 15-23.

[2] A. Nicolaidou (2003) Observations on the re-establishment and tube construction by adults of the polychaete Lanice conchilega. Journal of the Marine Biological Association of the United Kingdom 83: 1223-1224.

[3] B. De Smet, A.-S. D’Hondt, P. Verhelst, J. Fournier, L. Godet, N. Desroy, M. Rabaut, M. Vincx and J. Vanaverbeke (2015) Biogenic reefs affect multiple components of intertidal soft-bottom benthic assemblages: the Lanice conchilega case study. Estuarine, Coastal and Shelf Science 152: 44-55.

[4] R. P. Dales (1955) Feeding and digestion in terebellid polychaetes. Journal of the Marine Biological Association of the United Kingdom 34: 55-79.

[5] R. S. Wotton (1996) Colloids, bubbles and aggregates: a perspective on their role in suspension feeding. Journal of the North American Benthological Society 15: 127-135.

[6] https://www.marlin.ac.uk/species/detail/1642

500-year-old clams?

We can all name several National Parks, and many other designated conservation areas, in our various countries. It's a fair bet that all of those named will be terrestrial, as marine conservation areas are much less familiar to us – not surprising as many are rather difficult to visit.

The Faroe-Shetland Sponge Belt MPA (Marine Protected Area) [1] is, as its name suggests, a region of ocean bed rich in sponges and it also has good populations of ocean quahogs (Arctica islandica), a type of clam (see above) that is collected by dredging in some areas. They are not overfished and may be sold as mahogany clams for steaming, or used in the preparation of canned chowders and other products [2]. Like all clams, they have two shell valves that are held closed by the contraction of adductor muscles and gape when the muscles are relaxed, the valves being forced apart by the elastic hinge. When open, the animal feeds by drawing in a current of water through an inhalant siphon and this passes over the gills before exiting through an exhalant siphon. In addition to their use in respiration, the gills also trap particles and these are conveyed to the mouth in mucus-bound packages. Particles identified as being of food value are ingested while those that are not are rejected into the exhalant current. Sorting occurs on organs called labial palps and the whole collecting + sorting mechanism is sophisticated and driven by tiny cilia on the surface of the gill and on the palps. The millions of cilia on the gill create the respiratory currents.

There are male and female clams and reproduction occurs after sperm and eggs are passed out in the currents of water passing from the exhalant siphons. Fertilisation occurs in the water column and there is then a larval stage that lasts between 30 and 60 days [3]. In this time the tiny larvae feed on minute particles and swim by means of cilia on their body surface, undergoing a transformation within their larval life to a different form that has developing shell valves. The larva moves down in the water column and, if a suitable location is found, it settles and develops into the form with which we are most familiar.

Like most bivalves, ocean quahogs are sedentary and they bury themselves just below the surface, with the siphons protruding. On occasions they burrow down further and then stay, with the shell valves closed, for between 1 and 24 days [3]. No-one can explain this behaviour and it is accompanied by a depression of the rate of metabolism, a trait that enables some unrelated freshwater bivalves to remain closed up for two years.

The fame of ocean quahogs comes in the age achieved by some specimens. After measurement of the growth rings on the shell, one clam was found to be > 400 years old [4]; a more detailed analysis extending this to > 500 years [5]. This is of interest to those studying aging [6] and it has been suggested that ocean quahogs may provide good models for the study of the aging of tissues in humans. We cannot resist being anthropocentric.

It is the life history of ocean quahogs that fascinates Natural Historians. The chances of fertilisation are good if sperm come into contact with ova, but there is the risk that gametes, or young embryos, might be drawn in by feeding currents of nearby quahogs and become captured on their gills. Next come the risks of larval life and metamorphosis, with predators of many kinds feeding on the larvae. If they survive, each must then find a place to settle; by no means easy if the planktonic larva has been carried on currents to areas that have unsuitable substrata. There is then the challenge of sinking, and swimming, down through the water column and beginning the process of turning into an adult. If there are many settlers, competition for available space will result and only once this is overcome can the growth of the adult animal begin. 

Ocean quahogs can thus live for seconds, for hundreds of years, or any time in between these limits. When you have steamed clams, or open a can of clam chowder, give a thought to the age of the animals that you are eating and the miniscule chance they had of becoming adults and of reproducing. It might also be that some of them were living on the sea bed before you were born.

[1] http://jncc.defra.gov.uk/page-6479

[2] http://www.seagrant.umaine.edu/maine-seafood-guide/hard-clams

[3] I. D. Ridgway and C. A. Richardson (2011) Arctica islandica: the longest lived non colonial animal known to science. Reviews in Fish Biology and Fisheries 21: 297-310

[4] Alan D. Wanamaker Jr., Jan Heinemeier, James D. Scourse, Christopher A. Richardson, Paul G. Butler, Jón Eiriksson and Karen Luise Knudsen (2008) Very long-lived mollusks confirm 17th Century AD tephra-based radiocarbon reservoir ages for North Icelandic Shelf waters. Radiocarbon 50: 399-412.

[5] Paul J. Butler, Alan D Wanamaker Jr., james D. Scourse, Christopher A. Richardson and David J. Reynolds  (2013) Variability of marine climate on the North Icelandic Shelf in a 1357-year proxy archive based on growth increments in the bivalve Arctica islandica. Palaeogeography, Palaeoclimatology, Palaeoecology 373: 141-151.

[6] Danuta Sosnowska, Chris Richardson, William E. Sonntag, Anna Csiszar, Zoltan Ungvari and Iain Ridgway (2014) A heart that beats for 500 years: age-related changes in cardiac proteasome activity, oxidative protein damage and expression of heat shock proteins, inflammatory factors, and mitochondrial complexes in Arctica islandica, the longest-lived noncolonial animal. Journal of Gerontology: Biological Sciences 69: 1448-1461.