Monday, 17 October 2016

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.

[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.

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