Tuesday, 25 October 2016

Algae that glide

Microscopists in the 19th century were fascinated by frustules [1], the patterned siliceous coatings of diatoms, varying from one species to another (see below). When the algae die, frustules do not readily decompose and they fall to the sea, or lake, bed, joining those from diatoms that live there.

Experts arranged the frustules of different species on slides to produce kaleidoscopic patterns and this art form continues today in the wonderful work of Klaus Kemp [2] and others. However, it wasn't just the frustules that interested the many microscopists among the developing middle classes, as living diatoms were also observed, including those that move by gliding. This movement was described by Philip Henry Gosse [3] in one of his books that popularised Natural History and microscopy;

In some cases, as in the genus Bacillaria,.. ..this movement of sliding goes on till the frustules are on the point of separating, which then retrace their course till such a catastrophe seems equally imminent in the opposite direction.

We can see what Gosse means when we look at the video clip above and, for an explanation of these movements, he refers us to a paper by Wallich [4] who states:

The normal motion of the Diatomaceous frustule is in two opposite directions, which accord with its longest diameter. It is of a smooth, gliding nature, devoid of jerks or interruptions, and exhibits itself at tolerably regular intervals. The rate at which it travels is not uniform, being subject to variation on increase or diminution of light and warmth. The rate is also materially influenced by the condition of the endochrome, the motions being invariably more active and energetic when the frustule is full.

Wallich links the gliding movement to "elongated prehensile filaments", as particles are dragged along behind, or pushed in front of, the moving diatoms. We now know that motile diatoms, of which there are free-living as well as "colonial" types, move by exuding slime from what is termed a raphe. The slime produces a force against the substratum, or another diatom (should they be formed into a chain), that results in propulsion. At first sight, this might seem an expensive process, but the slime consists largely of carbohydrates that are generated in quantity during photosynthesis and these chemicals become much expanded when mixed with water. By exuding concentrated material through pores in the raphe, the rapid hydration then gives the observed propulsive force. The accurate observation by Wallich [4] that movement is associated with light and a "full" cell are evidence of an excess of carbohydrate from photosynthesis that can then be readily exuded.

When we observe gliding diatoms, we focus on the organisms, as we cannot see the slime that is produced, unless a chemical stain is used to show its presence. Interestingly, slimes from diatoms and many other organisms, from bacteria to mammals, have a most important role to play in the functioning of ecosystems [5].

For Henry Gosse, the beauty of diatom frustules, and the gliding movement shown by some of these algae, are further evidence of the wonder of God's Creation. For those of us that believe in evolution, there is a puzzle in thinking about how frustules developed through time and how the physiology of the algae evolved, allowing an excess of carbohydrates to be used as exudates that hydrate to produce a propulsive force. No-one would think that the variety of frustules evolved to please us and our approach to the puzzle is affected by our inability to understand the time over which evolution has occurred. Also, we don't know whether mutations resulted in a series of small changes or whether changes were on a larger scale. We can only speculate – and wonder.

[3] Philip Henry Gosse (1865) Land and Sea. London, James Nisbet & Co.

[4] G. C. Wallich (1860) Observations on the distribution and habits of the pelagic and freshwater free-floating Diatomaceae. The Annals and Magazine of Natural History 25: 1-20.

[5] Roger S. Wotton (2005) The essential role of exopolymers (EPS) in aquatic systems. Oceanography and Marine Biology: An Annual Review 42: 57-94.

The video of diatoms is by Donald Ott of the University of Akron and is available on YouTube at https://www.youtube.com/watch?v=NvHF-YjDZBo.

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.

Wednesday, 5 October 2016

Walking with Gosse – almost...

My annual "Coming up for Air" trip to Torbay [1] was in September this year. Fortunately, the weather was good, so I walked along the coast to re-visit places that were so important to me when I was growing up. Returning home to land-locked Hertfordshire, I then read what Philip Henry Gosse wrote in Land and Sea [2] about the parts of the Torbay shore that I had strolled through, and this made me appreciate Gosse's wonderful enthusiasm even more than usual.

I began by walking along the beach at Paignton and followed the strand line, always an interesting place to a Natural Historian. Unfortunately, there was little to see on this occasion and it was quite different to the strand line after a storm, when masses of algae are washed up. Like Gosse, I had always enjoyed exploring this evidence of the marine world of the bay, particular treasures being blue-rayed limpets attached to straps of brown algae, now torn from their anchorage. Of all the organisms I saw, I don't know why these limpets so appealed to me, but they are certainly attractive to the human eye (see below). 

In Land and Sea, Gosse describes their appearance:

The shell is of unimpeachable symmetry, polish, and delicacy; it is of a translucent horn-colour, and its summit is marked with three fine lines of the most brilliantly-gemmeous azure.

Not all the creatures washed up are so obviously attractive and, even fifty years ago, there was also much evidence of human pollution, with tar balls and pieces of net being common. Flotsam and jetsam are a rich source of natural materials, like wood, to the avid beachcomber, but there is now much more plastic refuse on beaches, as we continue to regard the sea as a convenient dumping place. I am impressed that artists like Jo Sayer can turn these plastic objects into attractive works of art that tell stories about what we are doing to Nature [4].

After the sands, I walked over Roundham Head and then on to Goodrington Sands (the sequence is shown in order in the photographs below; a route that Gosse walked in the opposite direction [2]). There was no promenade at Goodrington in the mid-nineteenth century and, while I took the promenade walk and cliff path on this visit, as a boy I preferred to scramble over the rocks when the tide was low, just as Gosse would have done.

 Gosse has a vivid description of some fisherman he encountered on Goodrington Sands:

..away across the heavy sands, in which we sink at every step, away obliquely to the left, where another bold headland, Roundham Head, breaks the sweep of the bay, and for the present shuts out Torquay from our view.

There is our working ground, at the foot of those red cliffs. We diverge a little from a straight line, and approach the edge of the sands, in order to see what those two men are so busy about, as they trudge along the water-line with stooping backs and downward gaze. Oh! they are fishermen taking solens, or razor-fish, as they call them. Each carries a light, narrow, but deep spade in his hand, and, as he marks a little jet of clear water that spirts upward from a small hole in the sand, he rapidly thrusts in his instrument, and adroitly jerks out his prey.. .. The man scarcely deigns it a glance, thinks nought of its curious structure, cares only for the halfpence it will bring him in the fish-market, jerks it into his basket, and watches for the next jet of water with which the frightened and retiring mollusc shall betray its place of retreat.

Razor clams (see below) are still a prized delicacy and they are certainly very effective at burrowing. Being a Natural Historian, Gosse was fascinated by this, but, as he pointed out, the clams were merely a commodity to the fisherman.

It was not only the wonders of Natural History that inspired Gosse, but the knowledge that all he saw was evidence of God. To him [2]:

..the inimitable, unapproachable, incomprehensible impress of Deity is there. Augustine says, "The soul bending over the things Thou hast made, and passing on to Thee who hast made them, there finds its refreshment and true strength."

Thus would I desire to contemplate the works of God, as bringing to my sense ever-fresh proofs of His all-pervading care, of His wondrous skill and wisdom, of His glorious majesty and power. Above all, they are the productions of the august Word: it is not that they were made by One who is infinitely great, but far removed from me, so that I can only reverently admire Him at an immeasurable distance. No; they are productions of the mind and hand of the Word (John i 3)..

..Yet let me not be mistaken. The study of the creatures could never teach me this. Notwithstanding all that they eloquently declare of the eternal power and Godhead of the Creator, they are ominously mute when I ask them how He will deal with me, a sinner.

All this comes as no surprise to those that know Henry Gosse's work and life. However, my appreciation of him does not extend to the religious views that made him such a devout Creationist. I find it baffling, and it is one of the reasons why I have never read any of Gosse's books on religious themes (listed in [3]). I admire his work in Natural History and enjoy his company in my imagination during my nostalgic visits to Torbay, but why did he have such a need to proselytise? Was he trying to convince himself?

[2] Philip Henry Gosse (1865) Land and Sea. London, James Nisbet & Co.

[3] R. B. Freeman and Douglas Wertheimer (1980) Philip Henry Gosse: A Bibliography. Folkestone, Wm. Dawson & Sons.