Stand by, English ‘A’ level pupils of a certain age. You will not be able to read this article without a nostalgic whiff of formaldehyde tickling the nostrils. You will remember the first dissection in the series of vertebrate types, a dogfish, and how quickly your attention was drawn by the dissection guide to the structure in the intestine labelled the ‘spiral valve’.
For those reading this overseas I should start by saying that the dogfish sold for dissection by biological suppliers was the Lesser Spotted Dogfish, also known as the Rough Hound. The accepted name for the beast among the fishy cognoscenti is Small-spotted Catshark. The scientific name is Scyliorhinus canicula but it was long known as Scyllium canicula, both names coined by Linnaeus. Why anything named for a dog should then get ‘cat’ as a common name I do not know.
If you wanted to know the function of the spiral valve this is what was to be found in Parker & Haswell*: ‘Absorption takes place largely in the colonic region, which is equipped with an anterior spiral valve. This begins at the end of the duodenum; it is a richly glandular fold of tissue which runs spirally around the intestine and both retards the too rapid passage of food and affords a more extensive area for absorption of digested substances’. The later editions of J.Z. Young’s Life of Vertebrates describes the spiral valve as ‘a tightly wound flap or ridge’.
It is highly pertinent to quote from the text book co-authored by Thomas Jeffery Parker (1850-1897; FRS 1888) since in 1880 he published a seminal contribution to the form and function of spiral valve, as well as devising a fixation method for studying its shape in situ. At that time Parker was lecturer at Bedford College in London and demonstrator in Thomas Henry Huxley’s department at the Royal School of Mines. Shortly after he gave the paper at a meeting of the Zoological Society of London on 16 December 1879 he moved to the University of Otago in New Zealand. He was diabetic and died aged 47.
By studying a number of species of elasmobranch particularly skates and rays, Parker showed there was a considerable amount of variation in the form of the spiral valve; he also calculated the increase in surface area as a result of its presence, in an early example of a mathematical treatment of anatomical features, and argued that passage of digesta would be relatively slow through the ‘valve’.
A recent paper in Proceedings of the Royal Society takes Parker’s work of 140 years earlier on the spiral valve further both in terms of form and function. The 3D diagrams the authors constructed from CT scans are essentially similar to the 2D ones drawn by Parker using his method of distending and hardening the intestine with dilute chromic acid and then cutting holes through the wall to examine the retained 3D structure inside.
The authors confirmed the four types of spiral valve found in different species (and adapted Parker’s drawings to illustrate them): (a) Column, (b) Scroll, (c) Funnels pointed posteriorly, (d) Funnels pointed anteriorly. They then measured the flow-rate of liquids of different viscosities in both directions through fixed specimens of spiral valves of the different types. Flow through the spiral valve was lower in the posterior to anterior direction than in the anterior to posterior direction; statistically significant differences were found for the two types of funnel structure but not for the column or scroll types even though these was a trend in the same direction. I have some concerns about the method used to determine flow rate and the seemingly high degree of variation observed with what was dead, fixed tissue. Those looking at the paper might be mystified by the unexplained notation used to denote statistical significance; I was and still am.
 |
| From Leigh et al. 2021 |
 |
| Note: The Order of types of spiral valve is not the same as in the first diagram |
The authors suggest that the spiral valve in those elasmobranch fish which have the funnel types can act as a partially effective Tesla valve, favouring movement of digesta through the intestine compared with the reverse direction. In their own words:
The spiral intestine…would allow segmental contractions to better mix digesta in the spiral intestine without the risk of much back-flow. Little is known about intestinal motility in sharks. Typically, overall evacuation rate is used to estimate the length of time that digesta remains in the gastrointestinal tract of sharks. However, by understanding contractile capabilities of the different segments (proximal, spiral and distal intestines) of the shark digestive system separately, we can begin to establish transit rates at specific points throughout the gut. We have begun to do this by measuring the average number of contractions per minute and the average amount of time necessary to move material of a known viscosity through the spiral intestine of S. suckleyi.
And end with:
In conclusion, our flow rate data suggest that the spiral intestine is acting as a flapper-less Tesla valve, which would promote unidirectional flow without any parts that are susceptible to blockage. We have established, quantitatively, that flow rate is slowed in the spiral intestine. Additionally, the flow rate was slowed significantly more when the two funnel-shaped spiral intestines (anterior and posterior funnels) were subjected to flow in the posterior to anterior direction. This indicates that at least funnel-shaped spiral intestines are capable of producing unidirectional flow, although the spiral and scroll do to a lesser extent. This could explain why digesta transit rates vary among species and among different spiral intestine structures. Further investigation of these unique intestinal structures as a component of the digestive success of sharks is necessary to understanding their function and evolution. The new techniques produced by this project lay the groundwork for future investigations involving the spiral intestine, and for understanding the functional role of the digestive tract in sharks, fishes and vertebrates in general.
Much has been made of the claimed Tesla valve-like properties of the spiral valve in the popular scientific press, as did the authors in the introduction in a ‘now ask of the beasts, and they shall teach thee’ theme in which biological mechanisms can lead to applications in engineering. Nikola Tesla, incidentally, patented the no-moving parts valve in 1920. I remain somewhat sceptical. While the authors have shown that the various forms of spiral valve CAN act as a sort of Tesla valve there is still way to go before it is finally established that in real life, spiral valves DO act as a Tesla valve, since that function could be an epiphenomenon of a mechanism that evolved, as Parker suggested, to ensure a high efficiency of absorption by means of a slow transit time and a large surface area. Whether or not a Tesla type effect is physiologically relevant would depend on such factors as the the timing and magnitude of peristaltic waves, the muscular tone of the gut at rest and the host of other possible factors, including the thick sludge-liked digesta, that determine transit through the gut. Some good gut physiology seems called for.
The authors also had me mystified on another aspect of their work. They mapped the type of spiral valve onto a cladogram of shark families. The family Scyliorhindae was denoted as having the ‘Scroll’ type spiral valve since two species from that family CT-scanned showed such a structure. However, Parker showed clearly that our old friend the Lesser Spotted Dogfish, Scyliorhinus canicula (closely related, according to the source of the cladogram, to one of the species CT scanned), has the ‘Funnels pointed anteriorly’ type of spiral valve.
 |
| Parker's drawing of dogfish spiral valve |
This is what Parker wrote:
In a large specimen of Scyllium canicula [a synonym of Scyliorhinus canicula] I find an especially interesting type of valve…This intestine is shown in fig. 5, Pl. XI.: there are twelve turns to the valve , all but the last of which are strongly deflected forwards, producing a structure which must offer an immense amount of resistance to the passage of intestinal contents, and, of course, making a decidedly greater proportional increase of surface than in any of the cases recorded of the Ray. The difficulty of cleaning out the intestine afforded a good criterion of the forms of these points; the finely divided contents stuck so tightly between the successive “cones,” that a stream of water was often quite insufficient to dislodge them. In fact the chyle (if one may apply the term to what rather resembled fine mud) completely filled up the whole available space in the intestine, so that, although the animal was preserved entire in spirit, the gut and its valve were in as good a condition for examination as if the former had been carefully emptied and distended with spirit while fresh. The pyloric valve was very perfect, having the form of a short conical tube projecting into the bursa entiana, with a very small aperture at its apex. This, of course, brought about the result referred to, that only finely divided matter could find its way into the intestine.
Another point I may mention about this specimen is the great thickness of its walls at the posterior end ; the thickness was actually greater than the diameter of the lumen at that part. This may have been a mere individual abnormality; but it seems not impossible that this increase of muscular substance had relation to the great force necessary to drive on the contents in a gut with so peculiar a spiral valve.
In a smaller specimen of the same species there were eight turns to the valve, of which the first five had a forward, the last three a backward direction…
Given that description I can see why ‘valve’ is the right description for the spiral structure.
Parker raised the intriguing possibility that the form of the spiral valve may differ considerably either between individuals of the same species and/or during growth and development of an individual.
A ‘why’ question to be asked, of course, is why have different types of spiral valve evolved?
To conclude, although I found the authors of the new paper had failed to convince me on a number of points, they should, if nothing else, have excited more interest in the functions of the spiral valve and of such old (the spiral valve appears to have been first described in 1671) but important topics in how fish work.
…And, in case you are asking, no, I cannot remember any detail whatsoever of the spiral valve in the three dogfish I have dissected, other than the fleshy folds and their semi-solid contents.
*This is what the 7th edition of 1962 says. As I recall that is no different from the 6th edition, published in 1940.
The new paper:
Leigh SC, Summers AP, Hoffmann SL, German DP. 2021 Shark spiral intestines may operate as Tesla valves. Proceedings of the Royal Society B 288: 20211359. https://doi.org/10.1098/rspb.2021.1359
------------------
Parker TJ. 1880. On the intestinal spiral valve in the genus Raia. Transactions of the Zoological Society of London 11, 49-61 plus 2 plates.
Parker TJ, Haswell WA. 1962. A Text-book of Zoology (7th edition revised and largely rewritten by AJ Marshall). London: Macmillan.
Vélez-Zuazo X, Agnarsson I. 2011. Shark tales: A molecular species-level phylogeny of sharks (Selachimorpha, Chondrichthyes). Molecular Phylogenetics and Evolution 58, 207–217 doi:10.1016/j.ympev.2010.11.018
Young JZ. 1981. The Life of Vertebrates (3rd edition). Oxford: Clarendon Press.
No comments:
Post a Comment