Unveiling cloakfishes' cloaked filters

I stopped blogging, so what is this post doing here?

Well, I never said I would stop altogether, and I would return if there was something of special interest to report. Yesterday, I received my advance copy of  'Demain, les animaux du futur' from the authors, Marc Boulay and Jean-Sébastien Steyer. I am quite impressed and will return to write about it, in a week or so. Writing the present post is to get me in the mood again.

A main reason to reduce blogging was to spend more time on producing The Book, and that worked quite well: without blogging, I manage to produce one two-page spread every month, meaning one full painting, accompanying text, scale drawings and usually a minor illustration. At 24 pages a year there is definite progress (and I intend to increase the output). Sadly, Fishes I, II and III together only get one spread, while terrestrial hexapods get many. To illustrate the mechanics of some groups, I have stumbled on a three-spread theme: one spread for explanation, one to show diversity, and one showing a single species in a full painting. Groups that get this treatment are spidrids (half finished), rusps (all done), tetropters (not yet) and cloakfish: half done.

Click to enlarge; copyright Gert van Dijk

The early beginnings of cloakfish are shown here, and the latest instalment of their physique was posted here. Like it or not, that particular form, shown above, has now been scrapped. As you can see I played with putting the mouth in the cone forming the 'snout' of the animal. Well, not anymore. While sketching I drew a cloakfish cut in two and that gave me the idea of making a 'cutaway' version to explain how it works. Unfortunately, that meant that I could use very little 'handwavium'. Without a cutaway drawing I could just write something like this: (imagine an Attenborough-style voice-over) "Hidden from view by the animal's cylindrical body wall, its food rakes, next to the gills, steadily filter the nutritious plankton so abundant in these waters." How they look is left to the imagination.

With filters unhidden, the problem presented itself that I never really understood how filter feeding works, which is no wonder as I never looked it up. Many animals use it, from sharks and rays to bony fish and whales. So it works, but consider a whale shark or a basking shark as a gigantic sieve sweeping through the ocean. After a while, the filter will have sieved lots of food particles, now stuck against the sieve. The animal will have to scrape the food from it, not only to swallow it, but also to prevent the sieve becoming clogged. Remember that the gills are there as well, and you do not want to ruin respiration, not even for feeding. What bothered me is that whales might use their tongues to scrape clean their baleens, or so I supposed, but I was not aware of scrapers inside a whale shark's mouth.

Click to enlarge; Source: Brainerd, Nature 2001; 412: 387-388

Well, reading a few papers later I found out about something called 'cross flow filtration'. Naively, I had imagined the filter as a sieve at a right angle to the flow of water, allowing water to pass while particles get stuck. That's not how all filters work, though. The image above explains the process nicely. In 'cross flow filtration', the surface of the filter is parallel to the flow of water. Behind the filter there is a low pressure area, so water flows there. Apparently, particles move on parallel to the water, staying on one side of the filter, where they are  concentrated more and more. The papers then mention things like 'near the oesophagus', suggesting that the animal then merely has to swallow the concentrated particles and there you are. If you want to read more, I found a site where you can obtain a Nature paper for free here. Mind you, the fact that this was worthy of publishing in Nature in 2001 means that this is still all fairly new. The papers are somewhat vague on why the concentrated particles bunch up in a cul de sac waiting for the oesophagus to gulp them up, but I will accept this leap of faith; it cannot be easy to do an oesophagoscopy on a freely swimming whale shark.

So I sketched some more, filling in the inside of cloakfish contours, giving it a cross flow filter with a cul the sac leading to the oesophagus. Actually, since we are talking about a tetraradiate animal, there are four filters and four oesophagi leading to one stomach. I paint but am not a technical artist, so I needed some help with the perspective and also with visualising the insides of the cloakfish. I used Vue Infinite to provide me with as many perspectively correct views of the animal's inside as I wanted to help draw the cutaway.

Click to enlarge; copyright Gert van Dijk

What you see above are some aids in doing so. The holes help visualise the flow of water (but I must add that the gill design was changed afterwards). The painting, based on this design, is nearly finished, but I will not show it: there should be new material in The Book. My first look at the 'Demain' book showed a very large amount of previously unpublished animals, and that strengthened my resolve to keep much hidden. I must say that writing this post did remind me why I did it for a long time: it is fun; but time is short...