Sanderson, S.L., Roberts, E., Lineburg, J., Brooks, H., (2016). Fish mouths as engineering structures for vertical cross-step filtration. Nature Communications. 7:11092. doi: 10.1038/ncomms11092
When it comes to filters, humans have nothing on fish. Fish such as paddlefish and basking sharks feed on small planktonic prey by filtering large amounts of water with nary a clog in sight. Take a look at the mouths of basking sharks and paddlefish. Though they are not close cousins, they have evolutionarily converged upon the same mouth design and can be seen swimming mouth open wide, filtering the water for tasty particles.
What look like white ribs are called branchial arches and attached to the branchial arches are gill rakers—the dark fringy structures on the inside of the mouth.
Cross flow filtration utilized in the beer, dairy and biomedical industries (and by fish, or so it was thought) minimizes clogs by having the flow of unfiltered liquid travel parallel to the filter, preventing the build up of any particulates (Image 1). While a big step up from a dead end sieve system—think spaghetti strainer–cross flow filters still clog up inevitably and need to be backwashed.
So how do filter-feeding fish manage to remain clog free?
To find out, Sanderson et al. created plastic 3D models of paddlefish and basking shark mouth filters taking into account several anatomical measurements. These models resembled plastic cones with ribs representing branchial arches, and fine mesh in place of gill rakers. The models were then placed in a flow tank which is a tank of water set up so the flow of water is unidirectional, and brine shrimp cysts (dormant eggs) were added to the flow tank and observed to tease out how this fish filtration system worked. They also placed a preserved paddlefish mouth within the flow tank to check their work. It turns out; it is the combination and spacing of physical mouth structures and characteristics that result in something called ‘vortical cross-step filtration’.
Let’s break that down.
The closely spaced, tall branchial arches or ribs form a series of backwards facing steps. This means, as the water flows towards the back of the mouth, and off of a ‘step’ (rib surface), part of the main water flow is directed into a slot where some of it exits through the gill rakers, and some of it creates spiral movement or a vortex—Image 2 and 3, keeping the water circulating within the slot. It is this recirculation of water lengthwise along the slots and ribs that keep the filter clog free. By moving the gill flaps (represented by adding an external, asymmetrical skirt on one of the 3D models) the trajectory or axis of the vortex can be manipulated along the slot. The authors liken it to a ‘hydrodynamic tongue’ that filter-feeding fish can use to direct food particles to the back of the mouth. Even when portions of the mesh were removed from some of the 3D fish mouth models, the vortical cross-step filtration was still effective in trapping particles which is consistent with observations of young basking sharks and paddlefish feeding despite underdeveloped gill rakers.
Not only may other filter feeders such as tadpoles, some birds, goldfish and baleen whales benefit from vortical cross-step filtration, but industries (e.g. beer, wine, dairy and biotechnology) as well. This research could be applied to engineer better filters that clog less saving valuable time and money. And maybe, just maybe, those savings would be passed onto the consumer. Cheaper wine and cheese–who can argue with that?
There are many more examples of nature’s ingenuity. From shark denticles to sticky sandcastle worms, nature has had millions of years to fine-tune elegant and efficient solutions that we can learn a lot from.
I’m a past oceanbites writer, occasional editor and guest poster. I graduated with a Masters of Coastal & Marine Management from the University of Akureyri in Iceland, and am currently working in marine conservation. In particular, I’m supporting an Indigenous-led initiative to safeguard the largest inland sea in the world (Hudson Bay & James Bay, Canada). I love weird ocean critters and *sigh…I really do enjoy long walks on the beach.