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Biology

How fish can help design better filters

Paper:

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 mainstream flow (MF) of water moves towards the back of the mouth, but as it passes over each branchial arch (BA), some of the water enters the slots between the branchial arches and a spiral, or vortex (vo) of water is created. Some particles are trapped in zones (1) and (3) of the gillrakers (GR), while other particles remain suspended and circulated in zones (2) and (3). The filtered water (Fi) then passes over the gill filament (GF).

Image 1: The mainstream flow (MF) of water moves towards the back of the mouth, but as it passes over each branchial arch (BA), some of the water enters the slots between the branchial arches and a spiral, or vortex (vo) of water is created. Some particles are trapped in zones (1) and (3) of the gillrakers (GR), while other particles remain suspended and circulated in zones (2) and (3). The filtered water (Fi) then passes over the gill filament (GF). Credit: Virginia Greene, Creative Commons

Part of one of the 3D models submerged in a flow tank. The white bands represent the ribs or branchial arches and the mesh represents the gill rakers. Green dye shows the vortex created over a backward-facing step. Credit: S. Laurie Sanderson, Creative Commons

Image 2: Part of one of the 3D models submerged in a flow tank. The white bands represent the ribs or branchial arches and the mesh represents the gill rakers. Green dye shows the vortex created over a backward-facing step. Credit: S. Laurie Sanderson, Creative Commons

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.

Discussion

2 Responses to “How fish can help design better filters”

  1. We think that this article is very important for current advances in filtration devices. We think that the design of the filter on the shark, the multiple layers, could also help us advance in filtration devices because if the filter in the shark works so well in filtering the food out of the water, it should also work well in things we use. We also think that it is important to lower these products prices because many thing today are very expensive. After looking at the diagram, we wonder if there are any other factors that affect the way that these sharks. use there filters to get food?

    Posted by Andrew and Kian | May 24, 2016, 1:56 pm
    • Megan Chen

      Thanks for your comment, Andrew & Kian! That’s an interesting question–I hope I understand it correctly. Are you asking if factors such as temperature and/or salinity of the water, or size of prey affects the filtration system of sharks/paddlefish? If so, I don’t think I can answer your question because I have no idea! However, this would make for some very interesting investigation (e.g. does the stiffness of the filter change with temperature?)! One thing that might be worth restating is that sharks/paddlefish can control how the water moves through their filters, so they may choose to swim slower or faster, or choose to close their mouth more often to keep their filters clean. Sorry I couldn’t be of more help, but I do appreciate the time you took to read and respond.

      Posted by Megan Chen | May 26, 2016, 11:35 am

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