Over generations as humans have eaten softer and softer foods it has resulted in the recession of the lower jaw (von Cramon-Taubadel, 2011). This was a discovery that really shocked people but I’m not really here to talk to you about humans. We are here for the fishes and in fishkeeping we aren’t so interested in change over such long time periods. The question is here, how does the hardness of the food we feed effect our fishes?

There is no doubt that a lot of the commercial diets are particularly soft whether it be pellets, wafers or flakes, this is only increased as foods become saturated with water. The hardness of foods must also be taken in respective to the species of the fish. Mode of eating would likely hold an influence to the hardness of food particularly when it comes to species that eat snails, there is a difference between whether the fish is extracting snails or crushing them. The hardness of the food a species might experience in the wild would definitely depend on what it is feeding on not just as the diet but also the surfaces it might be extracting food from e.g. sponges or wood.

Similar to humans fishes do show change in their jaw shape and morphology when fed a softer diet. In black carp, Mylopharyngodon piceus developed reduced pharyngeal jaws and pharyngeal arches which are involved in breaking down of food (Hung et al., 2015). Pharyngeal jaws are found at the back of the mouth of many fishes as a secondary pair of jaws involved in breaking down food whereas oral jaws that are visible externally are generally involved in prey/food item capture. Pharyngeal jaws might have teeth on them aiding in the breaking down of food, these jaws can be as diverse in a fish group as oral jaws. M. piceus in the wild frequently consume reasonable amounts of snails and mussels, interestingly when they do not have anything in their stomach they contain a lot of parasites previously carried by snails (Poulton et al., 2019).

Other factors are important though, food item hardness doesn’t always mean it is more difficult to break apart, maybe they require a bit more force. It’s always one thing to create an amazing diet but more importantly is will the fish eat it? Food item hardness potentially does effect how much food is eaten but the difficulty in being able to break apart the food is correlated with intake (Aas et al., 2020), perhaps it’s because the fish spend more time breaking it down?

There are other physiological effects of food hardness and durability, as it is broken down more easily in the water this can effect how quickly the fishes stomach can empty and reducing feed intake, Aas et al. (2020) suggested that the harder food takes longer to pass through the gut. A rather concerning aspect of this is the accumulation of fats in the stomach of salmon resulting in osmoregulatory stress from those softer diets that take up more water (Baeverfjord et al., 2006).

Of course it is important to mention there is a lot more to this, some fishes feed on naturally softer diets like a lot of algaes. Pufferfishes have a beak that is constantly growing and requires hard food items to keep that beak from becoming overgrown. The issue with puffer fishes is providing that range of nutrition without having an insane amount of different frozen foods, dry and gel diets generally contain a lot more of that range of nutrition.

An important part of hard against soft diets is water stability as previously mentioned before, as expected gel diets have a very high water stability (Lal et al., 2023). While it does seem counterintuitive there are ways of increasing the hardness of gel diets whether it be including harder items that require more processing or for the case of some fishes the diet can be added to the inside of a snail shell, a crab etc. On solid food items like a crab you could even smear the gel over the food item.

I feel this article has deterred from the original topic and largely this is because as a topic there isn’t much research into it. It does make sense for morphology to change as captive diet changes as well. So maybe it is more of something to think about for the future.

References:

Aas, T. S., Sixten, H. J., Hillestad, M., Ytrestøyl, T., Sveier, H., & Åsgård, T. (2020). Feed intake, nutrient digestibility and nutrient retention in Atlantic salmon (Salmo salar L.) fed diets with different physical pellet quality. Journal of Fisheries, 8(2), 768-776.

Baeverfjord, G., Refstie, S., Krogedal, P., & Åsgård, T. (2006). Low feed pellet water stability and fluctuating water salinity cause separation and accumulation of dietary oil in the stomach of rainbow trout (Oncorhynchus mykiss). Aquaculture, 261(4), 1335-1345.

von Cramon-Taubadel, N. (2011). Global human mandibular variation reflects differences in agricultural and hunter-gatherer subsistence strategies. Proceedings of the National Academy of Sciences, 108(49), 19546-19551.

Hung, N. M., Ryan, T. M., Stauffer Jr, J. R., & Madsen, H. (2015). Does hardness of food affect the development of pharyngeal teeth of the black carp, Mylopharyngodon piceus (Pisces: Cyprinidae)?. Biological Control, 80, 156-159.

Lal, J., Biswas, P., Singh, S. K., Debbarma, R., Mehta, N. K., Deb, S., … & Patel, A. B. (2023). Moving towards Gel for Fish Feeding: Focus on Functional Properties and Its Acceptance. Gels, 9(4), 305.

Poulton, B. C., Kroboth, P. T., George, A. E., Chapman, D. C., Bailey, J., McMurray, S. E., & Faiman, J. S. (2019). First examination of diet items consumed by wild-caught black carp (Mylopharyngodon piceus) in the US. The American Midland Naturalist, 182(1), 89-108.

Samuelsen, T. A., Hillestad, M., Jacobsen, H. J., Hjertnes, T. J., & Sixten, H. J. (2021). Physical feed quality and starch content causes a biological response in Atlantic salmon (Salmo salar L). Aquaculture Reports, 21, 100791.