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Physiology of The Gills in Relation to Substrate


I have seen many people recently warn about the 'dangers' of sand; so I thought this article might be helpful in explaining the mechanism by which goldfish and their close relatives feed and why sand is a very natural and typically harmless element in their environment.

So lets really take a look at the goldfish for what it is; a species of carp. It's wild counterpart is the Prussian Carp (Carassius gibelio) which in the wild feeds on organic detritus, filamentous algae, small benthic animals, and pieces and seeds of aquatic weeds. The key word in that last sentence is the word benthic, meaning "Relating to the bottom of a sea or lake or to the organisms that live there."[1] Prussian carp and their relatives (including feral or hybrid goldfish) are found in a wide array of habitats, most of which are lakes and rivers. Lakes are low energy environments, meaning that not much energy is being used in the environmental system to transport sediment, and this results in only small sediments being moved.[2] Therefore lakes have silt, sand, or clay as their substrate, almost exclusively. Goldfish have evolved to feed and live in this kind of environment, and have done so for millennia. They possess gill rakers, which are a structure that occurs on the gill arch and vary greatly in structure between species of fish.[3] Here is a helpful diagram showing two "forms" of gill rakers:


The one on the left belongs to a species that is feeding on very small foods and is most likely a herbivore or omnivore. In order to get as much food as possible out of the substrate or water column they must have many gill rakers that are spaced closely together to trap food particles. The gill on the right belongs to a species that is feeding on larger prey; typically (but not always) fully carnivorous fish have gill rakers like this form. Goldfish however possess rakers that looks very similar to the left structure. Here is a picture of a goldfish's gill rakers (it's on a dead fish so if you're squeamish you might not want to click it). Fish move food through the rakers during a processing called winnowing. First the fish ingests a mouthful of sediment, then by moving several structures within the mouth, including the gill covers and hyoid apparatus, moves food back and forth across the rakers. Food bits are chewed and swallowed while debris is pushed out the mouth or gills. "An ability to extract food particles buried within loose sediments is common among unrelated lineages of teleost fishes that grub or root for buried items. For example, substrate grubbers (rooting with the snout within loose sediments to locate and ingest single food items) include the common carp (Cyprinus carpio), callichthyid and doradid catfishes of the Neotropics, and loaches (Cobitidae) of Asia. Digging and sifting (winnowing) behavior is observed among many, if not most of the diverse percomorph fishes" [4]

For goldfish the options that allow for the most natural feeding behaviors are particles classified as "sand", which is considered 1-2mm at the largest and 62.5–125 µm at the smallest; smaller grain sizes rainging from silt (3.90625–62.5 µm) to clay(< 3.90625 µm) are also acceptable however are not practical.[5] Large substrates do not allow for them to be winnowed through the gills, which can lead to the substrate getting lodged in the fish's mouth, leading to death. Sand typically passes without issue and will not impact a fish so long as it is not too large. Winnowing is one of the most natural behavior goldfish illustrate in home aquaria, even so far removed from the wild and highly modified from their original wild appearance. Keeping sand under a half inch will ensure that no anaerobic pockets form within it and allow it to be fully turned over by the fish. Sand is also beneficial because it forms an home for meiofauna to inhabit. These are a wide range of planktonic organisms that live in between grains (the pore spaces) in a sediment. These are hardly a meal for a goldfish but contribute to an overall balanced environment.

In conclusion sand is no more dangerous than any other aspect of an aquarium.It mimics their wild habitat closely and when set up properly is a great addition to any tank. The goldfish have the proper anatomical structures such as gill rakers to easily winnow through substrate and doing so is a natural and healthy behavior. Without proper evidence blaming a death or illness on one cause is a recipe for disaster because the real issue may get glazed over and go unnoticed. Thorough knowledge of the internal structures and functions of the animals being kept is beneficial to eliminating possible hazards and allows them to survive and thrive.







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  • Regular Member

This is wonderful , especially in conjunction with Alex's article on the Vagal System. I think this will help dispense with any sand myths. Thanks again :)

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