Specimen Showcase｜The Pinocchio Fish

Some species intrigue us due to their unusual appearance; and the long nose👃🏻 of today’s fish species is definitely peculiar not only for its appearance but also its function. The American paddlefish, 𝘗𝘰𝘭𝘺𝘰𝘥𝘰𝘯 𝘴𝘱𝘢𝘵𝘩𝘶𝘭𝘢 (Walbaum, 1792) belongs to an old lineage of fish present for more than 70 million years, similar to other fish such as the sturgeons. Growing up to 2 metres and weighing over 90 kg for the largest specimens, this species is currently found in 22 states along the Mississippi River drainage in the United States and in the Great Lakes Basin in southern Canada. The American paddlefish was originally found in 4 other states but the construction of numerous dams affecting its migratory breeding behaviour in addition to other disturbances of its habitat and overfishing has reduced its distribution range. Hence, the American paddlefish is listed as vulnerable in the IUCN Red List. It should be noted, however, that farms growing this species also exist in over 15 European countries where the species has thus been introduced since the mid-1970s, and with populations sometimes established in the wild.

The American paddlefish has two features that stand out from other fish. Firstly, it is covered with smooth and scale-less skin instead of scales. This reduces the resistance in the water and aids its streamlined body to glide smoothly underwater. This special locomotion primarily relies on the wavy motion of its long caudal fin, which is the long back fin that also helps to navigate through the river stream. The second unique feature is its ‘long nose’. What is actually the function of this ‘long nose’? It is its snout, a forward projection from the head of the fish including the upper and lower jaw, making up about one-third of its body length. This unique structure mainly serves as a communication organ and aids its feeding strategy. The American paddlefish uses passive electrosense⚡️. Its snout is covered with 70,000 electroreceptors that facilitate communication, mating, prey detection and avoiding predators. The electroreceptors are used to detect the low-frequency electric discharge produced by gymnotiform fish, which is also called the knifefish, showing interspecies communication to avoid potential competition or predation situations using electroreception. The electroreceptors also enable the American paddlefish to detect potential predators or prey buried in the sand as well as facilitate mating behaviour.

To add on to the mating behaviour, another intriguing aspect of the American paddlefish is its reproductive behaviour. Adults adopt a migratory spawning habit and take a long journey upstream to find an area with a strong current, facilitating the fertilisation and dispersal of eggs. The larvae will begin their growth and development downstream. This is the reason why the American paddlefish is found all along the river stream and why the construction of dams affects their survival. The metal parts⛓️ of the dam generate unnatural electrical fields that disrupt the electroreceptor perception of the American paddlefish. They can detect a metal object as small as 2.5 cm from a distance up to 38 cm, hence, when they detect the unnatural electric field, they swim away to avoid possible predators. This hinders their migratory spawning behaviour and affects their reproduction and survival.

Secondly, the electroreception is also important in the feeding behaviour of the American paddlefish. Unlike other fish, the American paddlefish is a filter feeder, which feeds on zooplankton and phytoplankton suspended in the water by swimming with its mouth open. The paddlefish’s rakers, the comb-like structures located in its gill, trap small organisms or microplankton. The electroreceptors allow the American paddlefish to detect small prey, whose sizes range from five millimetres long to just one-thousandth of this size, contrasting to the large size of the American paddlefish. It is impressive that the American paddlefish can still catch prey and avoid predators solely depending on the electro-sensing ability in murky environments where visibility is very limited. The American paddlefish also use their antennae-like structure to identify groups of tiny water fleas to increase their chance of catching the prey. This feeding pattern helps regulate the plankton populations and enables a balanced ecosystem.

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Text: Cherry

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