Re:Hi and feeding station

Pete Giwojna

Dear Caropr:

Yes, that’s right — the feeding tube (just a length of rigid plastic tubing of appropriate size) is meant to be portable. You move it around within the aquarium as needed when target feeding the ponies or depositing the frozen Mysis in a suitable feeding station. Afterwards, the feeding guide or feeding tube is removed from the aquarium until it is needed the next time. Most seahorse keepers keep the feeding tube in their aquarium cabinet or on top of the tank, and it’s a good idea to rinse it out with hot water thoroughly from time to time to keep it clean and sanitary.

You could always use suction cups to mount the feeding tube inside the aquarium, if you prefer, but most aquarists do not want to include an unsightly piece of apparatus as a permanent fixture for their tanks when it’s not necessary…

Yes, indeed — the noise that healthy seahorses produce when slurping up their food is distinctly audible to the human ear and does sound rather like castanets or the sound produced by snapping your fingers.

Seahorses are capable of making noises both in and out of water, Caropr. All seahorse keepers are familiar with the "snick!" they make when slurping up prey, but they will also sometimes emit a series of staccato clicks or high-pitched squeaks when foraging or when held out of the water (Discovery of Sound in the Sea, 2004). What purpose these noises serve or whether they are a form of communication is unknown.

Hippocampus erectus in particular is well known for its ability to "vocalize." Seahorses produce sounds by two different mechanisms — drumming (vibrating their air bladders) and stridulation (scraping the back of the neuroskull against the bones of the coronet). Snicking is produced by stridulation whereas a growling or croaking noise is produced by drumming. If you are holding the seahorse at the time, you can actually feel this "growling" because the vibration of the air bladder is transmitted to the surrounding tissues and into the water through its body, as we’ll discuss in more detail below.

Marie Fish conducted a study of the significance of sound production in seahorses at the Narragansett Marine Laboratory, University of Rhode Island in 1953 (Bellomy, 1969, p190). She used a hydrophone to monitor and record the sounds produced by a large female northern seahorse (Hippocampus erectus) over a period of several months. She found that spontaneous sound production was limited to first two days in a new environment (Fish, 1953). The seahorse would cruise the length of the tank one or more times in an exploratory manner and then anchor itself to a holdfast and emit a burst of sounds (Fish, 1953). These consisted of sharp clicks in bursts of 2-5 with each snap spaced about one second apart (Fish, 1953). She would then repeat this behavior every 1/2 to 3/4 of an hour for the first day or two (Fish, 1953). After the second day she would fall silent, making no more sounds despite attempts by the experimenters to elicit them. However, each time she was transferred to a new tank, she would resume the same exploring/sound-making behavior for the first day or two (Fish, 1953).

Fish concluded, "…for this one fish at least, sound may be used in new surroundings for orientation, perhaps to find the whereabouts of others of its species (Bellomy, 1969, p190)." Fish notes that the female had spawned recently (Fish, 1953); it may have been searching for its mate. These findings beg the question of whether seahorses are able to locate one another in the ocean using sound. That would certainly be a useful ability for cryptic animals with very patchy distribution and very limited swimming ability.

Interestingly, there are several reports that mating in some seahorse species is often accompanied by clicking and snapping sounds, but I have never witnessed this first hand. For example, in 1970 Fish and Mowbray reported that Hippocampus erectus emitted a high-frequency clicking during courtship, which became louder and almost continuous during the actual mating.

These reports are intriguing because vocalizations in many other fishes are known to play an important role in intraspecific communication, including courtship and territorial behavior (Evans, 1998). There are several mechanisms by which fishes, including seahorses, are known to produce sound. For example, some fishes use their swim bladders as resonance chambers to produce sounds (drumming), using muscles on or near the gas bladder to vibrate the gas-filled membrane. Jorge Gomezjurado has found that Hippocampus ingens can make a "croaking" sound using their air bladder this way (Mann 1998). However, sounds made by drumming are low frequency, ranging from approximately 75 to 150 Hz, so that is definitely not the mechanism seahorses use to generate clicking noises during courtship and mating (Evans, 1998). Instead, the seahorse’s high-frequency clicking is thought to be produced by stridulation, which is simply rubbing or scrapping two parts of the body together to make noise (Evans, 1998). (Crickets, for example, produce their characteristic chirping via stridulation.) Sounds made by stridulating are usually concentrated at the higher frequencies, ranging from approximately 3,000 to 8,000 Hz (Evans, 1998).

Marie Fish identified the bones she believes produce the stridulation in seahorses. She found "…a loose articulation between the posterior margin of the skull and the anterior margin of the coronet, which is a star-shaped ossified crest mounted in a socket like base. When the seahorse’s head was extended moderately, the articulating bony edges could be seen to rub together, but when elevated more sharply, the coronet overlapped the other bone. Dissection showed adequate muscular equipment to permit such movement in the living fish. It is suggested therefore that the ‘finger-snapping sound’ results when the skull edge slips forcibly under the coronet, or, more likely perhaps, when it snaps out. Vibrations thus set up may be transferred to and amplified by the air bladder (Bellomy, 1969, p190)." In other words, seahorses use friction between the back of the neuroskull and the coronet bone (Mann 1998) to produce high frequency sounds via stridulation.

During the mating embrace, both male and female seahorses are thus said to produce high-frequency clicking sounds by scraping or "snapping" the bony edges on two parts of their skulls together (Discovery of Sound in the Sea, 2004). I should hasten to point out that these presumed courtship noises are distinctly different than the usual "snick" that seahorses make when feeding. They are not associated with eating and the clicks are made in bursts consisting of several snaps in quick succession (Fish 1953).

In short, the feeding noises that Snoopy and Clara make when enjoying their Mysis is music to the seahorse keeper’s ears, because it is a sure sign of a strong, healthy seahorse with a hearty appetite.

Best of luck with your new Ocean Riders, Caropr!

Happy Trails!
Pete Giwojna

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