You’re very welcome! Congratulations on performing a successful air evacuation! That’s a tricky maneuver and it’s always awkward and intimidating the first time you do it. But you seem to have done a fine job and no doubt your male capensis is feeling much better now. Providing the gas build up and problems with positive buoyancy don’t recur, he should be good as new. Eating well and actively courting are a couple of very good signs to that effect.
As for the growling while you were burping his pouch, that’s a very interesting subject. Seahorses are capable of making noises both in and out of water. 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 the growling or croaking you noticed is an example of drumming. You were able to feel the 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. 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).
Although I have never been able to personally verify that seahorses use sound as part of their mating ritual, there is no doubt that the ”good vibes” seahorses give off during their displays of reciprocal quivering play an important role in courtship. The seahorses can certainly detect the vibrations produced their partners during these bouts of side-by-side quivering, and the stimulation this provides may be important in heightening arousal and advancing the courtship toward eventual mating.
Unfortunately, Elina, Ocean Rider does not ship livestock overseas. However, they are happy to provide supplies and dry goods (e.g., Vibrance, pouch kits, etc.) to overseas hobbyists, including Europe and Germany, of course.
As of now, my new book is still unavailable and it could be some time before it’s finally released. I know, I know — it was originally scheduled to be released last January and it’s already long overdue — but right now the matter is out of my hands…
The problem is that TFH just changed publishers several months ago and that has delayed everything. All of their production schedules have been pushed way back as a result. Christopher Reggio, the new publisher at TFH, is both brand new to the company and unfamiliar with seahorses. He therefore wants to defer any action on my book until he’s had a chance to get a better feel for how things work at TFH and has had a chance to further assess the market for an ambitious book on seahorses. He plans to "revisit" my book project sometime early next year and decide the best way to handle it then, after he’s up to speed.
At the moment, Christopher Reggio’s impression is that seahorses are very much a specialty market, and his concern is that that market may simply be too limited to support a major book about seahorses. That’s why he wants to conduct additional market research before he commits to the project, and that’s why TFH keeps talking about coming out with a condensed version of the book first. The abbreviated version of the book would be a trial run to test the market, if it sold well, then TFH would have no qualms about releasing the full-length addition of the book. At any rate, right now all that’s up in the air pending further research…
But don’t worry — one way or another, I will see that the book comes out in its entirety as soon as possible. The moment I have a firm release date, I’ll be sure to let you guys know. And I promise you it will be well worth the wait! In the meantime, you can always reach me online for help with your seahorse-related problems.
Best of luck with your Cape seahorses (H. capensis), Elina!