seahorse not feeding

Dear Helen:

I’m sorry to hear about the eating problems that your male seahorse has developed. This type of feeding difficulty is known as weak snick. Since only your male seems to be affected, I suspect that the loss of suction that characterizes weak snick is the result of a mechanical injury or muscular strain in this case, as explained in greater detail below. The following information will explain weak snick and related feeding disorders in more detail, Helen, as well as discussing treatment options.

Weak snick and related feeding disorders are usually due to either a mechanical injury or an infection affecting the seahorse’s hyoid bone trigger mechanism or the underlying musculature with which it generates the powerful suction that it uses when feeding. Such mechanical injuries can sometimes be caused by ingesting a foreign object while feeding, or the problem may be due to protozoan parasites that attack the gills and eventually affect the muscles that operate the buccal suction pump and/or the opercular suction pumps.

FEEDING DIFFICULTIES: WEAK SNICK, TRIGGER LOCK, & LOCK JAW

Seahorses suck! That’s a fact. Our amazing aquatic equines are supremely well adapted for suctorial feeding, which just means that their tubular snouts are designed for generating a powerful suction and slurping up small prey whole (Giwojna, Feb. 2004). Basically, their tubular mouths operate like slurp guns, a method of feeding that is often adopted by fish accustomed to taking prey from the bottom or plucking small crustaceans and larvae from the leaves of underwater plants (Evans, 1998). The anatomy of the seahorse’s head has evolved to accommodate this method of feeding (Giwojna, Feb. 2004).

For example, a tubular mouth is an advantage for suctorial feeding because it acts like a pipette and the narrow opening accelerates the inrush of water via the Venturi effect, thus maximizing the suction generated by the powerful head muscles (Giwojna, Feb. 2004). The seahorse’s oral or buccal cavity and gill chambers (opercular cavities) act as dual suction pumps that draw the water inwards with considerable force (Evans, 1998). Expansion of the buccal and opercular cavities causes a sudden drop in pressure within the mouth (Evans, 1998). The suction thus created allows the seahorse to suck up food through its slurp-gun snout faster than the eye can follow. In essence, the seahorse inhales its food in the blink of an eye, and cavitation caused by the sudden inrush of water traveling at tremendous velocity through the narrow snout and characteristic movements of its head and skull bones produce the distinctly audible "snick!" which announces the demise of its prey (Giwojna, Feb. 2004).

Every seahorse keeper is familiar with the seahorse’s "trigger," located at the underside of its jaws at the base of its throat, which moves downward sharply when the seahorse strikes, thereby expanding its oral (buccal) cavity and generating the suction to draw its prey inwards. This trigger is actually the seahorse’s hyoid bone, and it is pulled downward by contraction of the powerful sternohyoideus muscle that runs from the hyoid bone to the cleithrum (one of the bones of the pectoral girdle), which forms part of the seahorse’s bony exoskeleton (the cleithral ring) just behind its head (Evans, 1998).

The suction generated by the sudden downward contraction of the hyoid bone when a feeding seahorse "pulls the trigger" on its intended prey is greatly enhanced by the nearly simultaneous expansion of its gill chambers or opercular cavities (Giwojna, Feb. 2004). The additional suction thus created by the seahorse’s opercular pump is produced by contractions of the hyohyoideus muscles and dilator operculi muscles (Evans, 1998). The water pulled into the gill chambers this way is then expelled from the opercular cavity through a small pore. (This narrow opening accelerates the stream of water passing through it in the same way as its narrow tubular snout does.) The seahorse’s bony coronet evolved atop its head in part to provide solid anchorage and attachment points for the large muscles that operate its buccal suction pump and twin opercular suction pumps, which enable it to feed so efficiently (Giwojna, Feb. 2004). This is the perfect feeding mechanism for an ambush predator, and the seahorse is perfectly adapted for its role as the sniper of the seagrass jungle (Giwojna, Feb. 2004).

Of course the seahorse’s turreted, independently operating eyes are the perfect targeting system for this sophisticated feeding apparatus (Giwojna, Feb. 2004). Side-mounted, hemispherical eye turrets provide nearly 360 degrees of vision and allow the seahorse to look upwards and downwards (or forward and backwards) simultaneously in search of potential prey or possible predators (Giwojna, Feb. 2004). As soon as it detects a likely prey item, both eyes lock on it simultaneously and track it intently, thus providing excellent depth perception. This allows the seahorse to judge distances with remarkable accuracy as it draws a bead on its intended victim (Giwojna, Feb. 2004).

The hyoid bone is the trigger which fires the seahorse’s slurp-gun snout, and the moment its prey closes within striking distance, the powerful sternohyoideus muscle contracts and pull the trigger (Giwojna, Feb. 2004). The buccal cavity expands, followed the almost instantaneous contraction of the hyohyoideus muscles and dilator operculi muscles, which likewise expand the opercular cavities (Evans, 1998). The resulting drop in pressure creates a sharp inrush of water, which draws the prey irresistibly into the seahorse’s mouth (Evans, 1998). Once the prey has been sucked in, the mouth is closed. At this point, the buccal and opercular cavities are contracted and the excess water is forced out in a strong stream via the tiny opercular pores (Evans, 1998).

All this happens in an instant, faster than the eye can follow, and the powerful suction that is generated often macerates large prey (Giwojna, Feb. 2004). When the resulting debris is expelled from the gill chambers, it looks remarkably as if the seahorse is shooting smoke out of its ears, thus giving a feeding seahorse an uncanny resemblance to the legendary fire-breathing dragon (Giwojna, Feb. 2004).

However, when this remarkable feeding mechanism is injured or disrupted by parasites and/or secondary infections, a number of problems arise. Weak snick is an unusual affliction that results when a seahorse is unable to generate adequate suction to feed properly. Seahorses develop weak snick when their sophisticated feeding apparatus, or the muscles that operate it, are incapacitated as a result of injury or infection.

For example, I have often seen it in seahorses as a result of protozoan parasite infections (Amyloodinium, Cryptocaryon, Brooklynella, Uronema, etc.). I tend to suspect that’s the cause when the weak snick is accompanied by rapid respiration and labored breathing, or when more that one seahorse develops the condition, or when the weak snick victim’s tankmates are bothered by odd ailments such as "trigger lock," appetite loss, lockjaw, heavy breathing, or the first signs of snout rot, which all early indications of masked protozoan parasite infections (Giwojna, Dec. 2003). These organisms typically attack the gills first, from which they spread to the throat and mouth (oral or buccal cavity). As their numbers build up in the gills and they spread from within, invading the esophagus and oral cavity, symptoms such as rapid breathing, loss of appetite, weak snick, trigger lock, and snout rot begin to appear (Giwojna, Dec. 2003).

This is how I believe the disease progresses in such cases: the burrowing of the embedded parasites causes hyperplasia of the underlying tissue, and when sufficient numbers of them build up in the gills, we see the initial symptoms of respiratory distress, labored breathing, and huffing (Giwojna, Dec. 2003). During a heavy infestation, the parasites may attack the key muscles that expand the opercular cavity, or sheer numbers of the parasites can clog the gills to the extent that the opercular pump is impaired, resulting in weak snick due to a decrease in suction (Giwojna, Dec. 2003). In severe cases, this will eventually result in death by asphyxiation.

In less severe cases, the parasites will continue to spread from the gills into the throat, buccal cavity, and eventually the snout itself (Giwojna, Dec. 2003). When this happens, the irritation caused by the burrowing parasites and the hyperplasia of the infected tissue can cause loss of appetite or difficulty swallowing and the victim may go on a hunger strike (Giwojna, Dec. 2003). If the swelling and hyperplasia occlude the gills, throat and snout sufficiently to prevent the seahorse from generating adequate suction when attempting to feed, weak snick is the result (Giwojna, Dec. 2003). If the burrowing of the embedded parasites allows secondary fungal or bacterial infections to take hold, the seahorse can develop snout rot (Giwojna, Dec. 2003). When such secondary infection(s) affect the sternohyoideus muscle that controls the hyoid bone trigger mechanism, ailments such trigger lock, sticky trigger, or lockjaw result and again the seahorse is unable to feed (Giwojna, Dec. 2003). Weak snick can be caused in this way as well if the sternohyoideus muscle is affected to the extent that hyoid trigger still operates, but so feebly that the buccal pump can no longer generate sufficient suction to feed (Giwojna, Dec. 2003).

Another common cause of weak snick in many instances is a mechanical injury to the seahorse’s hyoid-bone "trigger" mechanism. This sometimes happens when a seahorse accidentally ingests a foreign object when feeding off the bottom. The offending particle is often a piece of gravel or crushed shell. When a hard, sizable foreign object such as this is ingested, it can lodge in the throat or snout, and the seahorse may have difficulty expelling it again. (The seahorse’s feeding mechanism is much better suited for sucking things in than spitting them out again.) When that happens, the seahorse is almost always able to clear the offending object eventually, but sometimes not before it causes considerable irritation or the repeated efforts to eject it cause a muscular strain to the hyoid trigger mechanism. The seahorse then acts as though it has a very bad sore throat. The suction it generates is weak and both the act of pulling the trigger and the act of swallowing appear to be painful. The seahorse feeds reluctantly or halfheartedly as a result, and may eventually stop feeding altogether. Such mechanical injuries can also open the door for snout rot.

Suspect a mechanical injury when the weak snick or sticky trigger is not accompanied by respiratory distress, when only one of your seahorses is affected and exhibiting unusual symptoms, or when you witnessed the seahorse struggling to expel a foreign object. In such cases, most often the problem clears up on its own after two weeks to two months as the injury heals. No treatment is necessary and the key to a successful outcome is keeping the patient eating while the healing takes place. That’s what treatment should concentrate on.

When these feeding difficulties arise, it’s a good idea to try tempting the affected seahorse with live adult brine shrimp. Seahorses suffering from weak snick induced by an injury may have better luck slurping up smaller, lighter, soft-bodied prey like brine shrimp; if so, that will be enough to keep them going while they heal. You’ll want to enrich the brine shrimp to maximize its nutritional value, and gutloading the shrimp with an enrichment product high in HUFA such as Vibrance is a good way to fortify it beforehand. Brine shrimp are filter feeders that will ingest whatever is suspended in the water with them, so all you need to do is add a pinch or two (or drop or two) of the enrichment formula to a small container of saltwater swarming with brine shrimp at least 30 minutes before you offer the shrimp to your seahorse.

Some hobbyists dealing with weak snick have had good success in coaxing the affected seahorse to feed by transferring the seahorse to a critter keeper or breeder net or similar enclosure that can hang within the main tank itself, and then adding a generous amount of live adult brine shrimp to the container. Within the enclosure, the affected seahorse does not have to compete with its tankmates for the live food, and it is easy to maintain an adequate feeding density within the confined space so that there is always a big juicy brine shrimp passing within striking distance of the hungry seahorse. Add one or two hitching posts within the critter keeper or breeder net so that your male can anchor in place and wait for a tasty brine shrimp to pass within easy reach, and give him an hour or two within the enclosure to eat him fill of the softbodied adult brine shrimp. You can monitor his progress from a nonthreatening distance away from the tank to see how she is doing. In most cases, the seahorse quickly becomes familiar with the routine of being transferred to the special enclosure at feeding time and associates it with tasty live foods and a full belly — positive reinforcements that make it a very nonthreatening, stress-free procedure for the affected seahorse — and, as a result, it may actually come to look forward to it after a few feedings. You can repeat this feeding process two or three times daily in order to fatten him up again, if your schedule allows.

In short, Helen, if the affected seahorse is experiencing respiratory distress or any other indications that suggest the problem could be due to protozoan parasites, please let me know right away and I will provide you with some detailed treatment options explaining how to treat your aquarium to eradicate such protozoan parasites. But if he is not showing any other symptoms other than a loss of suction when feeding, and your new female continues to thrive and eat normally, then you’re most likely dealing with a muscular strain or mechanical injury, and keeping your male eating by providing him with abundant softbodied adult brine shrimp to slurp up is probably the best approach to this problem.

In addition, please let me know what type of seahorses you are keeping and what the current aquarium parameters for your seahorse tank are (i.e., the water temperature, salinity or specific gravity, and the ammonia, nitrite, and nitrate levels). Once I have this information, I will be able to advise you if there is any need to adjust your aquarium conditions to better suit the seahorses and make them more comfortable in their new surroundings. How large is your seahorse tank and how long has it been set up, Helen? Are there any tankmates besides the seahorses in the aquarium?

Best of luck keeping your male eating until his injury heals and the problem with weak snick resounds itself, Helen!

Respectfully,
Pete Giwojna