Congratulations on your juvenile Hippocampus erectus! It’s good to hear that they are active, growing, and eating aggressively — all good signs of healthy youngsters that are thriving.
You needn’t worry that the juvies are growing too fast, Lelia, but seahorses can sometimes be real seagoing gluttons and it is possible for them to over eat. That point generally comes when the food is passing through their digestive tract too fast to be digested properly. Allow me to elaborate.
Ordinarily our galloping gourmets are ALWAYS hungry. Our amazing aquatic equines are famous (infamous?) for their endless appetites and are accustomed to eating more or less continually throughout the day.
This is largely the result of their unusual anatomy and the lack of a true stomach. The seahorse intestine is basically a continuous, convoluted tube leading from the esophagus to the anus, whose primary purpose is the absorption of nutrients from digested food. Hippocampus does have rudimentary stomach, but it is greatly reduced, a common characteristic of carnivorous fishes adapted for feeding continuously on small prey items (Tamaru, Aug. 2001). It is little more than a pouchlike expansion of the intestine that begins in the crook of the neck. The pylorous region of the seahorse’s stomach, which separates it from the rest of the intestine, is underdeveloped and lacks the strong, muscular sphincter that separates the two organs in humans and most other animals (Tamaru, Aug. 2001). As a result, food passes continuously through this simple stomach and it cannot store food between meals. Due their inactive lifestyle, seahorses do not need to stockpile energy in fat reserves or food stores, so this simple stomach ordinarily serves them well.
Like other carnivorous fishes, the intestinal tract of the seahorse is also relatively short (Tamaru, Aug. 2001). This fact, combined with the lack of a true stomach, combines to render their digestion very inefficient when they have an overabundance of food. Once a seahorse is full, there comes a point when it cannot take anything more in at one end of the digestive tract (the esophagus) without passing something out the other end (the anus). When that happens, the transit time through the G.I. tract is simply too short for the food they have ingested to be digested fully, and the seahorses don’t get the full benefit in terms of nutritional value from their food.
This sometimes presents a problem in captivity when rearing the babies, since the feeding instinct is so strong in seahorse fry that they do not stop eating once they’ve reached their full capacity. They will continue to gulp down food as long as live prey is present, forcing them to eliminate food that’s only partly digested (Warland, 2003). When provided with too heavy a feeding density, this can sometimes reach the absurd point where they are passing live Artemia in their feces, and the ravenous fry can literally starve to death in the midst of plenty (Warland, 2003)!
However, this is rarely a problem once the seahorses have become juveniles that are feeding on frozen foods. If your young H. erectus are 3-4 inches in length, Lelia, they are still developing and growing rapidly and normally need all the calories they can get to support that vigorous growth. (Think back to your adolescent days when you are going through your teenage growth spurt. Remember how you could eat anything and everything at that time, and in surprising quantities, without getting fat? It’s kind of the same thing for your juvenile seahorses at their current stage of development.)
If you want to be sure that your galloping gourmets aren’t overindulging, just monitor their feces closely for the time being. As long as they are producing their normal fecal pellets, all is well. But if they start to pass recognizable pieces of Mysis shrimp that haven’t been properly digested, then they may be eating too much too soon, and you’ll want to space their feedings further apart and/or reduce the amount you offer them at each feeding. But I suspect that that won’t be necessary as long as they are still growing quickly.
Once your young H. erectus hit sexual maturity and their growth rate as adults slows markedly, you’ll need to be a little more careful about rationing their food in regulating how much they eat on a daily basis. It will be important for you to fast them one day a week as adults in order to protect them against fatty liver disease (hepatic lipidosis).
Because of their lazy lifestyle, our pampered pets are susceptible to a debilitating affliction commonly known as "fatty liver disease" or hepatic lipidosis when they are given a diet that’s excessively rich in HUFA (highly unsaturated fatty acids) and other lipids. Mature seahorses that are no longer breeding are at greatest risk from hepatic lipidosis. Young seahorses need a high-fat diet to sustain their rapid growth and development, and breeding pairs that are churning out brood after brood of fry likewise need all the energy they can get. But once they reach sexual maturity, their growth rate slows markedly, and nonbreeding adults that receive a high-fat diet will begin to store excess fat in specialized cells called adipose tissue (Tamaru, Sep. 2001). Eventually these fatty deposits will begin to infiltrate the liver cells, hence the name fatty liver disease (Tamaru, Sep. 2001).
In severe cases, adipose tissue can become so thick that it can literally hides the internal organs, cloaking them within a cocoon of fat, and distending the abdomen (Tamaru, Sep. 2001). When the seahorse’s liver or hepatopancreas becomes badly infiltrated with fatty deposits, it interferes with the organ’s ability to perform its vital role in digestion, food absorption, and detoxification of the blood, which has dire consequences for the affected seahorse.
Avoiding overfeeding, fasting adults once a week and using relatively low-fat enrichment products such as Vibrance II for mature seahorses that are no longer breeding are simple ways to prevent fatty liver disease.
Best of luck with your well-fed juveniles, Lelia!