I am very sorry to hear that your female Hippocampus fuscus, Olivia, was unable to recover following her bout with the tainted Mysis. All my condolences on your loss.
Yes, assuming that the bed batch of Mysis that had thawed en route to you was spoiled or contaminated with bacteria, it’s certainly possible that the tainted Mysis could have caused her demise if she ate too much of it. As you know, H. fuscus seahorses are considerably smaller than H. erectus, and if Olivia was the most voracious eater and consumed more of the bad Mysis than the other seahorses, it may have had a greater effect on her and caused her death. As we discussed previously, most of the time the symptoms caused by eating tainted Mysis are similar to a mild case of food poisoning. It will cause loss of appetite and the seahorses will go off their feed temporarily, but usually manage to recover from a mild case. But just as severe cases of food poisoning can be fatal to humans, getting too much contaminated Mysis can cause more severe problems and can be potentially fatal.
It’s difficult to determine exactly what is going with Oscar at this point. If he and Olivia have been courting heavily for the past month, it’s certainly possible that he could be pregnant and that his huge belly is the result of a brood pouch full of developing young. Pregnant seahorses typically become more lethargic and reclusive as their pregnancy progresses, and it’s not unusual for them to go off their feed and miss a meal or two or to fail to show up at the feeding station at feeding time, particularly late in their pregnancy. But if it’s his abdomen that is swollen and distended rather than his brood pouch or marsupium, then that could be related to ingesting spoiled Mysis. And losing a mate is always a traumatic experience for pair-bonded seahorses, so the loss of Olivia could also be affecting his behavior. (If so, the negative effect of losing his partner is temporary, and he should eventually return to normal and be his old self again.) Other than pregnancy, the only other time when a male’s brood pouch is enormously swollen and distended is when gas builds up inside during episodes of pouch emphysema, and that is always obvious due to the associated problems with positive buoyancy. Since Oscar isn’t having any buoyancy problems and he and Olivia had been actively courting recently, my best bet is that Oscar may be pregnant, but there’s no way to know for sure at this stage…
About the only thing you can do for us to right now is to wait and see what develops. The gestation period for H. fuscus is typically around 14-18 days, and if his pouch is enormous and Oscar is really pregnant, he must be due to deliver anytime now. If he doesn’t deliver within the next couple of days, that will be a good indication that something else is bothering him and we can proceed accordingly. There isn’t any treatment for food poisoning or ingesting spoiled Mysis, so we’ll just have to try to treat his symptoms if it turns out that he is not pregnant after all.
Nitrate levels above 20 ppm can become stressful for seahorses, but the nitrates would have to be extremely elevated to actually cause the death of a seahorse and the affected individuals would be showing obvious symptoms of respiratory distress prior to their demise.
At any rate, Gidget, I would not consider getting another female fuscus to replace Olivia at this time. If Oscar is pregnant, you can consider finding him a new mate after he delivers. He would have no interest in a new female while he was gestating in any case, so let’s wait and see what develops in the meantime.
Best of luck with your seahorses, Gidget. Here’s hoping that Oscar is just exhibiting the typical finicky behavior of a broody male and will soon be giving birth.