As long as your new seahorse is able to swim in its normal upright swimming posture, can easily descend to the bottom of the aquarium, can swim against the current without difficulty, and displays its usual mobility and agility when chasing down food, then it’s unusual late-night swimming habits are probably nothing to be concerned about.
Seahorses are sometimes quite active in the late evening and after lights out. They have a specialized retina especially rich in rods, which gives them excellent visual acuity under twilight conditions and low light levels in general. Pair-bonded seahorses typically perform their daily morning greeting ritual at first light shortly after dawn and often conduct most of their courting and breeding in the early morning hours under twilight conditions.
In fact, seahorses see so well under low-light conditions that certain species (e.g., Hippocampus comes, H. ingens) are believed to have adopted nocturnal habits in order to escape heavy fishing pressure during the day. For instance, field research indicates that wild Tigertail seahorses (H. comes) are now strictly nocturnal in parts of the range. During the extended study period, Tigertails were never observed during the day, despite the fact that the divers had identified and marked the home ranges of the test subjects and knew exactly where to look for them (Perante et al, 2002). The seahorses apparently spent the day hidden among the corals, emerging from crevices among the corals at dusk to feed overnight. At night, each seahorse usually positioned itself on the same part of its particular holdfast and remained there for hours at a time ambush feeding (Perante et al, 2002). They then moved back into the corals at dawn, where their activities remained unknown. During the study, each seahorse maintained a small home range centered around a particular holdfast, and typically remained within 1 meter of its preferred hitching post (Perante et al, 2002). The seahorses were very familiar with their home range and favorite hitching posts and had no trouble navigating or feeding during the night.
So I don’t think that Spike’s nocturnal wanderlust is too unusual. Seahorses can be active swimmers after night under some circumstances, perhaps because they feel safer moving around under the cover of darkness, and it is customary for them to perch high up, well above the substrate, when they are settling down for the night. After lights out, seahorses naturally gravitate toward taller hitching posts and usually spend the night perched well above the substrate. Rudie Kuiter and many others, myself included, feel this is instinctive behavior and that Hippocampus does this to avoid nocturnal bottom-dwelling predators, such as crabs and lobsters, that are known to predate seahorses.
Providing Spike does not develop any symptoms of positive buoyancy, he is probably just exploring his new surroundings in the evenings. But when a seahorse curls its tail beneath its body, it is normally shifting its ballast and adjusting its center of gravity so that it can swim downwards, and if Spike is curling his tail while he is bobbing around at the surface, that suggests that there may be up-swelling currents near the top that he must struggle against. Otherwise, it sounds as though he is struggling against the tendency to float at these times, but that doesn’t make sense to me because such buoyancy problems typically do not come and go; rather, they tend to get progressively worse as excess gas continues to build up within the swim bladder or pouch of the seahorse. So it’s beyond my experience for a stallion to develop positive buoyancy problems in the evening and the floating at the surface, and then to be perfectly fine the next morning and for the next day or two, only to be struggling against the tendency to float at the top again the following evening. My best bet is therefore that spike may be encountering uplifting water currents when he ventures to the top of the tank during his evening explorations, but I would keep a close eye on him for any signs of gas bubble syndrome in any case to be on the safe side.
As shown in the picture below, when seahorses are swimming and want to ascend, they will lift their heads and extend their tail out straight beneath them and hold it extended in order to shift their center of gravity and make it easier for them to rise.
Photo by Alexander Sobolewski
Likewise, when a swimming seahorse wishes to descend in the water column, it will tuck its head and curl its tail beneath it in order to shift its center of balance and make it easier to swim downward, as shown in the following photograph:
Photo by Alexander Sobolewski
Best of luck with your seahorses, Scott. Let us know of immediately if Spike develops any lasting problems with positive buoyancy or any other indications of a developing problem with gas bubble syndrome.