Oceanic dispersal systems are very poorly understood. H erectus in Barnegat Bay are often able to select their preferred temperature because of major temp differences in small contiguous areas. There are a number of reasons for this, including the outfall of a nuclear power plant and the nature of the bay itself, but this is irrelevant. It is very clear from what I have observed over the years that given a choice erectus seem to prefer the high 60s F to low 70s. They will congergate in that 67 to 72 band every time, assuming comparable cover, eel grass and food. They become sluggish in the high 50s and dormant, more or less, in the low 50s.
I suspect that there are specific geographic populations that account for the genetic and behavioral variability you mention. H erectus in places like Tampa Bay, which I am familiar with, are probably a more static inbred population, with very limited recruitment from the outside. They also have a longer growing period than their northern cousins, and markedly different habitats.
It's possible that the sudden appearance of hordes of seahorses in summer in local northern waters reflects primarily a native self-sustaining population, but I doubt it. Barnegat Bay, my home base, is shallow, heavily bulkheaded and limited in terms of suitable habitat. All sorts of mechanized fishing, including excavation of crabs wintering in deep water mud bottoms fails to turn up a single seahorse. Trawling eelgrass for grass shrimp will produce zero seahorses in May. Where can they be? The bay is relatively small and has been thoroughly examined and explored. There is almost no cover, no vegatation in winter. But anything is possible. We know so little about so many things.
I have seen seahorses in substantial numbers a couple of times far off shore, riding currents that are wamer than the surrounding waters, heading west toward land. Once, about 50 miles off Barnegat Light I was able to snorkel in a giant patch of clear blue water. The water beneath me was about 400 feet deep, and I could see down into it more than 10 meters. There were floating weed patches, a few Loggerheads, all sorts of wonderful things. The water was about 70F in June. I saw a shoal of C. shoepfi, Spiny Boxfish, possible 100, all about an inch long, and numerous drifting seahorses, essentially planktonic, of various sizes, but mostly small and thin, perhaps 2 inches on average. Tiny Jacks, filefish, a bewildering variety of fishes. Schools of predators were having a field day. That was a magic experience I'll never forget. Seahorses, btw, are eagerly eaten by predator fishes.
On the other hand the bay is alive with tiny newborn seahorses all summer long as the resident seahorses, much more active than the ones I see in Florida, mate incessantly, producing at least three batches of young over the summer/early fall. There are places where simply filling a 5 gallon bucket will get you a tiny seahorse or two as often as not.
It could be that the physical differences between Tampa Bay and Barnegat Bay erectus reflect different populations because those found in the north are recruited from areas other than Florida and the US east coast. Perhaps the outer edge of the stream, the Bahamas, even the Sargasso. Nobody can say for certain, but it's a fascinating question.
Nevertheless, I'm reasonably sure there are no permanent populations north of Long Island and Rhode Island. Whether they exist over the winter in places like Long Island Sound and Barnegat Bay is unknown. Perhaps some do, and others, related to the same genetic stock, are carried in from offshore every summer. So much beauty and mystery in the sea.
