I agree with the guys from your local fish store — if you found a group of 14 seahorses washed up on the beaches of Long Island, they can only be Lined Seahorses (Hippocampus erectus), which are the native seahorses in that area. In fact, Hippocampus erectus was originally referred to as Hippocampus hudsonius, because it was commonly found in the brackish portions of the Hudson River.
In this case, we can be quite sure that the seahorses are not Short-Snout Seahorses (Hippocampus breviceps), which is a temperate or cold-water seahorse that is endemic to Southwest Australia. There is no way for H. breviceps to conceivably make the journey from Australia to Long Island, and if they did, they would not survive and breed in a warm water aquarium. H. breviceps is much smaller than H. erectus and has very different coloration and markings. So it’s safe to assume that the seahorses you rescued from the shores of Long Island that are doing well in your 20-gallon aquarium at standard aquarium temperatures are Hippocampus erectus, which just happen to have relatively short snouts for their species.
In other words, I would raise the offspring of the seahorses that you rescued the same way that you are raising the fry produced by the H. erectus seahorses in your 55-gallon aquarium.
Here is some additional information on the breeding habits of the Short-snouted seahorse (H. breviceps), Mandy, just for fun:
Breeding Habits: polygamous
Breeding Season: usually from October to January.
Gestation Period: 23-26 days.
Brood Size: average brood is about 35 fry.
Size at Birth: ~1/2" (14 -16 mm).
Onset of sexual maturity: 5-6 months.
Pelagic/Demersal (benthic): fry are primarily pelagic for the first 2-4 weeks although they will hitch occasionally.
Ease of Rearing:
Thanks to their high-capacity brood pouch, breviceps males give birth to an average of 35 amazingly large babies (1.4-1.6 cm). The newborns go through a pelagic phase but are able to eat baby brine shrimp from the first, making them intermediate in difficulty. Rearing H. breviceps fry is similar to raising H. erectus, but perhaps a little easier due to the smaller number of hungry mouths you must feed (a few dozen fry versus a few hundred newborns).
Inhabits weedy inshore waters (Ref. 7300) and occupies much the same habitat as the Weedy Seadragon (Phyllopteryx taeniolatus).
Hippocampus breviceps is similar to the dwarf seahorse (H. zosterae) in the fact that it is a colonial seahorse that rarely occurs singly or in lone pairs. It is typically found in groups that range from small aggregations to large colonies living in various weedy habitats. It is often found among patches of Sargassum attached to rocks on a sandy bottom (Ref. 9002), and the specimens that live amidst the Sargassum typically have elaborate, well-developed cirri along their heads and backs. In deeper water, H. breviceps may be found on sponge reefs.
Rudie Kuiter reports they feed close to the sand or rubble by day, hunting mysids that congregate along the sandy substrate. By night, however, they seek shelter well above the substrate to avoid bottom-feeding predators such as crabs, and often assemble in small groups high in the weeds (Kuiter 2000).
The pelagic fry have proportionally much longer snouts that the snub-nosed adults (Kuiter 2000). Interestingly, Kuiter has noticed that the fry which remain pelagic the longest tend to develop longer snouts than the fry which settle out and assume a benthic existence at an early age (Kuiter 2000). He reports that during the summer months large numbers of H. breviceps fry can be observed clinging to rafts of floating vegetation carried on the strong outgoing tides that occur near the full moon (Kuiter 2000).
Perhaps because it is found in group situations in which partners are readily available, H. breviceps is known to be polygamous both in the wild and in the aquarium, showing no mate fidelity. Egg diameter is 1.6 mm.
The following readings are based on Tracy Warland’s rearing program for Hippocampus breviceps at the seahorse farm in Port Lincoln, Australia:
Temperature = optimum 65°-70°F (19-21°C).
Specific Gravity = range 1.020 – 1.026, optimum 1.024
pH = 8.2
Ammonia = 0
Nitrite = 0
Nitrate = < 20 ppm
Suggested Stocking Density: 1 pair per 4 gallons (15 liters).
H. breviceps is a smallish seahorse best suited for small aquaria of 10-30 gallons (38-114 liters). Aquaria of this size can be set up as either a basic tank with undergravels (or sponge filters) or as a standard SHOWLR tank, and be should heavily planted with Caulerpa or macroalgae to simulate the weedy habitat this species prefers. A protein skimmer and UV are always advisable, but may need to be confined to a sump on small tanks. These are temperate seahorses, and I recommend using a mini aquarium chiller to keep the water temperature in the upper 60s to perhaps 70°F (19°C-41°C) at all times.
They would thrive as a colony in a species-only tank, but also make excellent tankmates for other Australian seahorses with similar temperature requirements such as H. abdominalis and H. whitei.
Juvenile Rearing Tanks:
Newborn H. breviceps can take newly hatched brine shrimp (Artemia nauplii) as their first food (Tracy Warland, pers. comm.) and are suitable for the "easy" rearing method. The fry undergo a free-swimming stage for 2-4 weeks and will thus do best in kreisel-style nursery tanks designed for pelagic fry such as those described earlier for rearing H. ingens or H. reidi. In-tank nurseries with drum-type, goldfish-bowl inner chambers that produce a circular flow are another good alternative for rearing this species.
All things considered, Mandy, you can be quite certain that your new wild seahorses are H. erectus, and even if they were H. breviceps, the two species are reared using the same techniques and methods, with the exception of water temperature. So simply raise the fry produced by your new seahorses as if they were H. erectus and you will be doing the right thing.
Best of luck with all your seahorses, Mandy!