I’m sorry to say that I am unfamiliar with Hippocampus semispinosus. That could be because it is a new species of seahorse that has only recently been described and named, or Hippocampus semispinosus could be an outdated synonym for a seahorse that is now known by different species name…
The closest seahorse species that I know of is Hippocampus spinosissimus, which is a large tropical seahorse from the South Pacific ranging from Sri Lanka throughout Southeast Asia (including Malaysia, Indonesia, Vietnam, Taiwan, and the Philippines) and the Torres Strait in Australia.
Hippocampus spinosissimus is commonly known as the hedgehog seahorse due to it’s very well developed spines, which may either be blunt tipped or quite sharp. The spines bordering the pouch are extremely long in large males. It has a well developed crownlike coronet with four or five spines.
They are variable in coloration being either plain or having a pale background coloration with darker saddles across the back and darker cross bands on the tail.
They are inhabitants of coral reefs but are also found on muddy or sandy bottoms. Little else is known about their natural history in the wild.
Familiar seahorses that are the most similar to Hippocampus spinosissimus are Hippocampus barbouri and Hippocampus histrix, which has a longer snout and even longer spines.
The information above regarding Hippocampus spinosissimus was taken from the book Seahorses: an identification guide to the world’s species and their conservation., which was written by Lourie, S.A.; Vincent, A.C.J.; Hall, H.J. (1999, Project Seahorse, London, UK).
Best wishes with all your fishes, TJ!
Pete Giwojna, Ocean Rider Tech Support