DISCOVERING SPECIES
Presenting the descriptions that define the 42 recognised species of seahorse, from the woman who has written the book about them. Here she tells us just how species are determined
It was completely by chance that the first pygmy seahorse species was discovered. In 1969 Georges Bargibant, a diver for the Nouméa Aquarium in
New Caledonia, collected a gorgonian sea fan and brought it into the aquarium. Imagine his surprise when he noticed that there were two miniature seahorses, exquisitely camouflaged, hiding among its branches. When they died, his colleague Dr Réné Catala sent the specimens to Gilbert Whitley, curator of fishes at the Australian Museum, who described them in 1970 in Bargibant’s honour, as Hippocampus bargibanti.
The second pygmy seahorse to be described was H. denise. I was working on an identification guide to the world’s seahorse species when I received a picture from underwater photographer Denise Tackett. I did not recognise it as a known species; however, without specimens to confirm my hunch I could not give it a new scientific name. The chance to look for specimens came several years later when I was invited to join a trip on a liveaboard during my PhD fieldwork on seahorses in Indonesia. I had never seen these seahorses before, but I knew from Denise’s photographs the type of sea fan they called home. On every dive I scoured the reef for sea fans, and any sea fans for seahorses. Amazingly, on day five of the trip, my patience and diligence were rewarded. A pair of tiny, smooth, orange seahorses! I could barely contain my excitement. Of course I felt sad to collect them, but I knew that without specimens I could not describe them. I also knew that they would go down in history as
“type specimens”.
When a new species is described, every aspect of its morphology (body shape, size, features, etc.) is examined
and recorded. Where possible other characteristics are also noted, including ecology, behaviour, and even genetic data. The most important thing for a taxonomist (literally, a person who moves names around) to do is to provide a “differential diagnosis”, i.e., a clear explanation of exactly how this purported new species differs from all other known species. In addition to the diagnosis, they must also make a “description” in which they cover as much information about the new species as possible.
The original specimen that is used to make the description is called the “holotype”. If the description is based on more than one specimen, one is selected as the holotype, and the others are called “paratypes”. These “types” are reference specimens that are deposited in a museum where they are kept very carefully. If there is ever any question about the exact nature of the species, a researcher can re-examine the type specimens (even many decades or hundreds of years later). The oldest specimens that I have examined are dried ones from the collection of famous Swedish botanist Carl Linnaeus from the 1750s!
Sometimes, species are not so easy to tell apart. In 2003 I was sent several specimens and photographs of what appeared to be new species. For several years I struggled with the question of whether these specimens represented one, or more species. I tried many times, without success, to extract DNA from the little creatures in order to look at their genetic makeup. I measured them and made counts of their fin rays, body and tail rings. I noted differences among the specimens and between the specimens and previously described species. However I still couldn’t decide. What constitutes within-species variation, and what is truly diagnostic of a new species? This is one of the essential questions with which a taxonomist must always grapple. And sometimes we make mistakes.
In collaboration with Rudie Kuiter, an underwater photographer and fish enthusiast in Australia, I finally published a paper (in 2008) describing the specimens as three new species:
H. pontohi, H. severnsi and H. satomiae.
A couple of years later, I heard from my colleague Healy Hamilton that she had managed to get DNA from other specimens and proved conclusively that H. pontohi and H. severnsi are genetically the same. When two species turn out to be the same, the more recently described one is synonymised with the earlier name. In this case, both names were published in the same paper, but H. pontohi appeared earlier in the paper and thus remains the valid name, and
H. severnsi becomes a synonym.
This is what happens during a taxonomic revision. More data become available and our understanding of biodiversity changes. The classification of seahorse species has been confused for many years. My work has aimed to address this, but is still an area of active research. Seahorses are relatively similar in their overall features, yet they can show wildly different colours and develop and lose skin fronds depending on their habitat. Consistent differences are usually found in fin rays, trunk and tail rings, spines, and coronet development. The table below outlines some of these differences for the different species. The species are grouped roughly (as in my new book) by genetic similarity.