One of the three groups of benthic cnidarians that I study is the family Stylasteridae. Stylasterids are essentially calcified hydroids, meaning that the hydroid has evolved an internal calcium carbonate skeleton. Only two of the 78 hydrozoan families have independently evolved a calcareous skeleton: the Milleporidae (fire corals) and the Stylasteridae (sometimes called lace corals).
The stylasterids comprise 314 species and 29 genera, making them the family with the second highest number of species and highest number of genera among the 78 hydrozoan families. Calcification clearly confers adaptive advantages to the styalsterids relating to protection of its polyps, as well as support for larger colonies.
Evolutionary experimentation since the Late Cretaceous has resulted in a bewildering range of colony shapes, as well as gastrostyle (Fig. 2) and dactylostyle (Fig. 3) shapes, gastropore tube shapes (Fig. 4), coordination of gastro- and dactylopores (the cyclosystem, Fig. 5), ampullar shape and placement (Fig. 6), coenosteal texture (Fig. 7), and the development of other protective structures such as fixed (Fig. 8) and hinged (or free) lids (Figs. 9,10), gastropore lips, and dactylopore spines (Fig. 11).
But calcification cannot be the only answer to the stylasterid’s evolutionary success, since the Milleporidae also have calcified skeletons, but they consist of only 13 shallow-water species. An important difference between these two families is that the Milleporidae is found only in shallow tropical waters where competition is fierce in the reef environment, whereas the stylasterids are primarily deep-water animals (as deep as 2700 m), exploiting the niche of slope-depth ridges and seamounts. I therefore hypothesize that the combination of the key innovation of a calcified skeleton and the exploitation of the deeper water environment has led to their evolutionary success.
This hypothesis may seem logical and even somewhat self-evident, but it took me 35 years of working with this group to reach it. And, of course, it is only an hypothesis, and I see no way to prove it one way or the other. You might think that this formed the basis for a break-through paper for me, but in fact it was just part of one page of the introduction of a recently published 361-page paper in which I describe the 99 stylasterid species that occur in the New Caledonian region (Fig. 1). I don’t think this is unusual in taxonomic revisions or monographs. Now, you may ask, where is New Caledonia and who cares about what stylasterids occur there? My partial response to that is that once the Philippines were thought to be the center of biodiversity for marine life, but now that the staff of the Paris Museum has made 35 expeditions to New Caledonia it has become clear to me that the highest diversity, the epicenter, of benthic cnidarians lies in the New Caledonian region. Thus, knowing the fauna of this region is the key to knowing the fauna of the entire Indo-West Pacific.
I have studied stylasterids for a long time, having named over half of the species, and sometimes I feel that I have “seen it all.” But, in the recently published New Caledonian paper alluded to above, 57 of the 99 species were undescribed, as well as two new genera. It is always fascinating to determine a new species or genus because by definition it has a novel character or new set of characters that have never been seen before. In this paper alone I discovered a species that is flexible (having an internal chitinous axis), one that has a green skeleton, the smallest known stylasterid (only 1.5 mm in branch diameter and less than 15 mm in height), a species with a dactylopore spine arrangement resembling a frog’s face (Fig. 11), and many, many variations on a theme. I have spent a good deal of time closely examining the stylasterid skeleton with the scanning electron microscope and finding dozens of new morphological characters that can be used to identify and distinguish species. I think that finding new characters (not species) is the most fulfilling aspect of my profession. (New Caledonia is about 900 miles north of New Zealand, the two island groups being united by the submarine Norfolk Ridge).