No Bones

A Close Up Look at Deep Sea Calcified Hydroids: An Evolutionary Success Story

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). 

Fig. 1. Cover and back pages of recently published paper on New Caledonian stylasterids.

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. 

Fig. 2. A gastrostyle (SEM, magnification x288).

Fig. 3. A dactylostyle (SEM, magnification x474).

Fig. 4. Longitudinal section of a cyclosystem and its gastropore tube (SEM, magnification x 28).

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). 

Fig. 5. A cycloststem, showing 3 polyp types (SEM, magnification x80).

Fig. 6. A polyps with a large female ampulla (SEM, magnification x 47).

Fig. 7. Linear-imbricate coenosteal texture of branch surface (SEM, magnification x 700).

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.

Fig. 8. A fixed cyclosystem lid (SEM, magnification x 36).

Fig. 9. A free gastropore lid of species illustrated in figure 1 (SEM, magnification x 139).

Fig. 10. Lateral view of a detached gastropore lid from figure 9 (SEM, magnification x 200).

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. 

Fig. 11. Curious arrangement of dactylopore spines so as to resemble a frog’s face (SEM, magnification x 122).

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).

By Stephen Cairns

Unraveling the Evolutionary Relationships among Sea Slugs Using Genomic Data -- Just Getting Started

My colleagues and I (including Adam Bazinet and Michael Cummings of the University of Maryland and Allen Collins of NOAA/NMNH-IZ) recently published a paper in the open-access journal Royal Society Open Science. (And it got the cover! --Woohoo!) The title of this paper is “Relationships within Cladobranchia (Gastropoda: Nudibranchia) based on RNA-Seq data: an initial investigation”, and it focuses on the gastropod group Cladobranchia, a clade of nudibranchs (shell-less marine snails) that can be found worldwide. From a handful of these sea slugs, we sequenced whole transcriptomes to generate a set of orthologous groups (a.k.a. genes). We then analyzed these data to create a hypothesis of the evolutionary history of Cladobranchia. This is the first study that uses transcriptomes to study the phylogenetics of nudibranchs, and my first genomics project! This is also the second chapter of my dissertation.

Figuring out the evolutionary history of this group has often been a real challenge. Identifying cladobranch relationships and defining subgroups, using either morphological or molecular data, has always been problematic. Molecular-based attempts have used just a handful of genes commonly used in nudibranch systematics, and these markers were just not informative enough to generate a strongly supported evolutionary hypothesis for Cladobranchia. Morphological hypotheses were just as inconsistent, and some groups were defined by what they didn’t have, rather than the characters they did. Our work shows that these issues have now become less difficult to deal with when using RNA-Seq data, and our paper illustrates that we can now get a better idea of the evolutionary history of sea slugs. 

The nudibranch Melibe leonina from Morro Bay, CA, USA. The red arrow is pointing to the large oral veil of the animal.

One group in particular, the genus Melibe, has been really difficult. This group is made up of slugs with a very large oral veil (indicated by the arrow) that they use to trap their crustacean prey. In morphological phylogenies, Melibe was considered a definite member of Cladobranchia, but in molecular phylogenies it was all over the place. This group had really long-branch lengths (indicating a lot of divergence from the rest of the taxa in Cladobranchia, and in one study Melibe was found to be outside of Cladobranchia all together! Through this new data, though, Melibe is back within Cladobranchia and fits very nicely with the rest of the members of this group.

I’m really excited about what this study means for the field of nudibranch phylogenetics, and I’m looking forward to including more data in the future. New data are already in the pipeline, so keep a look out for the next paper!

by Jessica Goodheart [Edited a bit by Allen Collins]