Close brush with a comb jelly

NOAA discovers new species based on video evidence

Duobrachium sparksae. A new species of ctenophore first observed in 2015 by a NOAA Office of Ocean Exploration and Research team north-northwest of Puerto Rico. Source: NOAA Ocean Exploration web site [Click link to visit NOAA web page].

In April, 2015, 38 km north-northwest of Puerto Rico, a NOAA expedition, using an unmanned, remotely operable vehicle, encountered three individuals of a most unusual comb jelly.  Found near the seafloor, at about 3,900 meters below the surface, the specimens were unlike any seen before.  The comb jelly was somewhat rectangular and oblong in shape.  On the end opposite the mouth, it had two thick, conical tentacle arms giving it the appearance of an expanded pair of trousers.  Its thin tentacles were as long as the body and retractile.  Further study of images captured confirmed that the species was not easily assigned to any known genus, much less species, and, even more, it could not be placed confidently anywhere within the existing classification of the phylum Ctenophora.  This was a truly exciting discovery.  It was the new species now known as Duobrachium sparksae.  The genus name refers to the two-branched tentacle arms, and the specific epithet honors the wife of one of the authors.  This remarkable animal may be seen in a NOAA video. [Click here to visit site with video]

Ctenophora are invertebrate marine animals superficially resembling, but not closely related to, jellyfish.  They vary tremendously in size from a millimeter or so to about five feet in length.  You may recall cilia, movable hairlike structures, from high school biology, perhaps watching a protist like Paramecium caudatum use them to swim about on a microscope slide. In ctenophorans, cilia are consolidated into comb-like bands.  Most species possess eight of these linear combs, and ctenophorans are the largest animals that use cilia as a means of locomotion.

Although rare, fossils indicate that comb jellies have been around since the early Cambrian, 525 million years ago.  Their relationship to other animals remains hotly debated, but they represent an early branch in animal phylogeny, diverging either before or after the sponges.  Among known ctenophores are an impressive diversity of forms and habits.  Cydippids are described as egg-shaped with long, paired tentacles. Lobates have the body divided into a pair of lobes, with less conspicuous tentacles located at the corners of the mouth.  And beroids have no tentacles at all and feed on other ctenophores by means of an exceptionally large mouth equipped with stiff cilia that functionally resemble teeth.

Most ctenophores are bioluminescent, but the light they emit is faint blue or green and only visible in darkness.  Planktonic species are often described as having a brilliant rainbow appearance.  This is created by the scattering of light as their combs move, rather than bioluminescence.  Ctenophora occur worldwide, from cold waters at the poles to warm seas of the tropics, from shorelines to open ocean, and from the surface to extreme depths.  Nearly all ctenophores are predatory.  Collectively, their prey varies widely, including other ctenophores, plankton, crustaceans, molluscs, and larval fish. 

Fewer than two hundred species of comb jellies have scientific names, even though it is well known that others exist, including some encountered frequently enough to have common names.  Working at great depths in the ocean, where most species are found, is incredibly challenging, presenting limited opportunities to observe or collect these animals.  Further, as the common name jelly implies, their soft bodies are not easily preserved in a manner that makes their structure easily studied. The latter problem is not new to taxonomists, of course.  Using drawings or photographs in lieu of specimens should be avoided if at all possible and, in most instances, the best practice is to delay naming new species until such time as preserved specimens do exist.

With so many species undescribed, we have no way today to anticipate which morphological features will prove critical to studies of ctenophora in the future.  The attributes that make this new species utterly different from all others now could turn out to be shared by many species as knowledge grows.  And the discovery of some other new species might suddenly make a bit of morphology that seems uninformative today of great significance.  This is why preserving type specimens is important; so that they can be reexamined again and again as knowledge grows.  And why a photograph is a poor substitute for a specimen.  The latter can be examined from many angles, using different instruments, and even dissected if need be to determine its attributes. Species are hypotheses and preserved specimens are the means by which scientific names are anchored to physical evidence, and the means by which their status can be repeatedly tested. 

I agree with the scientists who described this comb jelly that naming species in the absence of museum specimens should neither be encouraged nor frequently done.  I agree, also, that their unique circumstances justified their departure from this rule.  It may be many years before a specimen is actually collected and the fantastically different morphological form of this animal deserves to be made known to encourage additional studies and efforts to secure and preserve specimens.

Declaration 45 of the International Code of Zoological Nomenclature states that preserved specimens should stand behind new species whenever feasible, but cautiously allows that photographic evidence, when justified by special circumstances, such as when capture or preservation of specimens is infeasible for technical reasons or conservations concerns, or when specimens must be destroyed to reliably diagnose a species.  This NOAA discovery is a posterchild for such the judicious recognition of such special circumstances and its thoughtful approach is a model for the responsible use of this exception.

If you will pardon the pun, in order to fully understand early animal evolution, the diversity and morphological details of comb jellies cannot be brushed aside.  Increased knowledge of ctenophorans is key in this respect and the discovery of Duobrachium sparksae represents an important next step for bringing animal phylogeny and marine biodiversity into sharper focus.

Further Reading

Ford, M., Bezio, N. and A. Collins (2020) Duobrachium sparksae (incertae sedis Ctenophora Tentaculata Cydippida):  A new genus and species of benthic ctenophore seen at 3,910 m depth off the coast of Puerto Rico.  Plankton Benthos Research 15: 296-305.