It’s hard to miss, standing tall above the general canopy height of corals on a Caribbean reef, looking somewhat like an ancient castle with multiple turreted towers. If a castle had been designed by corals, that is. Multiple tall columns, sometimes more than 10 cm in diameter, unbranched and extending as much as a couple of meters above the base of the colony; it is distinctive indeed. Although conspicuous, it is also rare in most of its range. You rarely see more than one or two on a dive. Distinctive, refreshingly different to the other corals, rare, rewarding, really a bit of a unicorn.
Dendrogyra cylindrus, a multi-turreted castle on a Caribbean reef. Photo © Paul Humann/Corals of the World.
From an evolutionary perspective, Dendrogyra has few close relatives among other living corals. D. cylindrus is the only species within its genus and Dendrogyra is one of just four genera within its family, the Meandrinidae. All four genera occur only on Caribbean reefs, and two of the other three genera are also ones with just one species. When you see a pillar coral you know you are seeing something special. To put this into perspective, humans are the only living species within our genus, Homo, but our family, the Hominidae, includes three other genera: Pongo, Gorilla, and Pan. There are three Pongo species (orangutans), two Gorilla species, and two Pan species (the chimpanzee and the bonobo). So far, some similarity, but humans are hardly unicorns; we are depressingly common all over the planet.
Architecturally, with its tall, stout columns, Dendrogyra is unlike any other coral. Its also one of the few corals in which polyps are usually expanded during the day. The long tentacles, all extended and waving about, give Dendrogyra a decidedly woolly appearance. In fact, come to think of it, Dendrogyra resembles a wookiee more than it does a unicorn (but then, wookiees are unicorns, too).
Dendrogyra also has a behavioral trick up its sleeve that can surprise and delight the passing diver. Like all corals, Dendrogyra lacks a nervous system as such. Instead, it has a nerve net, a diffuse network of neurones scattered through its tissues that give it some ability to sense and to respond to its environment. While we typically think of the polyps as the individuals in a coral colony, the tissues of neighboring polyps are connected and the nerve net extends across the colony. Transmission of stimuli is generally rather slow in animals with a nerve net and signals die out rather quickly. If you touch one or two extended polyps in most corals, they will slowly contract and a few of their immediate neighbors may contract also, but Dendrogyra is far more reactive than this. Touch one or two polyps on a Dendrogyra column, or even just wave your hand to generate a pulse of water against them, and there is a rapid response from virtually all the polyps in the colony. The response is so rapid that you have to watch closely to see that it is, indeed, a wave of responses by individual polyps further and further away from the ones that have been disturbed. The retraction of the polyps changes the appearance of the columns. Suddenly their fluffiness is gone, and their color becomes paler. The overall effect is a sudden flash as the darker brown polyps are replaced by the paler surface of the colony between polyps.
A sudden flash! The first time I saw this it startled me, and I suspect it has a similar effect on predatory fishes who might be thinking of nibbling on some polyp tentacles. A few minutes later, the polyps slowly expand again and resume their feeding on plankton. Dendrogyra can perform this trick because its nerve net is more effectively interconnected than are those of other corals. Unicorns often have magical tricks up their sleeves.
In 2002, when I was teaching my coral reef field course at Calabash Cay, on the eastern side of Turneffe Atoll in Belize, I was cruising along a reef with two novice undergraduates in tow, doing my best to periodically point out to them some unique feature or another (and lamenting all the while about the near impossibility of underwater conversation while on SCUBA). I spied a towering Dendrogyra off in the distance and swam towards it, beckoning them to follow. I pointed to it and with sign language tried to indicate that this was something special. Being novices they were finding everything special, including just managing their buoyancy and their breathing. Once I thought I had their attention, I swam forward and touched the Dendrogya. It did its trick. Big flash. I was impressed once more, but when I looked around, both my disciples were looking in other directions. They had missed the magic, and Dendrogyra would not perform again for several minutes. Back on shore I tried to tell them about it, but that was far less impressive than seeing it for themselves.
Dendrogyra column with its polyps retracted. Photo © Douglas Fenner/Corals of the World.
Most unicorns are now extinct, if they ever existed, and Dendrogyra appears to be well down the path towards extinction. In 2008, it was classified as Vulnerable on the IUCN Red List. At that time its wide distribution across the Caribbean and its presence in several marine protected areas, coupled with lack of evidence of overall population decline were important factors in determining this classification rather than Endangered or Critically Endangered. But times have changed since 2008.
Reproductively extinct in 2013, locally extinct by 2020
In 2011, the state of Florida listed Dendrogyra as Threatened as did the US EPA in 2014. With this heightened concern funding for research appeared and, among other things, an effort was made in 2013 to locate all living colonies of this coral along the ~500 km length of the Florida reef tract from the Dry Tortugas to north of Port Everglades (Fort Lauderdale) and to track their growth and survival. That this was even considered possible testifies to how rare the species is.
The sampling effort involved a number of individuals and institutions. Papers published in 2021 by Karen Neely of Nova Southeastern University and colleagues, and by Nicholas Jones, also of Nova Southeastern and colleagues, provide the main results. Altogether, 819 colonies were found and their size and condition tracked. Genetic research, by Andrea Chan of Penn State University and colleagues, on a subset of 217 of these colonies that occurred in 51 sites along the length of the Florida Reef Tract had been done early in the project. That work revealed that apparently separate colonies within 70 meters of each other were invariably separate ramets of the same genet – that is, they were genetically identical clones of a single genotype. The 217 colonies sampled belonged to just 56 separate genets. Each genet occurred in only one site, but three sites contained two genets each and one site contained three. Extrapolating from these results, Neeley and colleagues estimated that the 819 colonies across the Florida Reef Tract represented about 190 ramets – that’s just 190 genetically distinct individuals of this species, a pretty small population. I’ll state that again. Throughout the Florida Reef Tract from the Dry Tortugas to north of Port Everglades, just 819 colonies of Dendrogyra were known in 2013. Genetically they comprised just 190 genetically distinct individual ramets. Nearby colonies were invariably asexually produced clones of a single ramet; no evidence of successful sexual reproduction was found.
The fate of these colonies was tracked from 2013 through 2020. The total of 819 colonies is effectively a total count of the species in Florida at 2013 although a small number of additional colonies may yet be found in the Lower Keys. Chan’s genetic analyses indicated that the population was essentially stable in size over the years prior to 2013, which conforms with other data suggesting this has been a rare species for a long time. However, bleachings late in 2014 and again in 2015 caused some mortality directly but likely also made these corals more susceptible to diseases. A new disease which impacts at least 20 species of Caribbean corals was first observed at Virginia Key, FL, in September 2014 following the first of these bleachings. Named Stony Coral Tissue Loss Disease or SCTLD, this highly virulent and rapidly spreading disease has substantially changed the population of Dendrogyra along the Florida Reef Tract, while also affecting several other species. As SCTLD spread north and south from Virginia Key all Dendrogyra colonies within its path experienced extreme mortality. All colonies in the Upper Florida Keys and further north were lost by the end of 2018. In the Middle Florida Keys, there was 98.2% tissue loss by end of 2020. In the Lower Florida Keys, Dendrogyra colonies were unaffected until SCTLD arrived in 2018; by end of 2020, they had lost 99.8% of living tissue. In Florida, a unicorn has gone away.
Rarity is Common
Rare species are the norm in rich tropical ecosystems. One of the mysteries of coral reefs is how so many extremely uncommon species manage to be present. How do they find reproductive partners so their species can continue present? Dendrogyra is one of those uncommon species, and its ability (like most corals) to reproduce asexually through fragmentation must be an important part of why it has continued to be present on reefs throughout the Caribbean until now. (Its lack of sexual reproduction on the Florida Reef Tract in the early years of this century is likely simply due to two facts: colonies are almost invariably either male or female, producing only eggs or sperm, and colonies from different ramets are seldom near enough to each other for fertilization of gametes to be possible.)
Rare species can be very susceptible to local extinction, because they only need to lose a few individuals to disappear completely. That is what has happened to Dendrogyra in Florida with the arrival of SCTLD. Dendrogyra is large and wonderful, and so we have noticed its disappearance, but what about all those other rare species on coral reefs that are not conspicuous – the myriad crustacea, and tiny gobies, and more cryptic corals? They could be going extinct locally before our very eyes as coral reefs degrade, without us noticing anything at all until it is far too late to do anything about it. And with a few local extinctions in different parts of its range, suddenly another coral reef species has disappeared forever. This is the silent, unseen process by which biodiversity declines.
Fortunately for Dendrogyra, efforts to collect gametes and rear resulting larvae in the lab have been successful. This opens two possibilities: artificial propagation for out-planting to increase numbers or replace lost individuals at selected locations, and long term cryostorage of viable gametes or larvae as a way of archiving the biodiversity they represent. Neither possibility solves the problems posed by climate change or SCTLD, but they buy time, keeping the species alive until these problems are behind us. It is sad to think we may be heading towards a world in which much of the biodiversity has been lost and much of what remains is kept in cold storage for a future that may not come. Remember that the next time you see a Dendrogyra, because, thankfully, they still occur elsewhere in the Caribbean. Rare, unusual, unicorns.