[Coral-List] Coral immortality
fish2live at acsalaska.net
Wed Mar 23 19:21:15 EDT 2011
I'm so glad that people are talking about corals again! Even better, somebody finally expanded the discussion to the rest of the world's corals. The hermatypic slant at times feels a bit limited in scope.
Owen, your mention of bioerosion points toward many of my own observations and hypotheses about the life cycles of Alaskan Stylasters. They are stony too, though they are Hydrozoans, and they clearly suffer from some erosive process. They also recolonize eroded portions of the corallum.
I am curious about coral life spans from the perspective of an acceptable level of fishing pressure. There was a minor explosion of attention to deepwater Alaskan corals about 8-9 years ago, and some environmental groups were making claims of extreme longevity. That got me started reading the literature and looking for any discernible clues in all the pieces brought up by fishing boats.
While I admittedly have very little hard evidence, I've come to the conclusion that corals are both mortal and immortal, depending on how you frame the debate. The individual polyps certainly must die, inferred from the numerous Stylaster pieces where the remains of polyps partially protrude from new growth. Whether they died from senescence, disease or predation is an open question, but for me there is an even bigger question: Is the living tissue the same thing that built the earlier polyp?
There seems to be a presumption that a discrete piece of tissue builds a discrete corallum. To the contrary, look at Andrews et al, Hydrobiologia 471(1) pp 101-110 which indicates multiple settlement events on a Primnoa fan. As an analogy, nobody who lived in Paris in 1800 is still alive, but Paris remains as a mostly healthy and expanding organism of sorts. Andrews' corallum could have been recolonized by its own creator after an attack or episode of disease. It could just as easily have been recolonized by a genetically compatible relative who lived nearby. For an example of neighborly harmony, I think of a paper with the distinction of Absolute Best Title Ever, "Behind Anemone Lines" Ayre & Grosberg, Animal Behaviour 70: 97-110, which shows anemones to be tribal little beasts.
OK, so a coral could be more like a city than a single clonal creature, and you may not even realize it by doing DNA analysis. How old is it then? Do you even have an idea of which part you should be aging? This becomes a huge dilemma, because you need to keep in mind that Alaskan fisheries are well managed for the health of the species, but we're pretty eager to scoop up the individuals.
On the road to bioerosion, we should also touch on growth rates. Most animals (excepting Costco-fed Americans) have a fairly asymptotic growth pattern. It's fairly easy to find evidence of asymptotic growth for lots of animals, but I know of no evidence that corals follow this pattern. Indeed, I could argue the contrary, that these are animals that grow ever more mouths as conditions allow. If corals have asymptotic growth, isn't it conceivable that the asymptote would graph as a vertical line (with respect to time) rather than the traditional horizontal? I'd like someone to show me that corals add less mass as they age.
If corals were truly immortal it seems unlikely that there would be fossils of extinct species. Clearly they will eventually die. While there are undoubtedly many things that kill them, one big factor is bioerosion. It's pretty ordinary to find a Stylaster fan being undermined by an eroding base. It's equally common to find other parts chewed up and eroding away; some Stylasters take on wild grotesque shapes as they age. Sooner or later though, some insult will intrude on the integrity of a primary support and the party will be over. Some corals like our giant Alaskan Primnoas may set themselves up for bioerosion indirectly, as they seem to have a penchant for attaching themselves to clusters of giant barnacles. Since corals tend to show a preference for growth up and away from the base, it may well be inattention to foundation accretion that eventually dooms many of them.
Now, to make the bioerosion hypothesis more intriguing I can add some facts/myths about global warming. We're all worked up about the aragonite saturation state changing in the world's oceans, but I was told several years back (yes, "told"—no documents in my possession to verify this) that the aragonite horizon in the Aleutians is already up around 120 meters. All these eroding Stylasters that I've seen come from 300-800 meters or more, so either the shallow Aleutian aragonite horizon is a myth or Aleutian Stylasters are already fighting an enhanced erosion rate. I do have a list of aragonitic vs calcitic Alaskan Stylasters.
In the end, it seems that immortal or not, a great many corals have life spans that are determined by factors other than senescence but the tools you'd use to determine the answer are going to be misused or misunderstood. DNA and isotope analysis may both miss any history of previous occupants.
As a final oddity, my rarest piece of Stylaster coral—a single piece, of a species described from a single piece—has no discernible base. It has no up or down or any evidence of its orientation as it sat on the bottom. It was a healthy pre-spawner when I found it. A lost base is not necessarily a death sentence.
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