Seamount biogeography

gulf_alaska_seamounts_sm2.jpgWe don’t mention it often, but Craig and I publish regularly outside Deep Sea News, in the public arena of peer-reviewed scientific literature. Craig authors ~3 scientific journal articles per year since 2004. I author ~2/yr. The last two years were above average for both of us. This is amazing to me, because we spend so much time writing for DSN. Either our cups runneth over, or the glacial pace of scientific publishing obscures the impact of our extra-curricular reporting activities here at DSN. I tell you this so you know we are contributors to the field, not only journalists. We make the news and break the news.

The thing I like most about Craig’s work is that he’s a “big picture” guy. He likes to tackle sweeping ecological theories like the Island Effect, where big animals evolve smaller body sizes (think dwarf elephant) and small animals evolve bigger body sizes (think giant isopod). He likes to wrestle with broad hypotheses about seamount biogeography, wherein pundits conjecture about whether seamounts are more like isolated “islands” or interconnected “stepping stones”. Craig’s most recent “big-picture” paper was a guest editorial in the Journal of Biogeography entitled “Seamounts: identity crisis or split personality?” The paper is important because it helps to frame the debate about seamount biogeography. A brief review is given below.

A seamount is a large isolated submarine feature with relatively steep slopes compared to the surrounding seafloor. By the earliest definition, seamounts have more than 1000m vertical relief, so anything smaller would be considered a “mound”, a “rise”, or a “bank”. However, many biologists would tell you the 1000m relief definition is overly strict, because the benthic assemblages on these features can be similar. Nearly 14,000 seamounts have been identified using satellite altimetry and gravity data. Commercial and recreational fisheries target these features because of high productivity, and conservation groups have rallied around them because of localized depletions of long-lived fish species and high levels of non-target by-catch in bottom trawls and longlines.

One important question is whether these seamounts represent “islands” of isolated populations, whether they act as “island chains” of interconnected metapopulations, or whether they function more like “oases” for broadly dispersed and highly migratory species. In the last scenario, high productivity yields high biomass, which supports high species richness.

Of course, part of the answer depends upon which taxon you’re talking about. Craig cites different results depending on whether you are looking at bivalves, crabs, or corals. He notes that claims of seamount endemism are not well supported because the deep-sea often yields new species. Therefore, unless we have a good understanding of endemism in the surrounding abyssal plains and continental shelves, we have little to compare.

Consider collecting a single sea anemone that turns out to be a new species. How do you know it’s endemic? If you collected and preserved the only specimen known to science, for all we know, the species could now be extinct. Seamount endemism must be considered within the context of an overall deep-sea endemism. For instance, endemism at hydrothermal vents in nearly 75%. How do seamounts compare? Read the article to find out.

Ultimately, Craig provides 10 recommendations for further research, including a need for better biomass estimates, more molecular studies, more comparative studies of seamounts and non-seamount habitats, and an appropriate sampling design to test hypotheses of seamount endemism. This list will prove useful in coming years, because new research vessels will be coming online, and new shared enterprises will be forming using state-of-the-art technology. Census of Marine Life on Seamounts (CenSeam) is just one of the groups we need working on these problems.

There’s a lot of seamounts out there, folks. You can’t stop wondering… are they more like islands, or more like oases, or are they something else entirely?

9 Replies to “Seamount biogeography”

  1. Fascinating stuff- I couldn’t access the actual article since it is subscriber only, but I think your summary is plenty for my layman’s eyes.
    Is there a seamount analog to the Island Effect?
    I suspect future research will discover that seamounts offer a mixture of oasis environments and shelters for unique endemic permanent populations. And no doubt we will discover some surprising animals living there.

  2. No one has ever looked for the island effect with seamounts, vents, seeps, or whale-fall communities, but definitely and interesting question. As far as seamounts having endemic faunas…I think it is highly unlikely to be the case. Perhaps for the remotest seamounts in the middle of the Pacific. Ultimately more research is needed to test the extend to which any general theory can be applied to all seamounts.

  3. I haven’t read the paper yet, but certainly will. The problem I see with the statement “As far as seamounts having endemic faunas…I think it is highly unlikely to be the case”, and this goes back to the recommendation Peter wrote about for a better sampling design, is that seamounts are vertically stratified. Different species inhabit different zones and some species utilize the whole vertical habitat. Additionally, they are stratified with respect to prevailing currents. I would expect high species richness and a different community composition on areas of the seamount exposed to the prevailing current, and lower richness on the opposite side; the seamounts “shadow” as it were.

    So depending on where sample and what depth you sample along the seamount, you might get different answers. I haven’t read your sampling design yet so I don’t know if you have or can take this into account (i.e.due to logistical constraints). I would also hypothesize that seamount-abyssal plain shared species richness would decrease as you move up the seamount from its base (the vertical stratigraphy problem again).

    So I wouldn’t be surprised to see endemism at sea mounts, especially with sessile fauna, like corals, anemones, barnacles, bivalves and the like. But sampling design would be complex to really get at it and logistical constraints such as ship and submersible time may be the limiting factor to get statistically viable results.

  4. Gage and Tyler’s book Deep-sea Biology mentions a very interesting hypothesis that shallow seamount peaks should have more endemics and deep seamount peaks should have more cosmopolitan species. That would be a testable hypothesis that’s simplifies some of the confounding variables Kevin mentions.

    However, in Craig’s opinion, comparable sites off seamounts would be necessary to “confirm endemism”. I am not sure how one would do that, though, given some presumed peak effect.

  5. It is interesting that much of deep-sea science has focused on depth as a surrogate for several stratifying agents, but little is done with respect to depth and seamounts. Whereas, it is likely that depth imparts zonation on a seamount, I am still unconvinced, and there is little evidence for it, that seamounts host a unique community, regardless of taxon or guild.

  6. This may be oversimplification and/or ignorance, but I would expect high rates of gene flow between seamounts (thanks to gamete and larval dispersal mechanisms) and thus little endemism. If anything, I might expect a more rapid turnover in species composition. Are seamount communities relatively stable?

    I once knew something about Wright’s Fst, stepping stone vs. isolation-by-distance, etc, but I have been away from it for a number of years now so it all seems fuzzy. Maybe I should bone up on it. You know, just for fun.

  7. Jim,
    You have hit the nail on the head. Some have proposed distance and/or and localized current regimes around seamounts in effect isolate them, thus high levels of endemism. But this appears not to be the case. Neither of these prove to be an effective barrier to dispersal.

Comments are closed.