[Taxacom] an interesting paper on Long Distance Dispersal

Les Watling watling at hawaii.edu
Sun Jul 8 10:42:04 CDT 2018

Hi Michael,
Thanks for the comment. I realize that I was probably much too cryptic in
my note about cumaceans. The issue is not that some species cannot survive
the abyss, we've known about abyssal cumaceans since the time of the
Challenger. The problem is the shallow species. They cannot descend into
the abyss, walk across it, and then re-emerge in shallow water. Well, for
the most part. Where this generalization breaks down is exactly where you
point to, at high latitudes. That is because around the southern end of the
world, and to a certain extent in the northern end of the North Atlantic,
the water is cold top to bottom. However, we did an analysis of cumaceans
collected at all depths around the Faroes a number of years ago and we
found there seemed to be a deep barrier at about 1 deg C, irrespective of

Most people think of depth = pressure as a barrier, but that is only once
depths of about 3000 m are reached. Shallower than that, most animals have
physiological difficulties because of temperature. In the Antarctic ocean
increases in temperature of about 2 deg C can exert major physiological
problems and so most species get moribund when brought to the surface of
the ship where temperatures could be 3 or 4 deg warmer than the seawater.
But places such as the South Sandwich Islands have temperatures from the
surface to several thousand meters depth that are essentially the same.
That is, no shallow thermocline to limit distribution. I would have to
check, but I would suspect the cumaceans at SSI are merely deep cold water
tolerant species that can live at a wide range of depths due to a lack
temperature gradient. Same as NZ as I note below.

So, for cumaceans, and I suspect also for other groups of direct developers
that have no swimming or rafting capabilities, there is no way for shallow
water dwellers to get to the shallow waters of some places, such as Hawaii,
even though the hot spot producing the chain has a very long history, if
you include the Emperors as well. As you note, it would be interesting to
see what other isolated volcanic islands show for this and related groups.

A counter example of small crustaceans that have managed to get to Hawaii
and have done very well, that is, are well-established and to a certain
extent have Hawaiian endemics are the amphipods and tanaids. They do not
swim well and have direct development so cannot disperse via larvae. But
both groups are quite capable of living on drifting seaweed, palm trees,
whatever floats. I don't know the total numbers but there are at least 100
amphipod species documented from Hawaii and the tanaids are just now being
looked at, but I think 15 or so species are known. For the amphipods, it is
clear that most of the families present are associated with seaweed in one
way or another. Some sediment-dwelling species are present, however. These
two groups of crustaceans have the same issues confronting cumaceans, but
they can live on drifting things whereas cumaceans most likely cannot.

On the other hand, shallow water cumaceans most likely do and can get
carried around by drifting micro-continents, etc. Some years ago I wrote a
short paper about the cumaceans of New Zealand. It got subsumed into a
larger multi-author Species 2000 volume put together by Dennis Gordon so
its hard to find. But here is the essence of the story. There are only 8
families of cumaceans known. But two are very under-represented in New
Zealand. All families show very high endemicity at the genus level,
indicative of their long isolation with subsequent speciation. In the case
of the deep-water (bathyal) species, most were new to science and most are
members of widespread deep genera.

So, with that I would just note that in looking at the distribution of any
group we need to be cognizant of their natural history and not posit
mechanisms for their getting around that clearly are inapplicable. I don't
doubt vicariance is important, and likely the major mechanism spreading
life forms around the planet, but it is not the only mechanism as these
isolated islands show.

Hope this helps,

Les Watling
Professor, Dept. of Biology
216 Edmondson Hall
University of Hawaii at Manoa
Honolulu, HI 96822
Ph. 808-956-8621
Cell: 808-772-9563
e-mail: watling at hawaii.edu

On Sun, Jul 8, 2018 at 12:03 AM Michael Heads <m.j.heads at gmail.com> wrote:

> Hi Les,
> You wrote: ‘certain marine taxa, such as cumaceans, which are small
> benthic crustaceans with almost no swimming ability and no larvae, have not
> made it to Hawaii. Most likely that is because they could only get there by
> travelling along the bottom, meaning they would have to crawl through the
> abyss.... not going to happen, temperature and pressure’.
> However, samples from the Kuril–Kamchatka Trench and the adjacent abyssal
> plain at depths 4830–5780 m included 72 species of cumaceans from 23
> genera and 6 families (Lavrenteva & Mühlenhardt-Siegel in *Deep Sea
> Research II*, 111: 301, 2015). This makes the absence from Hawaii even
> more interesting.
> I’m curious to know if there are other young, isolated volcanic islands
> that do not have cumaceans (a quick search didn’t turn up any reference to
> them in French Polynesia, Cook Islands etc.). They *are* present on the South
> Sandwich islands - young islands, surrounded by abyss, and ‘how some
> sedentary taxa (e.g infaunal tanaids and cumaceans) get there is a mystery
> still’ (Kaiser et al., Antarctic Sci. 20: 281. 2008). I think they probably
> ‘got there’ by migrating with the migrating volcanic island arc.
> Absences from Hawaii are often as dramatic as the better-known endemics.
> For example, the plant *Begonia* thrives more or less throughout the wet
> tropics globally, but is absent from Hawaii (although there are naturalized
> species). Its intercontinental distribution is attributed to dispersal
> across the oceans (Moonlight et al., 2018 J. Biogeog), so the absence from
> Hawaii would be attributed to *lack* of transoceanic dispersal, i.e.
> ‘chance’. (Nevertheless, the sister group of *Begonia* is a Hawaiian
> endemic, *Hillebrandia*).
> Hawaii is often assumed to have been colonized by all its terrestrial
> groups since 30 Ma, as no subaerial land is thought to have existed between
> 34 and 30 Ma. However, this depends on a method for calculating the former
> heights of volcanoes that underestimated their current heights by up to
> 1000 m, and so it probably underestimated the former heights also (see my
> Tropics book, p. 319).
> On Thu, Jul 5, 2018 at 5:41 AM, Les Watling <watling at hawaii.edu> wrote:
>> Apropos the recent discussion re dispersal vs vicariance. As a recent
>> paper
>> in PeerJ makes clear, the size of the dispersing organism matters. As does
>> mobility, and a host of other factors.
>> In the case of tardigrades it was long assumed that wind was the major
>> dispersion agent, but the authors demonstrate, as much as one likely can,
>> that bird feathers are an effective agent for something as small as a
>> tardigrade.
>> https://peerj.com/articles/5035/
>> Not a too-likely method for primates other than in sci-fi stories(!).
>> Primates probably could wander long distances, but why would they?
>> Especially if their needs are being met where they are. In which case
>> rafting on continental chunks might be what carries them around.
>> But I think bird feathers also work for seeds of some species, and
>> something as unusual as terrestrial amphipods. In Hawaii, some animals,
>> such as terrestrial amphipods have no likelihood of dispersing over the
>> sea
>> on rafts or other floating objects because of their osmotic intolerance to
>> sea water. On the other hand, we also know that certain marine taxa, such
>> as cumaceans, which are small benthic crustaceans with almost no swimming
>> ability and no larvae, have not made it to Hawaii. Most likely that is
>> because they could only get there by travelling along the bottom, meaning
>> they would have to crawl through the abyss.... not going to happen,
>> temperature and pressure. But 3 species of cumaceans have now made it,
>> most
>> likely in ship ballast water.
>> As with cumaceans, shallow water octocorals, a regular feature of most
>> tropical coral reefs, are essentially absent from Hawaii. There are a few
>> (maybe 4?) species of very small soft corals that can be found in shallow
>> pools or in water a few meters deep. But the normal reef habitat has no
>> octocorals. However, at depths greater than about 350 m, octocorals become
>> abundant and diverse, exceeding more than 100 species, and inhabiting
>> depths to over 3000 m. So the deep sea species have made it, easily, but
>> the shallow species have not. Low dispersal capability in the latter, and
>> long distance larvae in the former?
>> In the end, I think the debate needs to be more carefully circumscribed
>> with respect to the organisms. And, from where I sit, I see both
>> panbiogeography and LDD each explaining some patterns.
>> Best,
>> Les
>> Les Watling
>> Professor, Dept. of Biology
>> 216 Edmondson Hall
>> University of Hawaii at Manoa
>> Honolulu, HI 96822
>> Ph. 808-956-8621
>> Cell: 808-772-9563
>> e-mail: watling at hawaii.edu
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>> Nurturing Nuance while Assaulting Ambiguity for 31 Some Years, 1987-2018.
> --
> Dunedin, New Zealand.
> My books:
> *Biogeography and evolution in New Zealand. *Taylor and Francis/CRC, Boca
> Raton FL. 2017.
> https://www.routledge.com/Biogeography-and-Evolution-in-New-Zealand/Heads/p/book/9781498751872
> *Biogeography of Australasia:  A molecular analysis*. Cambridge
> University Press, Cambridge. 2014. www.cambridge.org/9781107041028
> *Molecular panbiogeography of the tropics. *University of California
> Press, Berkeley. 2012. www.ucpress.edu/book.php?isbn=9780520271968
> *Panbiogeography: Tracking the history of life*. Oxford University Press,
> New York. 1999. (With R. Craw and J. Grehan).
> http://books.google.co.nz/books?id=Bm0_QQ3Z6GUC
> <http://books.google.co.nz/books?id=Bm0_QQ3Z6GUC&dq=panbiogeography&source=gbs_navlinks_s>

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