March 16 - On Silhouette


Silhouette has an interesting ecological history. Currently the entire island is protected as the Silhouette Island Special Reserve and one needs special permission from the Island Development Commission to live or even visit here. Up close, The Enchanted Island, a 10 km square remnant of Gondwana, is dramatic with its enormous granite cliffs jutting quickly from palm and takamaka lined soft sandy beaches. The steepness of the slopes rising to Mount Dauban, and Mont Pot a Eau, and the fact that the island is surrounded by a continuous coral reef kept the habitat from being replaced by extensive plantations and prevented its use as a port of call for the many pirates that plied the Seychellian seas in the late 17th Century. The higher elevations retain some of the largest tracts of virgin rainforest in the Indian Ocean

There are remnants of Arab tombs, and there once was a plantation of coconuts and breadfruit that provided for a population of about 1000 when the Dauban family colonized the island in the mid 1800s. However, the last Dauban left Silhouette half a century ago and only 130 residents remain. Counted among those are the Gerlachs who left England in the 1960s, having had enough of civilization, and settled in Mahe. Their son Justin was an avid naturalist from an early age (he has since received his doctorate from Oxford and is currently associated with Cambridge) and grew up collecting all of the strange and unusual critters that make their home here. Justin's father Ron established the non-profit Nature Protection Trust of the Seychelles and more recently the family moved to Silhouette in order to survey and manage the largely untouched biodiversity here. This includes a terrapin breeding project and their housing the only remaining females of the Seychelles Giant Tortoise Dipsochelys hololissa.

Justin and Ron have been extremely welcoming and helpful to us in the day and a half we have been here. Justin, of course, knows every invertebrate that makes its home on Silhouette having many times spent months on end camped out in the forests or traversing uncharted transects. As a result, we arrived to find that he had already collected a substantial number of Idiobdella prior to our arrival. What is bizzare about this is that they arrived in his malaise traps! These are simple contraptions that are part of the standard entomological toolkit in which one extends a fine conical net into the understory or canopy with a bottle of alcohol at its base. Most things flying then get stopped and many make their way down into the preservative. One need only set these up in choice locations and check them every couple of weeks to get a good sense of the diversity of winged insects. But leeches do not fly! What on earth are they doing in malaise traps?

We woke to an early morning rain. Perfect weather to whet our appetite for a hard morning climb towards Mont Pot a Eau. (We decided not to bother with the type locality, Mare aux Cochons as it was drained for a coffee plantation a decade ago.) Notwithstanding saturated ground and more snails than we could count, we had no success in finding live Idiobdella. But our discussions with Justin have been revealing.

Several times that he has found the leech in association with snails he has collected, he noticed them only after having returned to his lab at the NPTS Information Centre. This has us suspecting that the leeches are hiding inside the mantle cavity of the shells of gastropods. Moreover, when he has seen them inching around freely, it was on a protective tarp near a light at night. We are certain now that they are nocturnal, and that this explains our lack of success even with Mahebdella miranda. Late this afternoon, when the flying foxes begin their vigil overhead, we will return to our morning's destination with a hastily devised contraption of our own: a funnel inserted into a clear plastic tub. This we will sit in a hole in the ground. Next to this, underground, we will fix a headlamp that lasts for a 140 hours on new batteries. At the same time, this afternoon, we will sample as many of the arboreal molluscs we can find. Those snails and checking the makeshift pitfall trap over the next two mornings stand as our best chance at gathering fresh specimens of these elusive leeches.

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6.00 pm

We're back. The forest was sopping wet from the driving rains and the path upwards into the rainforest had become a torrent. Where water escaping the jungles crossed over through granite outcrops it was clear; a stark contrast to the brick red muddy flows we saw in Madagascar and a testament to the need for dense vegetation to prevent erosion of loose rainforest soils.

Other than Liz wiping out on the slick rocks a few times we managed to set up our make-shift pitfall trap in the leaf-litter, collect about 50 specimens each of Pandanus spp. and Stylonota spp. snails from the base of palms and in the litter. These will sit overnight with our hopes that an Idiobdella might crawl out.

We are having our doubts about the distinctions Richardson (a notorious "splitter") made with respect to the Seychellian leeches. He described Idiobdella daubani on the basis of one specimen; the only distinction being a slight difference in ventral annulation (ring) patterns. Unless one can demonstrate repetition of this pattern in some consistent way it seems to us a bit dubious to establish a new species for what may be an aberrent critter (lest we do the same for humans born with six toes).

Moreover, we doubt that Mahebdella miranda belongs in a separate genus (or even species). Silhouette and Mahe were connected as recently as the last ice ages about 20,000 years ago when much of the world's water was locked up in the polar ice sheets and ocean levels were much much lower. The position of gonopores, the lack of respiratory auricles near the caudal sucker (unlike typical haemadipsids, at left), even the annulation patterns are the same. Richardson, of course, did not see these alive and thought that Idiobdella species lack the dark middorsal stripe seen in Mahebdella. Obviously this is not correct as the picture above shows.

Justin has managed to dig out a Mahebdella miranda from his collections for us. We will try to extract DNA from that leech on the chance that we remain unsuccessful in the few days we have left on Mahe. Perhaps that will settle the matter.