Note: This post is being linked to a number of sites and discussion forums for hobbyists interested in the husbandry of the shrimps. I'd like to make it clear the post was never intended to be a guide on raising the animals but merely a report, as accurate as my understanding permits, of Frau Zitzler's research on the biogeography and systematics of the shrimps in the lake. Hence, please do not send me queries on how one should raise, much less acquire, the shrimps. And if you should feel disappointment that the post fails to address hobbyist-related questions, or the content prove uninteresting, or even untrustworthy in your opinion on account of its medium, the fault lies squarely in the wanton expectations (or prejudices) of the reader.
Straddling the fringes of Sundaland and the Australasian continental shelf, Sulawesi is a unique region that offers a motley mix of fauna from two exceedingly disparate zoogeographic lineages. The orchid-shaped island is home to endemic placentals such as black crested macaques, tarsiers and babirusas as well as two species of cuscus, which are arboreal marsupials resembling fat little bears. Oriental birdlife include staples such as hornbills, babblers, drongos and sunbirds, while an Australasian element is present via cockatoos and the maleos, large chicken-like birds which incubate their eggs with the thermal heat of volcanic soils.
For reasons of habit and morphology, my duck is interested in freshwater aquatic life, and here Sulawesi presents an unsurpassed vessel of endemic biodiversity. The inland lakes house several species of endemic ricefish (Oryzias spp.) and halfbeaks which face extermination by the appetites of introduced snakeheads and tilapia. Lesser known are the indigenous aquatic invertebrates, which were the subject of a talk by Kristina Zitzler of the Humboldt University of Berlin, Germany, on Wednesday, 2 November at the Department of Biological Sciences, National University of Singapore. Ms Zitzler is presently developing her PhD paper on atyid shrimp speciation in Sulawesi’s lakes and her talk entitled "Evolution and radiation of atyid shrimps (Decapoda, Caridina) in ancient lakes on Sulawesi, Indonesia - a molecular and morphological approach" provided an overview of her still unpublished research.
Alfred Wallace (the other discoverer of evolution) had sampled the extremities of the island, mainly for entomological and avian diversity in the mid 1800s, but it was Paul and Fritz Sarasin who first explored the island’s interior habitats in 1893 and revealed the existence of two ancient (1-2 million years old) lake systems in the central highlands: Lake Poso and the five-lake Malili system.
The lake systems are home to several endemic species of shrimp in the genus Caridina. Two primary questions confront the biogeographer: how many colonisations took place in these habitats; and what biogeographic patterns might be discerned by the distribution of these taxa? Alongside Ms Zitzler and in the same lakes, a colleague is conducting similar and related research on the phylogeny and biogeography of freshwater snails in the genus Tylomelania. In addition, several endemic freshwater crabs, isopods and sponges can be found in the lakes.
Extensive sampling of shrimp species from the island’s two lake systems and other habitats, along with molecular phylogeny (based on sequences of the mitochondrial genes 16S rRNA and cytochrome-oxidase-I (COI)) of Sulawesian shrimps as well as Caridina species from neighbouring regions revealed that at least three groups are present on the island: two consisting of riverine species with small eggs and a life cycle that involves downstream movement of the larvae, and a landlocked, lacustrine species flock with medium-sized eggs that hatch into larva without a transitional phase. Within this latter group, Poso contains a single clade while two clades were traced in Malili, indicating at least two separate colonisation events and evolutionary lineages.
Ms Zitzler notes that coevolution and a continuous arms-race between predator and prey has played a major role in adaptive radiation in the lakes. There are, for instance, species of fish with thick lips that enable to them to suck out shrimp dwelling in rock crevices. Endemic crabs have branched out as detrivores, omnivores as well as molluscivores with pincers strong enough to deal with hard-shelled snails.
There are at least 3 endemic Caridina species in Poso, while the Malili system is known to have 9 endemics and one non-endemic. Many other species of shrimp and snails remain undescribed. The lake-dwelling shrimps are also markedly more colourful and diverse in appearance compared to their less gaudy relatives in riverine environments.
Adaptive radiation in a monophyletic group is characterised by Schluter (2000) as taxa that demonstrate a common ancestry, rapid speciation and a phenotype-environment correlation. The Caridina distribution in Lake Poso and the Malili system fulfil all these criteria. In the Malili system, one species of shrimp was found to be morphologically identical with C. opaensis, a species first collected in a swampy region south of the lake system. This raised the questions of whether both shrimps were conspecific and if so, whether this meant that C. opaensis constituted the sole non-endemic Caridina in the Malili system. However, molecular phylogeny suggested that the lake-dwelling “C. opaensis” is a distinct taxa from the riverine counterpart.
From Baikal to Tanganyika, lakes have often served as fertile grounds for speciation by a pioneer species that comes across a habitat with unfilled ecological niches. In a largely enclosed, competitive environment, adaptive divergences in feeding preferences, predator avoidance strategies, sexual selection and mating rituals aid in phenotypic diversification and eventually, the erection of barriers to gene flow between divergent groups, leading to rapid speciation.
In the Malili system, there exists now only one Caridina species that is defined as a generalist, with a wide ranging distribution, no particular habitat preference and unremarkable appearance. In contrast, the other species display a strong phenotype-environment correlation, with taxa adapted for individual habitats such as shallow water sandy substrates, reed-overgrown regions, rocky shoals and even the cavities of freshwater sponges. They also tend to be very colourful and attractively patterned. Unfortunately, they are not easy creatures to keep in captivity as according to Ms Zitzler, their oxygen requirements are rather high.
Ms Zitzler’s summary of the results from her data are that:
• based on morphology, DNA taxonomy and colour patterns, the actual number of species in the Malili lake system (16+) and in Lake Poso (4+) is higher than presumed;
• 3 independent colonisation events took place in the lakes (2 in Malili and 1 in Poso) and all Caridina species in the lakes are endemic;
• the colour patterns found in both lake systems tend to be species specific; and
• based on trophic morphology and ecology, adaptive radiation appears to have taken a generalist vs. specialist dynamic in the Malili system.
Malili’s shrimp species consist of 2 lineages, with the sole generalist species occupying its own clade. The other clade has exploded into diverse specialised taxa: for example, C. pulchella is found only amongst rocks in deeper waters, while C. spongicola is an obligate sponge dweller that occurs in just one small bay of lake Towuti. The contrast in speciation trends might be attributed to the limited choices for specialisation available to the late-arriving clade. The species in both lake systems offer instances of parallel evolution, with independent radiations that occupied similar ecological niches.
The parallel study on snail biodiversity in the lakes found that the Malili system has 25+ species, while Poso has 12+. The greater snail diversity is attributed to the indication that at least 4 waves of snail colonisation took place (3 in Malili and 1 in Poso) as well as the lesser ability of gastropods to breach natural barriers that might have impeded gene flow. Conversely, the higher potential for dispersion in shrimps probably reduces the possibilities of allopatric speciation. The chances of hybridisation between shrimps (one possible mechanism for speciation) are also thought to be lower due to better mechanisms of species recognition and mate selection (this mechanism is now failing in Lake Victoria as sedimentation is increasing the lake’s turbidity, hindering conspecific mate recognition by female cichlids). The samplings also found that a line about halfway in between the two lake systems serves as a border between their respective shrimp species flocks. For snails, the line was a little lower.
A future avenue for research lies in habitat mapping of the lakes for a fine-scale assessment of habitats and dispersal barriers. The resulting data could also be used to test the correlation between spatial forces and both morphological and genetic population differentiation to gain an understanding of their relative importance in both groups: the shrimps and the snails. At the moment, the shrimps, which are monophyletic, exhibit high resolution at the species level, whereas the lakes’ snails, which are polyphyletic, display poor species resolution. [note: I am not too sure I understand what the last statement means – can anybody help?]