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Apsilochorema distribution 91 The distribution of Apsilochorema Ulmer, 1907: biogeographic evidence for the Mesozoic accretion of a Gondwana microcontinent to Laurasia Wolfram Mey Museum für Naturkunde, Institut für Systematische Zoologie, Humboldt-Universität Berlin, Invalidenstrasse 43, D - 10115 Berlin, Germany Key words: historical biogeography, Asia, Australia, Gondwana, zoogeography, aquatic insects, Trichoptera, Apsilochorema Abstract Apsilochorema is the only genus of the southern hemisphere family Hydrobiosidae that occurs on the Asian continent. The origin of the genus is obscure and enigmatic. A phylogenetic analysis of the species reveals the existence of 2 different subgenera: Archichorema and Apsilochorema sensu stricto. They show different distributional patterns on the Asian continent, and in the SW Pacific region including Australia. A hypothesis is proposed to explain the distribution based on the plate tectonic history of SE Asia. Ancestors of Apsilochorema reached Asia by rafting on terranes of Gondwana origin. This crossing of the Tethys ocean happened at least twice. The species migrated into Asia on the Lhasa and West Burma terrane in the Cretaceous (Archichorema) and on the Indian plate in the Tertiary (Apsilochorema sensu stricto). Introduction The distribution of the caddisfly family Hydrobiosidae has several times attracted the attention of Trichoptera specialists. One reason for this interest is the conspicuous distribution pattern (Fig.1) of the family, which comprises 47 genera, with a disjunction between South America and Australia. Only the genus Apsilochorema Ulmer occurs on the Asian continent. Secondly, the Hydrobiosidae are regarded as the sister group of the Rhyacophilidae (Frania and Wiggins, 1997), the most ancestral family of extant Trichoptera, which has a pronounced northern hemisphere distribution in contrast to the Hydrobiosidae (Fig.2). Within Hydrobiosidae Apsilochorema has a special position. It is regarded as representing the oldest evolutionary lineage of the family (Neboiss, 1962; Ross, 1951, Biogeography and Geological Evolution of SE Asia, pp. 91-98 Edited by Robert Hall and Jeremy D. Holloway © 1998 Backhuys Publishers, Leiden, The Netherlands 1956, 1967). The age of the family has been estimated as approximately 100 million years (Ross, 1951) meaning an early Cretaceous appearance. This assumption is corroborated by the discovery of a fossil forewing in Upper Cretaceous amber of North Siberia. It was assigned to Hydrobiosidae and described as Palaeohydrobiosis simberambra (Botosaneanu and Wichard, 1983). However, the family must be even older. The fossil record of the order indicates that the first true caddisflies appeared in the Permian (e.g., Ivanov, 1988), and in the Triassic the families Philopotamidae and Hydropsychidae were already in existence (Sukatcheva, 1991). According to proposed phylogenies of the Trichoptera (Frania and Wiggins, 1997; Ross, 1956, 1967; Schmid, 1989; Weaver, 1984; Wiggins and Wichard, 1989) these families are younger clades than the Hydrobiosidae. Thus, the Hydrobiosidae should be at least Triassic in age. Therefore the family is old enough to have been in existence during the breakup of Pangaea in the Mesozoic. Accordingly, the complicated history of Gondwana and its terranes in the SE Asian and Australian region in the Mesozoic and Tertiary (Audley-Charles, 1987; Metcalfe, 1996) should have affected the distribution of Hydrobiosidae. Biology of Apsilochorema The species are inhabitants of small rivers and brooks. In the north A. sutshanum Martynov lives in mountain streams as well as lowland streams. In SE and East Asia the genus is found 92 Wolfram Mey Fig.1. Distribution area of Hydrobiosidae. The main or core areas in South America and Australia are filled with black, and their respective numbers of genera are indicated. Fig.2. Distribution area of Rhyacophilidae, the sister family of Hydrobiosidae. Apsilochorema distribution 93 in mountains at heights from 1500m to 4000m (Nepal). The larvae are free-living predators, with 5 instars (Kotcharina, 1986). Pupation takes place in a case, made of small stones and attached to larger stones in the current. The adults are active in the daytime. There are no pronounced flight periods. The problem The south hemisphere disjunction of the family Hydrobiosidae appears very pronounced, if the ranges of Atopsyche Banks and Apsilochorema Ulmer are omitted from the map of the distribution area of the family (Fig.1). The majority of genera is confined to certain parts of South America, Australia, New Guinea and the Southwest Pacific. In contrast, the areas of Atopsyche and Apsilochorema are large and seem to be simply northern extensions of the main range. If this were the case, we might expect the more primitive or ancestral forms to occur in the south and more derived ones in the north. Surprisingly, the situation in Apsilochorema is the reverse. The ancestral species live in the north. This exactly is the problem, because we have to find an interpretation for this mysterious pattern. Up to now, two hypotheses have been proposed to explain the occurrence of Apsilochorema in Asia. Both asserted the ancestry of the Asian species. Ross (1951, 1956) assumed a SE Asian origin for the ancestor of Hydrobiosidae. According to this author the species spread into the southern continents via Sundaland or across an Alaska connection. After having reached Australia and South America, the ancestor started to differentiate. Some populations remained in Asia, became isolated and gave rise to the Apsilochorema line, which subsequently extended its range towards Australia. Neboiss (1962) agreed with Ross (1956) concerning the dispersal of Apsilochorema from the north into Australia. In contrast, Schmid (1989) proposed a Gondwana origin and Gondwana differentiation of the Hydrobiosidae. In coping with the problem of Apsilochorema he briefly discussed the possibility that the northward-moving Indian plate could have carried ancestral Apsilochorema species, which dispersed in Asia after the collision with Laurasia. However, he rejected this idea, because it implied a Jurassic or Cretaceous age of Apsilochorema. He was not prepared to accept such a great age. However, he admitted the differentiation of the genus had taken place in South Asia. Fig.3. Cladogram of the phylogenetic relationships within Apsilochorema Ulmer. The numbers refer to suggested synapomorphies. The arguments These studies on Apsilochorema were started with an examination of the morphology of the species. The objective was to elucidate the phylogenetic relationships within the genus and to construct a cladogram. The characters were polarised based on comparison to all other genera of Hydrobiosidae as defined in Schmid (1989). The phylogenetic reconstruction is based on cladistic principles. The second issue was the documentation of the range of the genus and the production of a more detailed distribution map. The phylogenetic relationships depicted in the cladogram of Fig.3 show a clear separation of the genus into 2 groups. For practical reasons they were given subgeneric rank and were named Archichorema and Apsilochorema sensu stricto (Mey, 1998). The morphological basis for this distinction is the presence or absence of a chorema on the underside of the male forewing (1 in Fig.3). The rather complex structure is illustrated in Figs. 4 and 5. It is regarded as a synapomorphy of Apsilochorema sensu stricto. Some additional 8 characters provide evidence for the further branching of the subgenera into 7 species groups. The character descriptions are listed in Mey (1998). The mapping of the distribution of all Apsilochorema species revealed a wide range, extending from Tasmania in the south to the island of Sakhalin in the north, and from the Iranian Elburz Mountains in the west to the oceanic islands of Fiji in the east (Fig.6). However, the distribution is by no means homogeneous. It is a mixture of smaller and larger areas with continuous distribution and a number of widely scattered localities of a single species. Regions with 94 Archichorema annandalei Apsilochorema indicum Fig.4. Male forewings of A. annandalei Martynov (Himalaya) and A. indicum Ulmer (Tianshan). The chorema of A. indicum is situated below the discoidal cell (DC). Wolfram Mey enigma of the ancestral species in South Asia we now have the additional problem of an explanation for the disjunction of the subgenus Apsilochorema. The progress of plate tectonic theory in the last decades has provided a firm geophysical framework against which the evolution and biogeography of Apsilochorema can be viewed (Hall and Blundell, 1996). Continental SE Asia is an assemblage of small and large terranes, which were rifted off the northern margin of Gondwanaland and were accreted to Laurasia and/or to Protochina. Similarly, a number of islands in the SW Pacific are continental terranes too, derived from Gondwana or Australia (Audley-Charles, 1987; Burrett et al., 1991; Metcalfe, 1996). There are several models for how the SE Asian and SW Pacific region became assembled. They differ mainly from one another on the timing of rift, drift and collision histories of the terranes. However, all agree that the amalgamating process forming SE Asia was due to a series of arrivals of northward moving microcontinents of Gondwana origin. The earliest arrived blocks are of Palaeozoic age. Further accretion occurred during the Mesozoic and the Tertiary. Fig.5. Structure of the chorema of Apsilochorema indicum Ulmer at the underside of male forewing. the highest number of species are the southern slope of the Himalayas (8 species; Schmid, 1970), New Guinea (8 species; Schmid, 1989) and Borneo (5 species; Huisman, 1992). After separating the distributional records according to the subgenera and plotting a map for each subgenus, the following picture emerges (Fig.7): Archichorema has a relatively compact range with its ancestral species in the Himalaya and Assam region. Surprisingly, the range of Apsilochorema sensu stricto shows a disjunction between India/Middle Asia and Sundaland/ Australia; there is a large gap in the distributions in SE Asia. These are the new arguments. Besides the Fig.6. Distribution area of Apsilochorema sensu lato, Ulmer 1907. Hatched areas indicate continuous distributions. Numbers indicate the numbers of species in each area. Apsilochorema distribution Archichorema 95 Apsilochorema Fig.7. Distribution of the subgenera Archichorema and Apsilochorema sensu stricto. The model Before the separation of India from Australia/ Antarctica was accomplished in the late Jurassic (Veevers, 1991) a microcontinent or several continental fragments detached from the northern edge of eastern Gondwanaland and moved northwards. The rifting started in the late Triassic with the Lhasa block, followed by the West Burma and Woyla terranes (Metcalfe, 1996). They might have carried populations of the ancestor of Apsilochorema, which was possibly distributed in northeastern Gondwanaland in that time. Drifting with one of these microcontinents the species reached the Pan-Laurasian continent in the Cretaceous. This would be the most simple explanation for the occurrence of Apsilochorema in Asia. However, it does not provide an explanation for the disjunction of Apsilochorema sensu stricto. It is questionable if the differentiation of the species had occurred before or during the journey with the Gondwana terrane, or if it developed after the establishment of the species in SE Asia. Both possibilities appear to be plausible, but they offer an explanatory potential only for the splitting and spreading of Archichorema. The basal separation into the 2 subgen- era must have happened in another way. Returning to the rifting of terranes from the margin of northern Gondwana, the process can be regarded as removing only a part of the distributional area of the Apsilochorema ancestor (Fig.8). The remaining part stayed on Gondwana. The subsequent isolation enabled the beginning of an independent evolution and divergence into Archichorema and Apsilochorema sensu stricto. In the Gondwana species the development of the chorema on the male forewings probably started. It could have been either an anagenetic or cladistic process. The species with this evolutionary novelty was distributed over the northern margin of Gondwana. In the late Jurassic/early Cretaceous a western part of the range was broken off with the Indian continent and started to move northwards. With the accretion of the Indian continent to Asia the second immigration of Apsilochorema species into Asia took place, this time by a species with a wing chorema, i.e., a member of Apsilochorema sensu stricto. It remained isolated on the Indian subcontinent and the adjacent northwestern area. The species in eastern Gondwana were confined to the Australian continent and gave rise to the development of 2 species groups. 96 Wolfram Mey Fig.8. Hypothetical range of an ancestral species of Apsilochorema in Eastern Gondwana. 140 Ma (Late Jurassic) map adapted from Smith et al. (1981). This scenario gives an explanation for the stepwise formation of the distribution pattern of Apsilochorema sensu lato. In the light of this hypothesis the different dispersal routes of Apsilochorema species become recognisable. They are depicted in Fig.9. According to this notion the species of Apsilochorema sensu stricto known from northern Borneo originated from Australian stock. Such a deep penetration of Australian elements into the Oriental region is rather unusual for the Trichoptera. However, the species in North Sulawesi is A. gisbum Mosely, which has a wide range in the Australian region including Tasmania. The high dispersal capacity of this species points possibly to an intrinsic character of the species of the subgenus. It enabled the colonisation of various islands in the SW Pacific (Fig.6), as well as Borneo on the Sunda shelf. But in contrast to this dispersal-based interpretation it is possible to suggest an alternative explanation. The Gondwana terranes which have been recognised in Sundaland offered the same potential and possibility for transport of Apsilochorema species in the past. Unfortunately, faunistic research in the region is at quite a low level and does not permit the distinction between a mobilistic or vicariant explanation in the case of the Borneo species. The question is left open for future research. Conclusions The proposed hypothesis allows some predictions. The most interesting one is the possible occurrence of Apsilochorema in the southern part of Africa and especially on Madagascar. That island was very close to the Indian conti- Apsilochorema distribution 97 Archichorema Apsilochorema Fig.9. Presumed dispersal routes of Apsilochorema sensu lato in the SE Asian and SW Pacific/Australian region nent and to western Gondwana at the time when the two plates separated. If an Apsilochorema species really survived, than it should belong to Apsilochorema sensu stricto and not to Archichorema. The predatory larvae of Apsilochorema feed mainly on other aquatic insects (Kotcharina, 1986). Thus, the persistence of Apsilochorema requires a diverse and rich community of aquatic insects offering enough prey species in adequate quantities. The co-evolution of the aquatic biota should have resulted in a similar persistence of other species or groups, not only in Apsilochorema. In consequence, further congruent patterns of distribution should exist. The most promising insect groups, which could provide further examples, should be found within the ancient orders Ephemeroptera (Mayflies) and Plecoptera (Stoneflies). In conclusion, if the hypothesis proves to be valid, we have an example of a very old genus with two subgenera of at least Cretaceous age. Further, the small morphological differences between nearly all Apsilochorema species points to a remarkable property of this evolutionary line: cohesion and stasis as successful mechanisms for survival through the Mesozoic and Cenozoic until today. Acknowledgements Special gratitude is extended to H. Duffels and R. Hall for their encouragement and help in preparing the manuscript. Thanks also to A. Neboiss, H. Malicky and L. P. Hsu for sending specimens of Apsilochorema. Most of the Asian material studied comes from field work support- 98 ed by the Deutsche Forschungsgemeinschaft (Me 1085/1, Me 1085/3, Me 1085/5). References Audley-Charles, M. G. 1987. Dispersal of Gondwanaland: relevance to evolution of the angiosperms. In Biogeographical evolution of the Malay Archipelago. pp. 5-25. Edited by T. C. Whitmore. Botosaneanu, L. and Wichard, W. 1983. Upper Cretaceous Siberian and Canadian amber caddisflies. Bijdr. Dierk. 53: 187-217. Burrett, C., Duhig, N., Berry, R. and Varne, R. 1991. 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