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A new genus for Melanochromis labrosus, a problematic Lake Malawi cichlid with hypertrophied lips (Teleostei: Cichlidae)
Abstract and Figures
Melanochromis labrosus is the only one of the 800 or so endemic cichlid species in Lake Malawi whose placement with mbuna or non-mbuna has been disputed on morphological grounds. Here, we redescribe M. labrosus using all obtainable specimens collected since its original description. In addition, we extensively survey vertebral counts in the mbuna. To determine if M. labrosus belongs with the mbuna, we review the morphological traits often considered to define the mbuna as a natural group, concluding that none is a uniquely shared specialization. Two features of body coloration (a relatively small number of ocellate spots in the anal fin and a possible polychro-matism in one population) suggest that M. labrosus is best considered a species of mbuna. Although its placement within the mbuna group remains somewhat equivocal, we unambiguously demonstrate that M. labrosus is not a Melanochromis, since it lacks the synapomorphic striped color pattern typical of that genus. Melanochromis labrosus also does not fit within any other described genus. Therefore, we designate it the type species of Abactochromis, new genus. Abactochromis is unique in the mbuna in having greatly enlarged, lobate lips and several other aut-apomorphies. Contrary to published opinion, the hypertrophied lips of A. labrosus express high densities of taste buds. We review the little that is known about the biology of A. labrosus and emphasize its noteworthy distribu-tion within Lake Malawi, on rocky shores lakewide excepting the southern arms. We underscore the importance of including this problematic, and therefore potentially informative, species in future evolutionary genetic and genomic studies of the Lake Malawi cichlid species flock. Finally, we provide a key to the now thirteen genera of mbuna inhabiting Lake Malawi.
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... Cyrtocarina). There has been no definitive resolution, as no known morphological character unambiguously distinguishes one group from the other; no (non-molecular) synapomorphy has been found for either clade (see discussion in Oliver and Arnegard (2010)). In the present study, I found that the total number of anal pterygiophores in the Pseudotropheina is 8-10 (mode 9); in the Cyrtocarina, it is 9-13 (mode 10). ...
... Fig. 7k). Although a morphological synapomorphy for the mbuna is still lacking, this finding is compelling evidence corroborating the placement of Abactochromis labrosus in the mbuna clade, Pseudotropheina, as was tentatively suggested by Oliver and Arnegard (2010). ...
... In addition, the minimum number of anal pterygiophores proved to be only eight in Pseudotropheina, but nine in Cyrtocarina. This corroborates the tentative assignment of Abactochromis to the mbuna clade (Oliver and Arnegard 2010), as this enigmatic thick-lipped cichlid usually has only eight anal pterygiophores. ...
Background
The iconic freshwater cichlid fishes (Cichlidae) comprise about 1750 validly named species and hundreds more that are known, but not yet described and named. Cichlids are an important source of protein for millions of people on several continents, are model organisms in studies of evolution, speciation, ecology, development, behaviour and physiology and are popular as aquarium fishes. Yet, comparative studies of cichlid internal anatomy are rare. Even their osteology has not been taxonomically surveyed. The cichlid postcranial skeleton has been especially neglected.
New information
Here, I provide the first survey in cichlids of the considerable variation in numbers of vertebrae, supraneurals and dorsal- and anal-fin supports (pterygiophores), as well as the patterns with which the pterygiophores insert between the neural or haemal spines. The study includes some 1700 specimens of nearly 400 cichlid species. Focusing on the largest subfamily, the African cichlids or Pseudocrenilabrinae, the survey furnishes data from species in all but one of its 166 genera. Limited data from species in the other cichlid subfamilies (Etroplinae, Ptychochrominae and Cichlinae) and from the related leaffishes, Polycentridae, are also presented. Key examples of pterygiophore insertion patterns from throughout the range of variation are illustrated and discussed. Detailed analytical tables and all raw data are provided in supplementary files.
A bizarre specialisation in Cyprichromis is noted, evidently for the first time. Uniquely in this Lake Tanganyikan genus, five to seven anal pterygiophores are abdominal in position, located anterior to the anal fin and inserting toward or between successive pairs of pleural ribs.
Taxonomic changes: The most speciose tribe of African cichlids, currently known as Haplochromini, is correctly called Pseudocrenilabrini. Based chiefly on the molecular phylogenetic findings of other workers, I propose four pseudocrenilabrine subtribes, one occurring in rivers and three endemic to Lake Malawi. I also re-assign the Lake Tanganyikan tribe Tropheini as another subtribe of Pseudocrenilabrini, in line with numerous molecular studies placing tropheines firmly within this tribe. The remaining genera of Pseudocrenilabrini remain incertae sedis in this tribe pending clarification of their phylogenetic relationships.
The character complex here surveyed is a promising source of taxonomically and phylogenetically informative characteristics distinguishing or uniting cichlid taxa at multiple hierarchical levels, from species through subfamily. This reference set of novel character data can also provide information for palaeontological studies of African cichlids. These attributes are skeletal features potentially available for study in well preserved fossils and may help determine their correct taxonomic placement.
... Cichlids have acquired a huge diversity of trophic morphologies that are specialized for different feeding niches, and hypertrophied lips are one of the most easily recognized phenotypes that have evolved multiple times independently (Burress et al. 2013;Colombo et al. 2013;Manousaki et al. 2013;Henning and Meyer 2014;Baumgarten et al. 2015;Henning et al. 2017). This distinct trophic innovation found in both Neotropical and African cichlid lineages is typically associated with fish that forage in rocky substrates where the lips may act as a seal to help suck prey from in between narrow cracks and crevices (Ribbink et al. 1983;Baumgarten et al. 2015), absorb stress from repeated contact with hard and rough surfaces (Fryer and Iles 1972;Greenwood 1974), and/or enhance prey detection by providing an enlarged area for taste receptors (Oliver and Arnegard 2010). This morphology also exhibits strong feeding tradeoffs with hypertrophied lip fish being more efficient at extracting prey from crevices but less apt at capturing evasive prey in open water (Elmer et al. 2010;Colombo et al. 2013;Machado-Schiaffino et al. 2017). ...
... The hypertrophied-lip phenotype is also one of the iconic examples of a trait that has arisen independently in all three major cichlid adaptive radiations that are endemic to East Africa's largest great lakes. This specialized morphology is found in Haplochromis chilotes from Lake Victoria, Lobochilotes labiatus from Lake Tanganyika, as well as eight species (including one undescribed species) native to Lake Malawi (Fryer and Iles 1972;Oliver and Arnegard 2010). Given the recurrent evolution of hypertrophied lips across these major East African cichlid lineages, it is plausible that the phenotype has also evolved independently multiple times in Lake Malawi (Hulsey et al. 2018). ...
... Additionally, all but one species of hypertrophied-lip taxa are classified as nonmbuna haplochromines and have been assorted into different genera largely on the basis of body pigmentation patterns (Fryer and Iles 1972;Arnegard and Snoeks 2001;Snoeks 2004;Konings 2007). The one putative mbuna hypertrophied-lip species, Abactochromis labrosus, is an evolutionarily enigmatic cichlid (Trewavas 1935;Eccles and Trewavas 1989;Oliver and Arnegard 2010). Only a single phylogenetic study has included A. labrosus along with a limited number of taxa (15 Lake Malawi species in total) and this was based solely on mitochondrial control region sequences that inferred this species split at the base of the mbuna radiation (Meyer et al. 1996). ...
Cichlid fishes have repeatedly evolved an astounding diversity of trophic morphologies. For example, hypertrophied lips have evolved multiple times in both African and Neotropical cichlids and could have even evolved convergently within single species assemblages such as African Lake Malawi cichlids. However, the extremely high diversification rate in Lake Malawi cichlids and extensive potential for hybridization has cast doubt on whether even genome-level phylogenetic reconstructions could delineate if these types of adaptations have evolved once or multiple times. To examine the evolution of this iconic trait using protein-coding and non-coding single nucleotide polymorphisms (SNPs), we analyzed the genomes of 86 Lake Malawi cichlid species, including 33 de novo resequenced genomes. Surprisingly, genome-wide protein-coding SNPs exhibited enough phylogenetic informativeness to reconstruct inter- and intra-specific relationships of hypertrophied lip cichlids, although non-coding SNPs provided better support. However, thinning of non-coding SNPs indicated most discrepancies come from the relative smaller number of protein-coding sites and not from fundamental differences in their phylogenetic informativeness. Both coding and non-coding reconstructions showed that several “sand-dwelling” hypertrophied lip species, sampled intraspecifically, form a clade interspersed with a few other non-hypertrophied lip lineages. We also recovered Abactochromis labrosus within the rock-dwelling “mbuna” lineage, starkly contrasting with the affinities of other hypertrophied lip taxa found in the largely sand-dwelling “non-mbuna” component of this radiation. Comparative analyses coupled with tests for introgression indicate there is not widespread introgression between the hypertrophied lip lineages and taken together suggest this trophic phenotype has likely evolved at least twice independently within Lake Malawi.
... The cichlid species or ecomorphs with hypertrophied lips evolved a new foraging strategy in rocky crevices thereby occupying a new ecological niche. In particular, hypertrophied lips increase suction power by sealing cracks and grooves and also might protect the head from injury from prey on hidden benthic organisms (Baumgarten et al., 2015;Oliver & Arnegard, 2010). Thickened lips may also allow these cichlid species to achieve higher numbers of taste receptors (Manousaki et al., 2013;Oliver & Arnegard, 2010;Schemmel, 1967). ...
... In particular, hypertrophied lips increase suction power by sealing cracks and grooves and also might protect the head from injury from prey on hidden benthic organisms (Baumgarten et al., 2015;Oliver & Arnegard, 2010). Thickened lips may also allow these cichlid species to achieve higher numbers of taste receptors (Manousaki et al., 2013;Oliver & Arnegard, 2010;Schemmel, 1967). ...
The evolution of convergent phenotypes is one of the most interesting phenomena of repeated adaptive radiations. Here, we examined the repeated patterns of thick-lipped or “rubberlip” phenotype of cyprinid fish of the genus Labeobarbus discovered in riverine environments of the Ethiopian Highlands, East Africa. To test the adaptive value of thickened lips, identify the ecological niche of the thick-lipped ecomorphs, and test whether these ecomorphs are the products of adaptive divergence, we studied six sympatric pairs of ecomorphs with hypertrophied lips and the normal lip structure from different riverine basins. Trophic morphology, diet, stable isotope (δ15N and δ13C) signatures, as well as mtDNA markers and genome-wide SNP variation, were analyzed. Our results show that thick-lipped ecomorphs partition trophic resources with generalized ecomorphs in only one-half of the examined sympatric pairs despite the pronounced divergence in lip structure. In these thick-lipped ecomorphs that were trophically diverged, the data on their diet along with the elevated 15N values suggest an invertivorous specialization different from the basal omnivorous–detritivouros feeding mode of the generalized ecomorphs. Genetic data confirmed an independent and parallel origin of all six lipped ecomorphs. Yet, only one of those six thick-lipped ecomorphs had a notable genetic divergence with sympatric non-lipped ecomorphs based on nuclear SNPs data (FST = 0.21). Sympatric pairs can be sorted by combinations of phenotypic, ecological, and genetic divergence from an ecologically non-functional mouth polymorphism via ecologically functional polymorphism to a matured speciation stage via divergent evolution.
... The cichlid fauna of this habitat is dominated by the rock-frequenting haplochromines known by the Chitonga name 'mbuna.' The mbuna consist of 13 genera of brightly colored, highly stenotopic fishes, notable for the extensive genetic, morphological, and behavioral divergence among allopatric populations (Ribbink et al., 1983a;Oliver and Arnegard, 2010;Pauers and McMillan, 2015;Conith et al., 2020). While some of these genera have received extensive attention from ichthyologists, with numerous species having been added since Ribbink et al.'s (1983a) survey of these fishes, others remain in need of taxonomic attention (Pauers, 2010). ...
... The genus Labeotropheus is one genus of the mbuna common to the rocky nearshore areas of Lake Malaŵi, and has, until recently, been overlooked by taxonomists (Pauers, 2010). It is distinguished from the other mbuna due to a steeply sloping head; broad, fleshy snout; wide jaws with retrognathous lower jaw; and an inferior and subterminal mouth (Oliver and Arnegard, 2010). First described by Ahl in 1927, Labeotropheus had long been considered to contain only two species, L. fuelleborni and L. trewavasae, despite extensive evidence suggesting the existence of numerous species within this genus (Ribbink et al., 1983a(Ribbink et al., , 1983bPauers, 2010). ...
Labeotropheus is a small genus of rock-dwelling haplochromine cichlid fishes endemic to Lake Malaŵi in the Great East African Rift Valley. As currently recognized, Labeotropheus contains five species: L. artatorostris, L. chlorosiglos, L. fuelleborni, L. simoneae, and L. trewavasae. Despite increased recent attention from taxonomists, there are still several undescribed species within this genus. Here, based upon morphological and meristic data, as well as differences in male nuptial color pattern, we describe six new species. Additionally, we update the type locality of L. simoneae. The descriptions of these new species are in line with current recommendations to better define and delimit the taxonomy of cichlids from Lake Malaŵi, which will hopefully lead to increased efforts to conserve these fishes.
... Lake Malaŵi, located in the East African countries of Malaŵi, Tanzania, and Mozambique harbors some 850 species of cichlids (Konings 2016), with only about half of the species described (Stauffer et al. 2013). Within this species flock is a diverse group of rock-dwelling cichlids, locally known as mbuna, that include the following genera: Abactochromis, Chindongo, Cyathochromis, Cynotilapia, Genyochromis, Gephyrochromis, Iodotropheus, Labeotropheus, Labidochromis, Melanochromis, Metriaclima, Petrotilapia, Pseudotropheus, and Tropheops (Trewavas 1935, Oliver & Loiselle 1972, Stauffer et al. 1997, Oliver & Arnegard 2010, Li et al. 2016. Historically, the genus Pseudotropheus Regan-with type species P. williamsi-contained many of these mbuna species among which Ribbink et al. in 1983 discerned several species groups such as the P. zebra species-complex, the P. tropheops species-complex, the P. williamsi species-complex, and the P. elongatus species-group. ...
The rock-dwelling cichlids from Lake Malaŵi, known as mbuna, comprise a diverse group of haplochromine fishes that are placed among 14 genera. Within the mbuna, Pseudotropheus is a polyphyletic genus, which has served as a catch-all for many of these fishes. Recently, many of the species-groups within Pseudotropheus have been elevated to separate genera. Herein, we describe an elongate form that was originally placed in the Pseudotropheus elongatus species group but is now described as a member of Metriaclima.
... African cichlids are one of the well-known and popular families of ornamental fish among breeders, local retailers and aquarists (Saemi-Komsari et al., 2018;Mousavi et al., 2020;Rahmati-Holasoo, Hobbenaghi, et al., 2010;Rahmati-Holasoo, Mousavi, et al., 2010). The albino pindani (Chindongo socolofi Johnson 1974) is a freshwater perciform ornamental fish, which belongs to the Cichlidae family (Stauffer Jr. et al., 1997;Oliver & Arnegard, 2010), and this family is described by a wide range of colours and hued patterns. The origin of pindani is Lake Malawi in Africa. ...
A female albino pindani (Chindongo socolofi) and female cobalt‐zebra (Maylandia callainos) were referred to the Ornamental Fish Clinic with a soft tissue mass on their upper jaw. Also, during clinical examination, an unencapsulated intraoral soft tissue mass was observed in the cobalt‐zebra cichlid. Digital radiography showed soft tissue masses and osteolytic lesions. Computed tomography (CT) imaging confirmed that osteolytic lesions occurred in skull bones of cobalt‐zebra. Also, soft tissue mass invaded to the intraoral space. Histopathologically, a non‐encapsulated, multilobulated and densely cellular neoplastic mass was identified. Extensive infiltration of neoplastic cells to the dermis and subcutaneous tissue was observed in both cases. The masses were composed of multiple coalescing nests, sheets, lobules and trabeculae of neoplastic cells within a fibrovascular stroma. The palisading basaloid neoplastic cells were markedly extended from the basal cells of the epidermis into the dermis and subcutis. The majority of the neoplastic cells had scant amounts of eosinophilic cytoplasm with indistinct margins, round to ovoid nuclei and typically a single small basophilic nucleolus. There was invasion of basaloid neoplastic cells into the bone tissue that caused osteolytic lesions. Mitotic figures were three to seven per high‐power field. Epidermal basement membrane was periodic acid‐Schiff (PAS) positive and neoplastic cell extension into the dermis was identified by the PAS staining. Fibrovascular stroma of the mass was also stained blue with Masson's trichrome. Based on histopathological and histochemical findings, the masses were diagnosed as infiltrative type of basal cell carcinoma (BCC).
... Hypertrophied lips have repeatedly evolved in the cichlid lineage, providing a striking case of convergent evolution (Kocher et al. 1993;Oliver and Arnegard 2010;Colombo et al. 2013;Manousaki et al. 2013;Burress 2014;Henning et al. 2017;Machado-Schiaffino et al. 2017). Species with hypertrophied lips (also referred as thick-lipped species) can be found in the three African great lakes, among South American riverine cichlids of the genera Crenicichla and Gymnogeophagus, and in the Midas cichlid species complex. ...
The Nicaraguan Midas cichlid species complex is a natural experiment where fish from a large source population from turbid and shallow great lakes very recently (<20,000 years ago) colonized eight small crater lakes. The colonizers experienced completely novel environments in the clear and deep calderas. So far, 13 Midas cichlid species have been described, but more genetic clusters were identified. Although some of these species arose in allopatry, many more evolved in the absence of barriers to gene flow within two crater lakes. They contain small radiations of four and six endemics, respectively. These radiations constitute one of the few generally accepted empirical examples for sympatric speciation making them an ideal system for studying repeated evolution of adaptations and speciation at different levels of biological organization, including the genome level. Diversification occurred repeatedly in parallel including body morphology, coloration, color perception, and trophic structures such as pharyngeal jaws and hypertrophied lips. Additionally, parallel speciation happened in the two small crater lake radiations, where ecomorphologically similar species evolved repeatedly. Genomic differentiation associated with oligogenic traits (e.g., hypertrophic lips and coloration) is shallow, remaining polymorphisms, but much higher for polygenic traits (e.g., body shape and pharyngeal jaw morphology) that distinguish new species.
Cichlid fishes inhabiting the East African Great Lakes, Victoria, Malawi, and Tanganyika, are textbook examples of parallel evolution, as they have acquired similar traits independently in each of the three lakes during the process of adaptive radiation. In particular, "hypertrophied lip" has been highlighted as a prominent example of parallel evolution. However, the underlying molecular mechanisms remain poorly understood. In this study, we conducted an integrated comparative analysis between the hypertrophied and normal lips of cichlids across three lakes based on histology, proteomics, and transcriptomics. Histological and proteomic analyses revealed that the hypertrophied lips were characterized by enlargement of the proteoglycan-rich layer, in which versican and periostin proteins were abundant. Transcriptome analysis revealed that the expression of extracellular matrix-related genes, including collagens, glycoproteins and proteoglycans, was higher in hypertrophied lips, regardless of their phylogenetic relationships. In addition, the Wnt signaling pathway, which is involved in promoting proteoglycan expression, was highly expressed in both the juvenile and adult stages of hypertrophied lips. Our comprehensive analyses showed that hypertrophied lips of the three different phylogenetic origins can be explained by similar proteomic and transcriptomic profiles, which may provide important clues into the molecular mechanisms underlying phenotypic parallelisms in East African cichlids.
The genomic loci generating both adaptive and maladaptive variation could be surprisingly predictable in deeply homologous vertebrate structures like lips. Variation in highly conserved vertebrate traits such as jaws and teeth in organisms as evolutionarily disparate as teleost fishes and mammals are known to be structured by the same genes. Likewise, hypertrophied lips that have evolved repeatedly in Neotropical and African cichlid fish lineages could share unexpectedly similar genetic bases themselves and even provide surprising insight into the loci underlying human craniofacial anomalies. To isolate the genomic regions underlying adaptive divergence in hypertrophied lips, we first employed genome wide associations (GWA) in several species of African cichlids from Lake Malawi. Then, we tested if these GWA regions were shared through hybridization with another Lake Malawi cichlid lineage that has evolved hypertrophied lips seemingly in parallel. Overall, introgression among hypertrophied lip lineages appeared limited. Among our Malawi GWA regions, one contained the gene kcnj16 that has been implicated in the convergently evolved hypertrophied lips in Central American Midas cichlids that diverged from the Malawi radiation over 50 million years ago. The Malawi hypertrophied lip GWA regions also contained several additional genes that cause human lip-associated birth defects. Cichlid fishes are becoming prominent examples of replicated genomic architecture underlying trait convergence and are increasingly providing insight into human craniofacial anomalies such as cleft lip.
Trophic specialization plays a major role in ecological speciation. Multiple adaptive radiations among cyprinid fish of the genus Labeobarbus were recently discovered in riverine environments of the Ethiopian Highlands, East Africa. These radiations contain parallel diversifications of trophic morphology, including thick-lipped or ‘rubberlip’ phenotype, whose trophic function remains largely unexplored. To test the adaptive value of thickened lips, to identify the ecological niche of the thick-lipped ecomorphs, and to test of these ecomorphs are the products of speciation we studied six sympatric pairs of ecomorphs with hypertrophied lips and the normal lip structure from different riverine basins. Trophic morphology, diet, stable isotope (15N and 13C) signatures as well as mtDNA markers and genome-wide SNP variation were analyzed. Our results show that thick-lipped ecomorphs partition trophic resources with generalized ecomorphs in only one half of the examined sympatric pairs despite the pronounced divergence in lip structure. In these thick-lipped ecomorphs that were trophically diverged, the data on their diet along with the elevated 15N values suggest an insectivorous specialization different from the basal omnivorous-detritivouros feeding mode of generalized ecomorphs. Genetic data confirmed an independent and parallel origin of all six lipped ecomorphs. Yet, only one of those six thick-lipped ecomorphs had a notable genetic divergence with sympatric non-lipped ecomorphs based on nuclear SNPs data (FST = 0.21). Sympatric pairs can be sorted by combinations of phenotypic, ecological, and genetic divergence within the speciation continuum from an ecologically non-functional mouth polymorphism via ecologically functional polymorphism to completed speciation via divergent evolution.
In Lake Malawi cichlids, comparisons of feeding morphology have typically been performed at the genus level, with little emphasis on interspecific comparisons. Variation in jaw shape can be a powerful predictor of feeding performance and habitat preference. Jaw morphology and habitat partitioning were examined among species in the Lake Malawi rock-dwelling species complex, Tropheops. Although Tropheops species varied in a variety of craniofacial characters, lower jaw shape was the focus of this study because of its close association with feeding performance in bony fishes. Significant differences were observed among five microhabitat variables measured in situ for six Tropheops species at two field sites in southern Lake Malawi in July and August, 2001. Depth was identified as a major axis of trophic niche partitioning, and species-specific differences in lower jaw shape and foraging strategy were correlated with depth distributions among species. These data are consistent with the hypothesis that competition for trophic resources (epilithic organisms) has led to habitat partitioning and the establishment and maintenance of distinct trophic morphologies within the Tropheops species complex. Detailed morphological analyses of inter- (and intra-) specific trophic variation in Lake Malawi cichlids will likely lead to important insights into the proximate mechanisms that underlie cichlid trophic diversity.
The diagnosis of the genus Labidochromis is revised to include species with bicuspid outer teeth. Sixteen species, of which 13 are new, are described and illustrated and the validity of five others is discussed. The interrelationships between members of the genus and the relationship of the genus to other genera are discussed. No clearly defined sister group to the genus has been found either within or outside the Lake Malawi cichlid species flock. A key based on the coloration of known species is included.
Mitochondrial DNA (mtDNA) restriction fragment length polymorphisms were assayed among 40 species of Lake Malawi haplochromines (Cichlidae) including representatives of ecologically divergent genera. Six distinctive mtDNA lineages were distinguished, two of which were major clades, represented by a large number of species. The other four lineages were each represented by a single species with a divergent mtDNA haplotype. One of the two major clades was composed of the shallow-water, rock-dwelling mbuna species, whereas the other included a diverse array of sand-dwelling and pelagic species. A number of taxa, found to be firmly embedded within the mbuna clade, are quite distinct in morphology and generally inhabit deeper, sediment-rich areas rather than the rocky habitats typical of other mbuna. The mbuna group is generally thought to be a monophyletic assemblage, but these results suggest that it is actually paraphyletic. In contrast to the high morphological diversity among Malawi haplochromine species, mtDNA sequence divergence was found to be remarkably low. This finding underscores the unprecedented rapidity of speciation and evolutionary plasticity in this fish species flock.