Acanthurus Leucosternon Research Articles

Feeding kinematics of a surgeonfish reveal novel functions and relationships to reef substrata

Biting to obtain attached benthic prey characterizes a large number of fish species on coral reefs, and is a feeding mode that contributes to important ecosystem functions. We use high-speed video to reveal the mechanisms used by a surgeonfish, Acanthurus leucosternon, to detach algae. After gripping algae in its jaws, the species pulls it by ventrally rotating both the head and the closed jaws, in a novel use of the intra-mandibular joint. These motions remain in the plane of the fish, reducing the use of a lateral head flick to detach the algae. The novel ability to bite and pull algae off the substrate without bending the body laterally minimizes exposure to high water flows, and may be an adaptation to feeding in challenging reef habitats such as the crest and flat. Therefore, our results could potentially represent a key milestone in the evolutionary history of coral reef trophodynamics.

Neuromolecular responses in disrupted mutualistic cleaning interactions under future environmental conditions

BackgroundMutualistic interactions, which constitute some of the most advantageous interactions among fish species, are highly vulnerable to environmental changes. A key mutualistic interaction is the cleaning service rendered by the cleaner wrasse, Labroides dimidiatus, which involves intricate processes of social behaviour to remove ectoparasites from client fish and can be altered in near-future environmental conditions. Here, we evaluated the neuromolecular mechanisms behind the behavioural disruption of cleaning interactions in response to future environments. We subjected cleaner wrasses and surgeonfish (Acanthurus leucosternon, serving as clients) to elevated temperature (warming, 32 °C), increased levels of CO2 (high CO2, 1000 ppm), and a combined condition of elevated CO2 and temperature (warming and high CO2, 32 °C, and 1000 ppm) for 28 days.ResultsEach of these conditions resulted in behavioural disruptions concerning the motivation to interact and the quality of interaction (high CO2 − 80.7%, warming − 92.6%, warming and high CO2 − 79.5%, p < 0.001). Using transcriptomics of the fore-, mid-, and hindbrain, we discovered that most transcriptional reprogramming in both species under warming conditions occurred primarily in the hind- and forebrain. The associated functions under warming were linked to stress, heat shock proteins, hypoxia, and behaviour. In contrast, elevated CO2 exposure affected a range of functions associated with GABA, behaviour, visual perception, thyroid hormones and circadian rhythm. Interestingly, in the combined warming and high CO2 condition, we did not observe any expression changes of behaviour. However, we did find signs of endoplasmic reticulum stress and apoptosis, suggesting not only an additive effect of the environmental conditions but also a trade-off between physiological performance and behaviour in the cleaner wrasse.ConclusionsWe show that impending environmental shifts can affect the behaviour and molecular processes that sustain mutualistic interactions between L. dimidiatus and its clients, which could have a cascading effect on their adaptation potential and possibly cause large-scale impacts on coral reef ecosystems.

Neuro-molecular characterization of fish cleaning interactions

Coral reef fish exhibit a large variety of behaviours crucial for fitness and survival. The cleaner wrasse Labroides dimidiatus displays cognitive abilities during interspecific interactions by providing services of ectoparasite cleaning, thus serving as a good example to understand the processes of complex social behaviour. However, little is known about the molecular underpinnings of cooperative behaviour between L. dimidiatus and a potential client fish (Acanthurus leucosternon). Therefore, we investigated the molecular mechanisms in three regions of the brain (Fore-, Mid-, and Hindbrain) during the interaction of these fishes. Here we show, using transcriptomics, that most of the transcriptional response in both species was regulated in the Hindbrain and Forebrain regions and that the interacting behaviour responses of L. dimidiatus involved immediate early gene alteration, dopaminergic and glutamatergic pathways, the expression of neurohormones (such as isotocin) and steroids (e.g. progesterone and estrogen). In contrast, in the client, fewer molecular alterations were found, mostly involving pituitary hormone responses. The particular pathways found suggested synaptic plasticity, learning and memory processes in the cleaner wrasse, while the client indicated stress relief.

Do differences in mating behaviour lead to differences in connectivity patterns of reef fishes? Insights from two sympatric surgeonfish species in the Indian Ocean

Disentangling the contribution of biotic and abiotic factors in the structuring of the genetic diversity of reef species is critical to illuminate the diversification of evolutionary lineages in marine environment. Howevr, previous studies have mainly focused on determining the influence of pelagic larval duration on the connectivity and demographic history of reef fishes, whereas few studies have examined the effects of other biotic factors, such as mating behaviour and habitat preference. Here, we use mitochondrial DNA (ATPase 6/8) and ten microsatellite loci to compare the population genetic structure and demographic history of the spawning aggregating Acanthurus triostegus with the monogamous spawning Acanthurus leucosternon. Pairwise comparisons and discriminant analysis of principal components showed that the genetic structuring patterns of the two species are not consistent with the influence of mating behaviour, suggesting the possible role of other biotic and abiotic factors. However, demographic history estimates revealed that these species may have responded differently to sea level fluctuations during the glacial maxima.

Diagnosis and treatment of multi-species fish mortality attributed to Enteromyxum leei while in quarantine at a US aquarium.

Enteromyxum leei is an enteric myxozoan parasite of fish. This myxozoan has low host specificity and is the causative agent of myxozoan emaciation disease, known for heavy mortalities and significant financial losses within Mediterranean, Red Sea, and Asian aquaculture industries. The disease has rarely been documented within public aquaria and, to our knowledge, has never been confirmed within the USA. This case report describes an outbreak of E. leei in a population of mixed-species east African/Indo-Pacific marine fish undergoing quarantine at a public aquarium within the USA. Four of 16 different species of fish in the population, each of a different taxonomic family, were confirmed infected by the myxozoan through cloacal flush or intestinal wet mount cytology at necropsy. Clinical and histopathological findings in this case are similar to previous findings describing myxozoan emaciation disease, e.g. severe emaciation, cachexia, enteritis, and death. Sequence analysis of the 18S rDNA of intestinal samples from a powder blue tang Acanthurus leucosternon and an emperor angelfish Pomacanthus imperator confirmed the parasite to have 99-100% identity with other E. leei sequences. Spore morphology and ultrastructure were consistent with previous reports of E. leei. Treatment of clinically affected fish by oral administration of the coccidiostats amprolium and salinomycin led to reduction of mortalities and resolution of clinical signs. This case report highlights the importance of thorough examination and surveillance of fish during quarantine, particularly with respect to enteric myxozoans.

Genetic connectivity in a herbivorous coral reef fish (Acanthurus leucosternon Bennet, 1833) in the Eastern African region

Knowledge of larval dispersal and connectivity in coral reef species is crucial for understanding population dynamics, resilience, and evolution of species. Here, we use ten microsatellites and one mitochondrial marker (cytochrome b) to investigate the genetic population structure, genetic diversity, and historical demography of the powder-blue tang Acanthurus leucosternon across more than 1000 km of the scarcely studied Eastern African region. The global AMOVA results based on microsatellites reveal a low but significant F ST value (F ST = 0.00252 P < 0.001; D EST = 0.025 P = 0.0018) for the 336 specimens sampled at ten sample sites, while no significant differentiation could be found in the mitochondrial cytochrome b dataset. On the other hand, pairwise F ST, PCOA, and hierarchical analysis failed to identify any genetic breaks among the Eastern African populations, supporting the hypothesis of genetic homogeneity. The observed genetic homogeneity among Eastern African sample sites can be explained by the lengthy post-larval stage of A. leucosternon, which can potentiate long-distance dispersal. Tests of neutrality and mismatch distribution signal a population expansion during the mid-Pleistocene period.

Homeward bound: fish larvae use dispersal corridors when settling on coral reefs

Homeward bound: fish larvae use dispersal corridors when settling on coral reefs

Molecular and morphological descriptions of Ceratomyxa collarae n. sp. and Ceratomyxa leucosternoni n. sp. from marine ornamental fishes of Indian waters.

Two novel species of Ceratomyxa infecting marine ornamental fishes from Indian waters are described. Marine ornamentals, Chaetodon collare and Chaetodon decussatus, collected from Vizhinjam, along the southwest coast of India and Acanthurus leucosternon collected from Lakshadweep islands of Arabian Sea revealed Ceratomyxa infections in their gall bladders. Mature spores of Ceratomyxa from Chaetodon collare and Chaetodon decussatus were elongate and slightly crescentic, with rounded ends, and measured 5.20 ± 0.32μm in length and 16.32 ± 1.29μm in thickness. Polar capsules spherical, equal in size and measured 2.23 ± 0.16μm long and 2.24 ± 0.20μm wide. Posterior angle measured 157.75 ± 8.650. Principle Component Analysis and molecular analysis using partial SSUrDNA sequences showed the isolates from these two hosts to be identical. Morphological, morphometric and molecular analysis using partial SSUrDNA sequences revealed the taxonomic novelty of isolates and are hence treated as Ceratomyxa collarae n. sp. Mature spores of Ceratomyxa from Acanthurus leucosternon were elongate, slightly tapering with rounded ends, and measured 7.34 ± 0.92μm in length and 24.37 ± 2.34μm in thickness. Shell valves were equal, joined by a narrow suture line. Polar capsules spherical in shape, equal in size, 2.59 ± 0.32μm long and 2.46 ± 0.32μm wide, and polar filament measured 18.68 ± 2.54μm. Based on morphological, morphometric and molecular analyses, the present species of Ceratomyxa is distinct, considered as a new species and named Ceratomyxa leucosternoni n. sp. The paper also discusses the prevalence of the recovered parasites and host specificity of Ceratomyxa collarae n. sp.

Surgeons and suture zones: Hybridization among four surgeonfish species in the Indo-Pacific with variable evolutionary outcomes

Closely related species can provide valuable insights into evolutionary processes through comparison of their ecology, geographic distribution and the history recorded in their genomes. In the Indo-Pacific, many reef fishes are divided into sister species that come into secondary contact at biogeographic borders, most prominently where Indian Ocean and Pacific Ocean faunas meet. It is unclear whether hybridization in this contact zone represents incomplete speciation, secondary contact, an evolutionary dead-end (for hybrids) or some combination of the above. To address these issues, we conducted comprehensive surveys of two widely-distributed surgeonfish species, Acanthurus leucosternon (N=141) and A. nigricans (N=412), with mtDNA cytochrome b sequences and ten microsatellite loci. These surgeonfishes are found primarily in the Indian and Pacific Oceans, respectively, but overlap at the Christmas and Cocos-Keeling Islands hybrid zone in the eastern Indian Ocean. We also sampled the two other Pacific members of this species complex, A. achilles (N=54) and A. japonicus (N=49), which are known to hybridize with A. nigricans where their ranges overlap. Our results indicate separation between the four species that range from the recent Pleistocene to late Pliocene (235,000–2.25million years ago). The Pacific A. achilles is the most divergent (and possibly ancestral) species with mtDNA dcorr≈0.04, whereas the other two Pacific species (A. japonicus and A. nigricans) are distinguishable only at a population or subspecies level (ΦST=0.6533, P<0.001). Little population structure was observed within species, with evidence of recent population expansion across all four geographic ranges. We detected sharing of mtDNA haplotypes between species and extensive hybridization based on microsatellites, consistent with later generation hybrids but also the effects of allele homoplasy. Despite extensive introgression, 98% of specimens had concordance between mtDNA lineage and species identification based on external morphology, indicating that species integrity may not be eroding. The A. nigricans complex demonstrates a range of outcomes from incomplete speciation to secondary contact to decreasing hybridization with increasing evolutionary depth.

Sequencing and analysis of the complete mitochondrial genome of the surgeon fish Acanthurus leucosternon BENNETT 1833 (Perciformes: acanthuridae) with phylogenetic consideration

In this study, the complete mitochondrial genome of Acanthurus leucosternon was determined. The genome is circular, double-stranded DNA molecule with 16,434 bp in length (EU 136032). The structure, gene organization, and gene order is similar as in other vertebrates. The overall base composition is A (29.29%), C (28.53%), T (26.37%), G (15.81%), and with 44.33% G + C content. Overlapping observed between contiguous genes is encoded in the opposite strands. The two 12S and 16S rRNA genes consist of 949 and 1690 nucleotides, respectively. Non-coding control region is 754 bp long and origin of replication is located in the WANCY cluster. Conserved domains in the control regions and secondary structures for the OL regions are also predicted. Molecular phylogenetic analysis using Maximum-Likelihood method of A. leucosternon based on the protein-coding gene sequence was also studied.

Microsatellite DNA markers to resolve population structure and hybridization of two closely related surgeonfish species, Acanthurus nigricans and Acanthurus leucosternon

Ten polymorphic microsatellite loci were developed for sister surgeonfish species: the white cheek surgeonfish (Acanthurus nigricans) which occurs primarily in the Pacific Ocean, and the powder blue tang (Acanthurus leucosternon) in the Indian Ocean. All loci were scored for widely separated collections (A. nigricans, N = 30, French Polynesia; A. leucosternon, N = 27, Republic of Seychelles; hybrids, N = 17, Cocos/Keeling Island, eastern Indian Ocean). For A. nigricans, A. leucosternon, and hybrids, respectively, the mean number of alleles per locus is A = 17.80, A = 16.60, and A = 14.60, while observed heterozygosity ranges from HO = 0.78–1.00, HO = 0.74–0.96, and HO = 0.69–1.00. With the exception of locus Ahy54, all loci conformed to Hardy–Weinberg expectations, most loci were in linkage equilibrium, and there was little evidence for confounding null alleles. These markers will be used to resolve range-wide population structure and patterns of hybridization.

Hybridization of reef fishes at the Indo-Pacific biogeographic barrier: a case study

Hybridization is recognized as an important source of genetic variation. In some reef fishes, including the Acanthuridae, hybridization has been detected due to intermediate colouration. This study used a molecular genetic approach to investigate hybridization in two Acanthurid species: Acanthurus leucosternon and Acanthurus nigricans, which have Indian and Pacific Ocean distributions respectively and are sympatric in the eastern Indian Ocean. In this area a putatitve hybrid, Acanthurus cf. leucosternon has been recognized based on intermediate colouration and restriction to the sympatric region of otherwise allopatric putative parental species. This study aimed to test this hypothesis using genetic tools. The three species were sampled from Cocos (Keeling) and Christmas Islands, the biogeographic boundary where many Indian and Pacific Ocean biota meet. Representatives from allopatric populations of both parental species and outgroups were also sampled. Mitochondrial COI and intron 1 of the nuclear ribosomal protein S7 were sequenced from 13 and 30 specimens respectively. Although sample sizes in this study are relatively small and more genetic data, including an extended phylogeographic sampling, is required to further evaluate these findings, the COI results support hybrid origins of Acanthurus cf. leucosternon, but S7 data are inconclusive due to the possibility of incomplete lineage sorting. The fourfold more abundant Acanthurus nigricans is most often the maternal parent. Inter-fertile hybrids apparently backcross with rare Acanthurus leucosternon males, transferring Acanthurus nigricans mitochondria to this species. These results suggest that Acanthurus leucosternon may eventually be lost from these islands, due to their relative rarity and introgressive hybridization.

Varying responses of herbivorous and invertebrate-feeding fishes to macroalgal reduction on a coral reef

The consequences of macroalgal overgrowth on reef fishes and means to reverse this condition have been little explored. An experimental reduction of macroalgae was conducted at a site in the Watamu Marine National Park in Kenya, where a documented increase in macroalgal cover has occurred over the last nine years. In four experimental 10 m by 10 m plots, macroalgae were greatly reduced (fleshy algal cover reduced by 84%) by scrubbing and shearing, while four similar plots acted as controls. The numerical abundance in all fish groups except wrasses and macroalgal-feeding parrotfishes (species in the genera Calotomus and Leptoscarus) increased in experimental algal reduction plots. Algal (Sargassum) and seagrass (Thalassia) assays, susceptible to scraping and excavating parrotfishes, were bitten more frequently in the algal reduction plots one month after the manipulation. Further, surgeonfish (Acanthurus leucosternon and A. nigrofuscus) foraging intensity increased in these algal reduction plots. The abundance of triggerfishes increased significantly in experimental plots relative to control plots, but densities remained low, and an index of sea urchin predation using tethered juvenile and adult Echinometra mathaei showed no differences between treatments following macroalgal reduction. Dominance of reefs by macrofleshy algae appears to reduce the abundance of fishes, mostly herbivores and their rates of herbivory, but also other groups such as predators of invertebrates (triggerfishes, butterflyfishes and angelfishes).

Anatomy and use of the caudal spines in the aggressive behaviour of a surgeonfish (Osteichthyes: Acanthuridae)

We examined the anatomy of the lateral caudal spines of Acanthurus leucosternon and their use during intraspecific agonistic interactions. A. leucosternon can actively and independently erect its spines to a maximum of 80° from the sagittal plane through contraction of an epaxial erector muscle which does not directly insert on any part of the spine but in the cutis caudally of its anchoring basal plate. Maintenance of erection of the spine is achieved through the mechanical support of a prominent lateral apophysis of the 22nd vertebra. When presented with a model of a conspecific inside their territory both male and female A. leucosternon engaged in escalating aggressive activities and performed tailbeats with erected spines that lacerated the model. A similar escalation to the point of tissue damaging tailbeats could be elicited by introducing live fish to an aquarium containing a conspecific resident. Field and laboratory observations of A. leucosternon revealed that non‐experimental agonistic interact...