Background: At depths below 10 m, reefs are dominated by blue-green light because seawater selectively absorbs the longer, 'red' wavelengths beyond 600 nm from the downwelling sunlight. Consequently, the visual pigments of many reef fish are matched to shorter wavelengths, which are transmitted better by water. Combining the typically poor long-wavelength sensitivity of fish eyes with the presumed lack of ambient red light, red light is currently considered irrelevant for reef fish. However, previous studies ignore the fact that several marine organisms, including deep sea fish, produce their own red luminescence and are capable of seeing it.
BackgroundWith about 120 colour morphs currently assigned to six nominal species, the genus Tropheus is an ideal model to study evolutionary divergence of populations in allopatry. The morphology of Tropheus has been described as relatively static, but reproductive constraints are sexually dimorphic due to mouthbrooding in females. We analysed phenotypic variation in six populations of T. moorii and one population of T. polli using geometric morphometrics to assess morphological differences among sexes in relation to the differentiation of populations and species.ResultsThe mean shapes differed significantly between sexes, populations, and species even though within-sex variation exceeded the divergence among populations. The first principal component of Procrustes shape coordinates revealed differences between populations and species in mouth position and ventral head shape. The second principal component reflected sex-specific shape differences, mainly comprising a relatively larger female viscerocranium and, in particular, a larger buccal area. While shape variation between populations and between sexes was primarily located in the cranial region, within-sex variation was relatively uniform across all landmarks.ConclusionsDeviations of the between-population and between-sex pattern of shape variation from that within sex indicate that the differences in head shape likely result from both adaptations to female mouthbrooding and population-specific foraging strategies.
Lake Tanganyika harbours the oldest and ecologically, morphologically and behaviourally most diverse species flock of cichlid fishes. Its species are excellent subjects for the study of explosive speciation and adaptive radiation. Many species are subdivided into numerous genetically and phenotypically distinct populations, often classified as distinct geographical races or colour morphs, which mostly differ in colour and much less in terms of morphology. This study for the first time quantifies morphological differences among such morphs by studying three populations of Tropheus moorii. We compared Ôtraditional morphometricsÕ (TM) and Ôgeometric morphometricsÕ (GM) to explore their potential for discriminating populations. So far species description and population discrimination are almost solely based on TM in the form of standardized measurements, although specialists are aware of their lack of diagnostic power for discrimination of closely related entities. Moreover, comprehensive TM measurements are time consuming and can best be done on dead specimens which have to be preserved in the case re-measuring is necessary. In contrast, GM can also be based on photographs and computer scans of anaesthetized fish, so that the same individual can be repeatedly analysed during its ontogeny. Here, we show that GM is more flexible in data acquisition and more powerful in the discrimination of species and closely related populations. While TM is restricted to distances and ratios of distances, GM not only includes these measurements indirectly, but also allows for body shape analysis using a semi-landmark approach. It can be equally standardized as TM by defining diagnostic landmarks. Data description by canonical variate analysis was most informative using GM data including semi-landmarks, whereas differences between populations were significant (p < 0.05) based on both morphological approaches.
Twenty‐one coral‐ and coral rock‐associated gobiid fish species were examined in the Gulf of Aqaba, northern Red Sea in autumn 2003 and spring 2004. They represent the seven genera Bryaninops, Eviota, Gobiodon, Paragobiodon, Pleurosicya, Priolepis and Trimma. All species showed clear spatial niche segregation. Branching corals of the genus Acropora were obligatorily inhabited by Gobiodon spp., while Paragobiodon echinocephalus was restricted to Stylophora pistillata. Three Bryaninops species showed species‐specific associations with Acropora spp., Millepora dichotoma and Cirripathes sp. Most of the five Eviota species had weak associations with live corals but frequented coral rock. Among the genus Pleurosicya, two species were encountered. Pleurosicya micheli frequented massive scleractinian corals, while P. prognatha occupied various species of Acropora. Priolepis semidoliata was only occasionally observed on coral rock. Weak associations with scleractinian corals were documented in Trimma avidori and T. mendelssohni, both of which inhabit coral rock. Principal component and cluster analyses showed several morphological features to be important ecomorphological traits. Gobiid species inhabiting encrusting or massive corals have a more depressed body than species in branching corals. Eye size was well correlated with preferred water depth, especially in the coral‐associated species. A typical gobiid feature – the pelvic disc – was found in very different character states, and corresponded to preferred microhabitats. A well‐developed suctorial pelvic disc was typical for obligate coral‐dwellers.
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