Morphology, endocrinology, and environmental modulation of gonadal sex differentiation in teleost fishes

Successful reproduction by an adult depends on the normal ontogenesis of the gonads, a complex process of cellular and histological differentiation that starts early in life. This process is theoretically predetermined by genetic factors and includes sensitisation of the bipotential gonads to endogenous endocrine factors prior to, during and even after commitment to maleness or femaleness. However, young fish are relatively vulnerable to a host of environmental (physical and chemical) factors that can affect this endogenous endocrine axis, disturbing or even overriding the putative developmental pathway. This sexually lability can be exploited to our advantage for the production of monosex fish populations of the most valuable sex for food production or aquarium fish trade. On the other hand, it represents also a potential path for undesirable influences from endocrine-disrupting chemicals and climatic factors, particularly environmental temperature. This paper provides a detailed account of the early histological process of gonadal sex differentiation, with special reference to gonochoristic species, and reviews the criteria employed to positively identify ovarian and testicular differentiation. It also reviews the development of endocrine competence and sensitivity of the differentiating gonads to exogenous influences in the context of the relative stability of genotypic sex determination in various fish species. Sex differentiation in some species seems to be under strong genetic control and may not require endogenous sex steroid production. Conversely, reliance on endogenous sex steroids for gonadal differentiation is observed in other species and this phenomenon is apparently associated with a higher incidence of environment (mainly temperature)-labile sex differentiation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evidence for genetic differentiation in the european conger eel Conger conger based on mitochondrial DNA analysis

ABSTRACT:   The European conger eel Conger conger is an important marine benthic fish in the North-East Atlantic and represents a valuable fishery resource. However, little is known about its reproductive biology. In an attempt to gain a better understanding of the conger eel population structure, mitochondrial DNA (mtDNA) sequences were examined. A region with 432 bp of the control region of the mtDNA was sequenced from 40 individuals from six different locations around the central and eastern North Atlantic Ocean. Thirty variable positions defined 28 distinct haplotypes. The average sequence difference within samples (1.3–4.2%) was comparable to those between samples (1.4–3.6%). MtDNA sequence-based statistical tests showed significant geographic differentiation between some local population samples, suggesting that the conger eel does not comprise a single panmictic population. However, given our sample sizes, these preliminary results should be interpreted with caution and more individuals from more sites, including the Mediterranean Sea, should be analyzed in detail. The genetic variability detected in this study is an initial step to elucidate the genetic background of the conger eel population structure.     

Genetics of neotropical fish: from chromosomes to populations

The Neotropical freshwater fish fauna is very rich—according to the most recent catalogue 71 families and 4,475 species have been described. However, only a small amount of general information is available on the composition of Neotropical marine fishes. In Brazil, 1,298 marine species have been recorded. General analysis of available cytogenetic and population genetic data clearly indicates research has been mainly concentrated on freshwater fishes. Thus, today, cytogenetic information is available for 475 species of Characiformes, 318 species of Siluriformes, 48 species of Gymnotiformes, 199 freshwater species that do not belong to the superorder Ostariophysi, and only 109 species of marine fishes. For the species studied, only about 6% have sex chromosomes and about 5% have supernumerary or B chromosomes. A review of the cytogenetic studies shows that these data have provided valuable information about the relationships between fish groups, the occurrence of cryptic species and species complexes, the mechanism of sex determination and sex chromosome evolution, the distribution of nucleolus organizer regions, the existence supernumerary chromosomes, and the relationship between polyploidy and evolution. In relation to populations in Neotropical marine waters, the studies have shown the presence of cryptic species, which has important implications for fishery management. Different levels of genetic structuring can be found among Neotropical freshwater migratory fish species. This raises important implications for fish population genetic diversity and consequently its sustainable utilization in inland fisheries and aquaculture, specifically for conservation of ichthyo-diversity and survival.     

春季和夏季杭州湾北部海域鱼类种群结构分析

为了解杭州湾北部海域鱼类种群组成及结构特点,利用相对重要性指数(IRI)、Cluster聚类和多元统计等方法对该海域鱼类种群结构进行分析。结果显示,春、夏季杭州湾北部海域共出现鱼类22种,隶属8目、12科、19属,鲈形目占比最高,为50.0%,其次为鲽形目和鲱形目,所占比例各为13.6%,生态类型以海洋性鱼类为主。调查共发现6种鱼类优势种,春季优势种为棘头梅童鱼(Collichthys lucidus)、鮸(Miichthys miiuy)和凤鲚(Coilia mystus),夏季优势种为棘头梅童鱼、白姑鱼(Pennahia argentata)、龙头鱼(Harpodon nehereus)、鮸和焦氏舌鰨(Cynoglossus joyneri)。Cluster聚类和非度量多维标度显示,杭州湾北部海域鱼类种群可分为春季组(Ⅰ组)和夏季组(Ⅱ组),春季组由5月站点组成,夏季组由8月站点组成,组间差异系数达到84.17%。ANOSIM分析表明,春季组和夏季组种群结构差异极显著(R=0.554,P0.01);BIOENV分析表明,春、夏季杭州湾北部鱼类种群结构与温度的相关性最高,相关系数为0.404。研究表明,杭州湾海域仍然是棘头梅童鱼、凤鲚等经济鱼类的产卵场和索饵场,鱼类种群结构的季节变化明显,棘头梅童鱼的生态洄游习性可能是造成种群结构变化的主要原因。     

Assigning individual fish to populations using microsatellite DNA markers

New statistical developments combined with the use of highly polymorphic microsatellite DNA markers enable the determination of the population of origin of single fish, resulting in numerous new research possibilities and applications in practical management of fish populations. We first describe three main categories of methods available, i.e. (i) assignment tests and related methods, (ii) discriminant function analysis and (iii) artificial neural networks. In all these, individuals can be assigned to the population from which their multilocus genotypes are most likely to be derived. Assignment tests are based on calculations of the likelihood of multilocus genotypes in populations, based on allele frequencies. Discriminant function analysis is based on multivariate statistics, whereas artificial neural networks formulate predictions through exposure to correct solutions. Assignment tests are the methods of choice when considering genetic data alone, whereas discriminant function analysis and artificial neural networks may be useful when genetic data are combined with, for instance, morphological and ecological data. Assignment tests can be used to assess the genetic distinctness of populations, for discriminating among closely related species and to directly identify immigrants or individuals of immigrant ancestry, and thereby study patterns of dispersal among populations, including sex‐biased dispersal. In a conservation context, assignment tests can be used to assess the genetic impact of domesticated fish on wild populations and for determining if extant fish populations are in fact indigenous or descendants from stocked fish or strayers, and they can be applied in forensics, for instance to reveal poaching. Assignment tests are at present most useful for studies of freshwater and anadromous fishes owing to stronger genetic differentiation among populations than in marine fishes. However, some genetically divergent populations of marine fishes have been discovered, which could be used as natural laboratories for studying dispersal and gene flow. It is foreseen that ongoing developments in statistical methods, combined with improved techniques for screening large numbers of loci, will permit assignment methods to become standard tools in studies on the biology of fishes.     

Ecological drivers of fish metacommunities: Environmental and spatial factors surpass predation in structuring metacommunities of intermittent rivers

Metacommunity theory is a new approach for explaining how local and regional processes contribute to community organisation and integrative studies are needed to fully characterise the processes underlying its structure and function. We analysed, through variation partitioning and distance decay relationships, how metacommunities of fish in pools of intermittent rivers are structured by environmental, species interaction and spatial factors. The results indicate that both species sorting and dispersal limitation (spatial factors) were important in shaping fish metacommunities. Species sorting was the most influential driver within fish metacommunities, but predation was much less relevant and did not show any pure effect in metacommunity structure. However, environmental factors were determinant among metacommunity patches.     

Population genetic studies of hilsa shad, Tenualosa ilisha (Hamilton), in Bangladesh waters: evidence for the existence of separate gene pools

Hilsa shad, Tenualosa ilisha (Hamilton), in Bangladesh is found in inland rivers, estuaries and the marine environment, throughout the year, but the peak catch period is during upstream migration. Tissue (white muscle, liver, brain) samples (total 640 specimens) were collected from three different localities, representing marine, brackish and fresh water, during the monsoon in the summer of the years 1993–1996 to identify genetic markers and study the population structure of this species. The samples were analysed by starch gel electrophoresis and isoelectric focusing, and stained for 15 enzymes and general muscle proteins. Only phosphoglucomutase, aspartate amino transferase, esterase and unidentified muscle proteins were found to be polymorphic. The allele frequencies for the samples collected in the marine environment deviated from corresponding samples from freshwater and estuarine localities, indicating that hilsa shad in Bangladesh waters comprise more than one gene pool.     

Genetic population structure of marine fish: mismatch between biological and fisheries management units

An essential prerequisite of a sustainable fisheries management is the matching of biologically relevant processes and management action. In fisheries management and assessment, fish stocks are the fundamental biological unit, but the reasoning for the operational management unit is often indistinct and mismatches between the biology and the management action frequently occur. Despite the plethora of population genetic data on marine fishes, to date little or no use is made of the information, despite the fact that the detection of genetic differentiation may indicate reproductively distinct populations. Here, we discuss key aspects of genetic population differentiation in the context of their importance for fisheries management. Furthermore, we evaluate the population structure of all 32 managed marine fish species in the north‐east Atlantic and relate this structure to current management units and practice. Although a large number of studies on genetic population structure have been published in the last decades, data are still rare for most exploited species. The mismatch between genetic population structure and the current management units found for six species (Gadus morhua, Melanogrammus aeglefinus, Merlangius merlangus, Micromesistius poutassou, Merluccius merluccius and Clupea harengus), emphasizes the need for a revision of these units and questions the appropriateness of current management measures. The implementation of complex and dynamic population structures into novel and less static management procedures should be a primary task for future fisheries management approaches.     

Realizing the potential of trait‐based approaches to advance fisheries science

Analysing how fish populations and their ecological communities respond to perturbations such as fishing and environmental variation is crucial to fisheries science. Researchers often predict fish population dynamics using species‐level life‐history parameters that are treated as fixed over time, while ignoring the impact of intraspecific variation on ecosystem dynamics. However, there is increasing recognition of the need to include processes operating at ecosystem levels (changes in drivers of productivity) while also accounting for variation over space, time and among individuals. To address similar challenges, community ecologists studying plants, insects and other taxa increasingly measure phenotypic characteristics of individual animals that affect fitness or ecological function (termed “functional traits”). Here, we review the history of trait‐based methods in fish and other taxa, and argue that fisheries science could see benefits by integrating trait‐based approaches within existing fisheries analyses. We argue that measuring and modelling functional traits can improve estimates of population and community dynamics, and rapidly detect responses to fishing and environmental drivers. We support this claim using three concrete examples: how trait‐based approaches could account for time‐varying parameters in population models; improve fisheries management and harvest control rules; and inform size‐based models of marine communities. We then present a step‐by‐step primer for how trait‐based methods could be adapted to complement existing models and analyses in fisheries science. Finally, we call for the creation and expansion of publicly available trait databases to facilitate adapting trait‐based methods in fisheries science, to complement existing public databases of life‐history parameters for marine organisms.