{"id":3835,"date":"2023-06-03T09:32:24","date_gmt":"2023-06-03T09:32:24","guid":{"rendered":"https:\/\/sciences-unamur.be\/bio\/system-ecotox\/?p=3835"},"modified":"2023-06-03T09:32:28","modified_gmt":"2023-06-03T09:32:28","slug":"a-field-expedition-in-the-mangrove-of-belize","status":"publish","type":"post","link":"https:\/\/sciences-unamur.be\/bio\/system-ecotox\/a-field-expedition-in-the-mangrove-of-belize\/","title":{"rendered":"A field expedition in the mangrove of Belize"},"content":{"rendered":"
Version fran\u00e7ais en bas de page
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In May 2023, Justine Belik (young teaching assistant), Ivan Blanco (FRIA doctoral researcher) and myself (Fr\u00e9d\u00e9ric Silvestre – professor) travelled to Belize, a small Central American country, to work on our favorite model fish: the mangrove rivulus. This killifish averages 3cm in adult size and is found exclusively in the red mangroves along the American and Caribbean coasts, from Florida to Brazil. Arriving in Belize after a 35-hour journey, the last leg of the trip is by small boat from the small town of Dangriga to the island of Tobacco Caye. Surrounded by birds such as the noisy grackle, with its loud call in the early morning, and the magnificent frigatebird, we set up our base camp here, some thirty minutes’ sail from Twin Cays, the chosen sampling site.<\/div>\n
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Sunrise and sunset from Tobacco Caye<\/figcaption><\/figure>\n<\/div>\n
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The loudy grackle (here, a female)<\/figcaption><\/figure>\n<\/div>\n
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A frigatebird stealing a fish to a cormorant<\/figcaption><\/figure>\n<\/div>\n
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Twin Cays was not chosen randomly, as its mangroves are home to a population of rivulus unique in the world. The highest genetic diversity is found here, the result of a high rate of reproduction between males and hermaphrodites. Indeed, the interest in studying this species, and what justifies our expeditions, is the fact that the rivulus is the only known vertebrate, along with a sister species living a little further south, to alternate between reproduction by self-fertilization of hermaphrodites and cross-fertilization with a male. The former quickly leads to quasi-clonal lines with very little genetic diversity, while the latter increases this diversity. Most rivulus populations, and particularly those found in Florida, have very few males, and therefore very little outcrossing and genetic diversity. In contrast, the Twin Cays population has proportionately many males and therefore high genetic diversity. The main objective of this mission is in line with Ivan Blanco’s thesis work, which aims to gain knowledge of the mechanisms controlling the sex ratio. For example, we will be analyzing epigenetic marks (DNA methylation) on Twin Cays males and comparing them with hermaphrodites to understand the role played by epigenetics in this particular reproductive system.<\/div>\n
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On our way to Twin Cays<\/figcaption><\/figure>\n<\/div>\n
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Arriving at Twin Cays, the northern sampling site (with an old empty cabine)<\/figcaption><\/figure>\n
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The mangrove at our destination (northern part of Twin Cays)<\/figcaption><\/figure>\n<\/div>\n
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This is our second mission to Twin Cays, a remote place that scientists have studied very little. In 2019, I had already come as part of the thesis work of Valentine Chapelle, now a doctor of science, and accompanied by our colleagues from the University of Alabama, Ryan Earley and Kristy Marson. At the time, we sampled some forty hermaphrodites and estimated their behavioral, genetic and epigenetic variability. This time, we’ll be focusing on the males.<\/div>\n
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Yes ! A mangrove rivulus !<\/figcaption><\/figure>\n
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Measuring oxygen, temperature, salinity and pH<\/figcaption><\/figure>\n<\/div>\n
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Mangroves cover almost the entire island, isolated from the other Cays and the mainland. It is dense and sometimes impossible to cross, made up of alternating red and black mangroves. The former are characterised by their spectacular stilt-like aerial roots, while the latter have vertical roots emerging from the ground, known as pneumatophores. Mangroves also have a particular mode of reproduction: they are viviparous and propagate by means of propagules, small plants ready to develop directly into mature trees. Mangrove trees are also able to eliminate excess salt through their leaves, visible as crystals on the surface. It has to be said that the organisms that live in mangroves have to find mechanisms to resist the very difficult and variable conditions of this ecosystem: sharp variations in salinity, high temperatures, very low oxygen levels and very high sulfide concentrations. However, in addition to the rivulus, many other animals have made their home here, such as crabs, lizards and abundant gastropod molluscs.<\/div>\n
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Red and black mangrove trees<\/figcaption><\/figure>\n
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Salt crystals on red mangrove leaves<\/figcaption><\/figure>\n
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View from the ground of black mangrove pneumatophores and red mangrove propagules<\/figcaption><\/figure>\n<\/div>\n
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For five days, we set out our homemade traps, half-submerged in the marshy waters, if possible on the descending tide. It’s sometimes difficult to find good spots. If there’s too much water, the traps are washed away and no rivulus can be caught. Conversely, too little water and the rivulus hide in crab burrows or crevices where they are difficult to extract. Three locations were sampled, roughly equidistant from each other. The first was to the east of the island, a site that had been surveyed in 2019, the second to the north, and finally the third to the south, near a rangers station. The latter was the most prolific in terms of rivulus, the eastern site having proved disappointing. In all, we were able to collect 95 fish, including 17 males, more than enough for Ivan to carry out his analyses and answer his questions. The icing on the cake was that 6 adult fish were brought back alive to Belgium, enabling us to develop a new line in our laboratory.<\/div>\n
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Mangrove crab climbing a tree<\/figcaption><\/figure>\n
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A common mangrove lizard following us<\/figcaption><\/figure>\n
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A huge hermit crab<\/figcaption><\/figure>\n
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Termites are also common in the mangrove<\/figcaption><\/figure>\n<\/div>\n
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Many gastropods found on the ground<\/figcaption><\/figure>\n<\/div>\n
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The next step is to go to Florida, to the Emerson Point reserve, where we’ll be studying a population that’s the opposite of the Twin Cays population, with its near-absence of males. But that’s for next July.<\/div>\n
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Twin Cays, Belize<\/figcaption><\/figure>\n<\/div>\n
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En ce mois de mai 2023, Justine Belik (jeune assistante doctorante), Ivan Blanco (doctorant FRIA) et moi-m\u00eame (Fr\u00e9d\u00e9ric Silvestre – professeur), nous sommes rendus au B\u00e9lize, petit pays d\u2019Am\u00e9rique centrale, afin d\u2019y travailler sur notre poisson mod\u00e8le favori: le rivulus des mangroves. Ce killifish mesure en moyenne 3cm \u00e0 l\u2019\u00e2ge adulte et se trouve exclusivement inf\u00e9od\u00e9 aux mangroves rouges le long des c\u00f4tes am\u00e9ricaines et carib\u00e9ennes, de la Floride au Br\u00e9sil. Arriv\u00e9s au B\u00e9lize apr\u00e8s 35h de voyage, la derni\u00e8re \u00e9tape se fait en petit bateau afin de rejoindre l\u2019\u00eele de Tobacco Caye depuis la petite ville de Dangriga. Entour\u00e9s d\u2019oiseaux comme le bruyant quiscaille, au cri puissant d\u00e8s le petit matin, ou la phr\u00e9gate magnifique, c\u2019est l\u00e0 que nous avons \u00e9tabli notre camp de base, situ\u00e9 \u00e0 une trentaine de minutes de navigation de Twin Cays, le site de pr\u00e9l\u00e8vement choisi.<\/div>\n
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Twin Cays n\u2019a pas \u00e9t\u00e9 choisi au hasard puisque ses mangroves abritent une population unique au monde de rivulus. On y rencontre en effet la plus haute diversit\u00e9 g\u00e9n\u00e9tique, r\u00e9sultat d\u2019un taux \u00e9lev\u00e9 de reproduction entre des m\u00e2les et des hermaphrodites. En effet, l\u2019int\u00e9r\u00eat d\u2019\u00e9tudier cette esp\u00e8ce, et ce qui justifie de telles exp\u00e9ditions, est le fait que le rivulus est le seul vert\u00e9br\u00e9 connu, avec une esp\u00e8ce soeur vivant un peu plus au sud, \u00e0 pouvoir alterner entre une reproduction par auto-f\u00e9condation des hermaphrodites et une reproduction crois\u00e9e avec un m\u00e2le. La premi\u00e8re abouti rapidement \u00e0 des lign\u00e9es quasi-clonales avec tr\u00e8s peu de diversit\u00e9 g\u00e9n\u00e9tique, alors que la seconde permet d\u2019augmenter cette diversit\u00e9. La plupart des populations de rivulus, et notamment celles rencontr\u00e9es en Floride, ont tr\u00e8s peu de m\u00e2les, et donc tr\u00e8s peu de croisements et de diversit\u00e9 g\u00e9n\u00e9tique. Par opposition, la population de Twin Cays a elle proportionnellement beaucoup de m\u00e2les et donc une diversit\u00e9 g\u00e9n\u00e9tique \u00e9lev\u00e9e. L\u2019objectif principal de cette mission s\u2019inscrit dans le travail de th\u00e8se d\u2019Ivan Blanco qui est d\u2019apporter des connaissances quant aux m\u00e9canismes qui contr\u00f4lent le sex-ratio. Il sera par exemple question d\u2019analyser les marques \u00e9pig\u00e9n\u00e9tiques (m\u00e9thylation de l\u2019ADN) sur les m\u00e2les de Twin Cays et de les comparer aux hermaphrodites afin de comprendre le r\u00f4le jou\u00e9 par l\u2019\u00e9pig\u00e9n\u00e9tique dans ce syst\u00e8me de reproduction particulier.<\/div>\n
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C\u2019est notre deuxi\u00e8me mission \u00e0 Twin Cays, endroit recul\u00e9 que les scientifiques ont tr\u00e8s peu \u00e9tudi\u00e9. En 2019, j\u2019\u00e9tais d\u00e9j\u00e0 venu dans le cadre des travaux de th\u00e8se de Valentine Chapelle, aujourd\u2019hui docteure en sciences, et accompagn\u00e9s de nos coll\u00e8gues de l\u2019Universit\u00e9 d\u2019Alabama, Ryan Earley et Kristy Marson. Nous avions alors \u00e9chantillonn\u00e9 une quarantaine d\u2019hermaphrodites chez qui nous avons estim\u00e9 la variabilit\u00e9 comportementale, g\u00e9n\u00e9tique et \u00e9pig\u00e9n\u00e9tique. Cette fois, ce sont surtout les m\u00e2les qui vont nous int\u00e9resser.<\/div>\n<\/div>\n
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La mangrove recouvre la quasi totalit\u00e9 de cette \u00eele, isol\u00e9e des autres Cays et du continent. Elle est dense et parfois impossible \u00e0 traverser, constitu\u00e9e d\u2019une alternance de pal\u00e9tuviers rouges et noirs. Les premiers sont caract\u00e9ris\u00e9s par ces spectaculaires racines a\u00e9riennes en \u00e9chasse, alors que les seconds montrent des racines verticales sortant du sol et appel\u00e9es pneumatophores. Les pal\u00e9tuviers ont \u00e9galement un mode de reproduction particulier puisqu\u2019ils sont vivipares et se propagent gr\u00e2ces aux propagules, sortes de petites plantes pr\u00eates \u00e0 se d\u00e9velopper directement en arbres adultes. Les pal\u00e9tuviers sont \u00e9galement capables d\u2019\u00e9liminer l\u2019exc\u00e9dant de sel \u00e0 travers leurs feuilles, visible sous forme de cristaux \u00e0 la surface de celles-ci. Il faut dire que les organismes qui vivent dans les mangroves doivent trouver des m\u00e9canismes pour r\u00e9sister aux conditions tr\u00e8s difficiles et tr\u00e8s variables de cet \u00e9cosyst\u00e8me: fortes variations de salinit\u00e9, temp\u00e9ratures \u00e9lev\u00e9es, taux d\u2019oxyg\u00e8ne tr\u00e8s faible et concentration en sulfure tr\u00e8s haute. Pourtant, outre les rivulus, de nombreux animaux y ont \u00e9lu domicile et nous suivent tout au long de notre travail comme des crabes et l\u00e9zards ou d\u2019abondants mollusques gast\u00e9ropodes.<\/div>\n<\/div>\n
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Pendant cinq jours, nous avons dispos\u00e9 nos pi\u00e8ges faits maison, mi-immerg\u00e9s dans les eaux mar\u00e9cageuses, si possible lors de la mar\u00e9e descendante. Il est parfois difficile de trouver de bons endroits. S\u2019il y a trop d\u2019eau, les pi\u00e8ges sont emport\u00e9s et aucun rivulus ne peut \u00eatre captur\u00e9. A l\u2019oppos\u00e9, trop peu d\u2019eau et les rivulus se cachent dans des terriers de crabes ou des enfractuosit\u00e9s o\u00f9 il est difficile de les y extraire. Trois lieux ont \u00e9t\u00e9 \u00e9chantillonn\u00e9s, r\u00e9partis \u00e0 peu pr\u00eat \u00e0 \u00e9quidistance l\u2019un de l\u2019autre. Le premier \u00e0 l\u2019Est de l\u2019\u00eele, site qui avait \u00e9t\u00e9 \u00e9tudi\u00e9 en 2019, le second au nord, et enfin le troisi\u00e8me au sud, pr\u00e8s d\u2019une station de rangers. C\u2019est ce dernier qui a \u00e9t\u00e9 le plus prolifique en rivulus, le site \u00e0 l\u2019Est s\u2019\u00e9tant r\u00e9v\u00e9l\u00e9 d\u00e9cevant. Au total, nous avons pu pr\u00e9lever 95 poissons, dont 17 m\u00e2les, nombres largement suffisants pour qu\u2019Ivan puisse r\u00e9aliser ses analyses et r\u00e9pondre \u00e0 ses questions. Cerise sur le g\u00e2teau, 6 poissons adultes ont pu \u00eatre ramen\u00e9s vivants en Belgique et nous permettront de d\u00e9velopper une nouvelle lign\u00e9e au sein de notre laboratoire.<\/div>\n<\/div>\n
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La prochaine \u00e9tape est maintenant de nous rendre en Floride, dans le r\u00e9serve d\u2019Emerson Point, o\u00f9 nous allons \u00e9tudier une population oppos\u00e9e \u00e0 celle de Twin Cays puisqu\u2019elle pr\u00e9sente une quasi absence de m\u00e2les. Mais cela, c\u2019est pour juillet prochain.<\/div>\n<\/div>\n
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