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SAGA AÐLÖGUNAR Hornsíli eru líklega flestum kunn. Nafn sitt draga þau af göddum sem þau nota sér til varnar, en gaddarnir eru umbreyttir uggar. Einnig hafa þau brynplötur úr beini í stað hreisturs á síðum. Þau eru smávaxin, oftast um 4 – 5 sm og lifa að meðaltali í 3 ár.

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The amazingly adaptable entirely adorable Sticklebacks

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Page 1: Stick Back5

SAGA AÐLÖGUNAR

Hornsíli eru líklega flestum kunn. Nafn sitt draga þau af göddum sem þau nota sér til varnar, en gaddarnir eru umbreyttir uggar. Einnig hafa þau brynplötur úr beini í stað hreisturs á síðum. Þau eru smávaxin, oftast um 4 – 5 sm og lifa að meðaltali í 3 ár.

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Sílin finnast víða um láglendi og á hálendi Íslands, jafnt í litlum tjörnum og lækjum sem og stórum og djúpum vötnum. Óhætt er að fullyrða að hornsíli er algengasti ferskvatnsfiskurinn á Íslandi.

Skjót aðlögun hornsíla að ólíkum búsvæðum gera fiskana frábrugðna öðrum fiskum í ferskvatni hér á landi. Sílin virðast geta lifað við öfgakenndar aðstæður sem aðrar tegundir þola illa eða ekki. Þar má nefna örar seltubreytingar í sjávarlónum eða hitasveiflur í grunnum vötnum.

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Eftir síðasta jökulskeið (~ 10.000 ár) tók landið að rísa þegar þunga íssins létti. Þá lokuðust sjávarlón af og árfarvegir til vatna breyttust. Við þetta einangruðust fiskistofnar sem héldu til í vötnunum. Sjávarhornsíli höfðu þá numið hér land en þau eru forfeður ferskvatnshornsíla eins og við þekkjum þau í dag. Sjávarsílin eru stór (~ 11 sm), hafa lengri og grófari gadda og eru fullbrynjuð, þ.e. hafa brynplötur sem ná frá höfði að sporði. Hálfbrynjuð síli kallast þau sem finnast í ísöltum vötnum (sjávarlónum) en brynja þeirra nær að rauf.

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Eftir áratuga dvöl í ferskvatni fækkar brynplötum í 3 – 4 og gaddar verða styttri og fínlegri. Dæmi eru um að þessar breytingar taki aðeins örfáar kynslóðir. Ein ástæða þess að brynplötum fækkar er m.a. talin vera vegna minna afráns í ferskvatni en sjó.

Tjörnin var líklega sjávarlón. Eftir að henni var lokað endanlega, um 1989, og sjór gekk ekki lengur inn í hana minnkaði seltan og annar fiskistofn (áll) minnkaði.

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Greinilegt er að aðlögun að þessum breyttu skilyrðum er nú í fullum gangi hjá hornsílunum því í dag má sjá mikinn breytileika á fjölda brynplatna eða frá einni brynplötu upp í fulla brynju. Ætla má að eftir 10 – 15 ár verði þau flest lítið brynjuð.

Hér hafa bein hornsílanna verið lituð til að undirstrika breytileika í fjölda brynplatna.

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Heimildir:Bjarni Kristófer Kristjánsson ofl. 2002. Morphological segregation of Icelandic threespine stickleback (Gasterosteus aculeatus L.). Biological Journal of the Linnean Society 76:247 – 257.

Bjarni K. Kristjánsson. 2005. Rapid morphological changes in threespine stickleback, Gasterosterus aculeatus, in freshwater. Environmental Biology of Fishes 74:357 – 363.

Guðbjörg Ólafsdóttir, Sigurður S. Snorrason og Ritchie, M.G. 2007. Postglacial intra-lacustrine divergence of Icelandic threespine stickleback morphs in three neovolcanic lakes. Journal of Evolutionary Biology 20(5):1870 – 1881.

Klepaker, T. 1993. Morphological changes in a marine population of threespine stickleback, Gasterosteus aculeatus, recently isolated in fresh water. Canadian Journal of Zoology 71:1251 – 1258.

Robinson, B.W. og Schluter, D. 2000. Natural selection and the evolution of adaptive genetic variation in northern freshwater fishes. Í: Adaptive Genetic Variation in the Wild. Ritstj. Mosseau, T.A., Sinervo, B. og Endler, J. s. 65–94. Oxford University Press, Oxford.

Sigurður S. Snorrason, Bjarni K. Kristjánsson, Guðbjörg Ólafsdóttir, Lisa Doucette, Hilmar J. Malmquist og Skúli Skúlason. 2002. Hornsílið. Í: Þingvallavatn: undraheimur í mótun. Ritstj. Pétur M. Jónasson og Páll Hersteinsson. Mál og Menning, Reykjavík.

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