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La riproduzione degli squali - Il giornale del mare
Skip to content Strategie per la riproduzione degli squali Le strategie riproduttive degli squali sono essenzialmente di due tipi: ovipara e vivipara . Nelle specie ovipare osserviamo, dopo essere state deposte, uova di forma e di dimensione che possono raggiungere 12 centimetri d’altezza. In genere le uova possiedono un robusto guscio di materiale corneo, permeabile all’acqua e all’ossigeno, all’interno del quale l’embrione cresce sino allo sviluppo completo. La nascita del piccolo squaletto, avviene in zone generalmente protette da cavità rocciose, o fra i rami di corallo o della vegetazione. Sono in grado una volta nati di nutrirsi autonomamente, mangiando, (come nelle specie più grandi), prede di dimensioni abbastanza grandi.
La riproduzione vivipara, è assai più complessa.
Alcune specie, i cui embrioni dipendono per il loro sviluppo dalle quantità di tuorlo associate all’uovo, come lo Spinarolo, in cui molte uova fecondate sono avvolte da una membrana ovigera comune, una volta giunte nell’utero dove si forma un fuso che dopo sei mesi si schiude liberando gli embrioni.
Questi continuano a svilupparsi in maniera indipendente uno dall’altro traendo nutrimento dal sacco vitellino, che fornisce loro nutrimento sino a poco prima del parto che avviene 22 mesi dopo la fecondazione. Questa è la gravidanza più lunga che si conosce.
In questo caso il rapporto tra madre ed embrioni diventa molto stretto, un poco, come i mammiferi più evoluti. In altre specie assistiamo invece alla presenza di una specie di placenta. In questo caso i nutrienti e l’ossigeno, passano tra la madre e l’embrione direttamente, utilizzando un cordone ombelicale. Nelle specie dotate di placenta, l’utero è suddiviso in camere, dalla trasformazione delle singole membrane ovigere.
In ogni camera si trova un embrione. Assistiamo anche alla cosiddetta “oofagia”, una sorta di cannibalismo tra fratelli. Le ovaie sono capaci di produrre molte uova, ma solo gli embrioni che si schiudono per primi avranno la possibilità di svilupparsi, nutrendosi delle uova via via prodotte. In ogni camera dell’utero si sistema un embrione. Le specie di squalo più famose che adottano questo sistema riproduttivo sono lo squalo Volpe e il Mako.
La nascita di uno squalo in natura. E’ la prima foto documentata Alcune femmine prima di partorire, si appartano in zone a ridosso della riva. Queste zone sono chiamate “ nursery area”. I piccoli che nascono hanno la possibilità di nuotare in acque calme e tranquille, a riparo anche dei predatori, per il tempo necessario poi di avventurarsi in mare aperto. Alcune specie d’uovo hanno come dei filamenti al margine, servono per ancorarsi su coralli, o altre parti, come anfratti delle rocce o fra la vegetazione marina.
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