Pseudothecosomata

Els Pseudothecosomata són un grup de mol·luscs gasteròpodes heterobranquis marins classificats tradicionalment entre els pteròpodes, coneguts habitualment com a papallones de mar. Com altres pteròpodes, estan adaptats a la vida a la columna d’aigua i utilitzen unes expansions del peu en forma d’ales, anomenades parapodis, per nedar. A diferència dels Euthecosomata, que solen presentar una conquilla externa ben desenvolupada, els pseudotecosomats adults generalment tenen la conquilla externa reduïda o no en tenen. El seu cos delicat i les estructures especialitzades per nedar reflecteixen la seva adaptació a la vida pelàgica, lluny del fons marí.

Corolla spectabilis, un Pseudothecosomata típic

Els Pseudothecosomata habiten en ambients marins d’arreu del món, especialment a mar obert, on formen part del zooplàncton. Es troben en aigües tropicals, temperades i, en alguns casos, més fredes, i la seva distribució està condicionada per factors com els corrents oceànics, la temperatura, la disponibilitat d’aliment i la profunditat. Moltes espècies són relativament petites i transparents, fet que en dificulta l’observació sense tècniques de mostreig especialitzades. Algunes viuen principalment en aigües superficials, mentre que d’altres es desplacen verticalment per la columna d’aigua en resposta als canvis de llum, les oportunitats d’alimentació i les condicions ambientals.

Una de les característiques més distintives del grup és l’estructura del cos i la manera com es desplaça per l’aigua. Els parapodis s’han desenvolupat en forma de lòbuls alars que baten rítmicament i permeten a l’animal nedar. En moltes espècies, el cos és gelatinós i transparent, i la conquilla és absent o s’ha reduït a una petita estructura interna. Alguns pseudotecosomats presenten un cos voluminós i gelatinós, amb un ampli aparell natatori, cosa que els confereix un aspecte molt diferent del de les papallones de mar amb conquilla, més conegudes. Aquestes diferències anatòmiques són importants per distingir les diverses espècies i comprendre les seves adaptacions al medi pelàgic.

Els pseudotecosomats presenten diverses estratègies alimentàries. Molts són depredadors especialitzats que capturen altres organismes planctònics, inclosos petits crustacis i altres animals gelatinosos o de cos tou. Algunes espècies utilitzen estructures especialitzades o mucositat per facilitar la captura de les preses, mentre que d’altres depenen de la natació activa i del contacte directe. El seu comportament alimentari els converteix en un component important de les xarxes tròfiques pelàgiques, ja que connecten diferents grups de zooplàncton i serveixen d’aliment a depredadors marins de mida més gran.

El seu cos gelatinós i el seu estil de vida planctònic també fan dels pseudotecosomats uns organismes interessants per als estudis d’ecologia oceànica. La seva distribució i abundància poden variar segons les masses d’aigua, els canvis estacionals i la disponibilitat de preses. Com a integrants del zooplàncton, contribueixen a la transferència d’energia a través de les xarxes tròfiques marines i formen part de les complexes relacions ecològiques que sostenen els ecosistemes oceànics. Tanmateix, la informació sobre la seva biologia i la dinàmica de les seves poblacions continua sent limitada per a moltes espècies, en part perquè són fràgils i difícils de recol·lectar i estudiar sense malmetre-les.

La taxonomia i les relacions evolutives dels Pseudothecosomata s’han revisat mitjançant investigacions anatòmiques i moleculars. La classificació tradicional dels pteròpodes en grans grups ha experimentat canvis a mesura que els estudis filogenètics han aclarit les seves relacions dins dels Heterobranchia. Per això, els límits i l’estatus de Pseudothecosomata depenen de la classificació emprada, i el grup s’ha d’entendre en el context de la sistemàtica, encara en evolució, dels gasteròpodes pelàgics. Malgrat el seu aspecte discret, aquests animals representen una part singular de la biodiversitat marina i ofereixen informació valuosa sobre l’evolució de la natació, l’alimentació i la supervivència a mar obert.

La classificació actual dels Pseudothecosomata, segons WoRMS és:

Algunes espècies de Pseudothecosomata citades a la Mediterrània o a la Península Ibèrica:

Corolla spectabilis

Cymbulia peronii

Peracle reticulata

Bibliografia

1246567 Pseudothecosomata 1 eunomia-revista-en-cultura-de-la-legalidad 50 creator asc 1 25096 https://opistobranquis.info/wp-content/plugins/zotpress/
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Bouchet, P., Rocroi, J. P. (2005). Classification and nomenclator of gastropod families. Malacologia, 47(1-2), 1-397.
Bouchet, P., Rocroi, J. P., Hausdorf, B., Kaim, A., Kano, Y., Nützel, A., Parkhaev, P. et al. (2017). Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia, 61(1-2), 1-526.
Bibliografia basada en els treballs de Steve Long, 2006. Bibliography of Opisthobranchia 1554-2000 i Gary McDonald, 2009. Bibliographia Nudibranchia, amb actualitzacions posteriors procedents d´altres fonts.