Los euthecosomata son opistobranquios pelágicos, nadadores y holoplanctónicos que pasan toda su vida en fase planctónica. Su nombre se refiere a que tienen el cuerpo protegido por una concha calcificada o theca (caja en griego). Como una adaptación a la vida pelágica, los euthecosomata tienen unas expansiones del pié que les permiten usarlas como aletas de natación. Por esta razón, comúnmente se llaman mariposas marinas.
La concha calcificada podría ser espiralada como en las especies del género Limacina o con forma de tubo cónico o piramidal, como en las especies de los géneros Cavolinia y Clio, pero la concha es siempre frágil, de baja consistencia, para reducir el peso del animal y facilitar su suspensión en agua. También pueden tener un opérculo córneo, por lo general con espinas, utilizado para envolver por completo y proteger al animal dentro de la concha.
Styliola subula, un Euthecosomata
La cabeza de los euthecosomata tiene un par de tentáculos con usos olfativos llamados rinóforos. La cavidad paleal está bien desarrollada en este grupo, pero generalmente no tienen branquias, sino que respiran a través del epitelio del manto. La rádula tiene de 3 a 5 dientes por fila y el tracto digestivo tiene una especie de placas masticatorias para moler los alimentos ingeridos.
Biología
Los euthecosomata son grandes nadadores y se ha calculado que pueden nadar a una velocidad de hasta 45 cm por segundo. Las migraciones verticales diarias son comunes en la mayoría de las especies de euthecosomata. Algunas especies epipelágicas como Clio pyramidata realizan migraciones verticales diarias en la columna de agua de hasta 1.000 m, por lo que durante el día viven aproximadamente a esta profundidad, mientras que cada noche ascienden a niveles más superficiales para alimentarse. En otras especies se ha observado que, aunque los adultos se encuentran en aguas profundas, los juveniles tienden a vivir en aguas poco profundas.
Los euthecosomata son micrófagos que se alimentan de plancton microscópico (dinoflagelados, diatomeas, radiolarios, foraminíferos, etc.) que se dirigen a la boca con las aletas natatorias y el epitelio ciliado. El alimento consiste en fitoplancton que es retenido por la mucosidad, formando un cordón que se introduce en la boca.
Los euthecosomata son hermafroditas y cuando dos individuos se encuentran ambos se fecundan mutuamente. Después de la fertilización, los huevos se liberan al agua dentro de una substancia gelatinosa o mucosa. Las cápsulas ovígeras pueden producir, al eclosionar, larvas veligeras con o sin concha. La etapa larval puede durar de 45 a 90 días y los adultos viven entre 1 y 2 años.
La acidificación progresiva de las aguas marinas como resultado del aumento de los niveles atmosféricos de CO2 afecta muy negativamente la tasa de supervivencia de los eutecosomados (no pueden desarrollar una concha adecuada) y existe la teoría de que, por esta razón, los mares del sur pronto serán inhabitables para estas especies de testáceos si la tendencia actual continúa.
Distribución
Los euthecosomata, como muchas otras especies de plancton marino, suelen tener una amplia distribución geográfica que depende en gran medida de las corrientes oceánicas. Se pueden encontrar desde aguas polares hasta mares tropicales pero hay cambios frecuentes de tamaño y morfología de muchas especies, relacionados con su ubicación geográfica, lo que ha dado lugar a considerar muchas formas o subespecies. Los euthecosomata de la costa catalana se citan principalmente a partir de conchas vacías varadas en las playas después de las tormentas o por capturas ocasionales de barcos comerciales de arrastre.
Clasificación
Se conocen bastantes especies de euthecosomata a nivel mundial. Se clasifican principalmente por la forma de las conchas, mientras que otros rasgos como la forma de las aletas, el manto o los apéndices orales son secundarios.
La taxonomía actual de los Euthecosomata propuesta por Bouchet et al. (2017) y publicado en WoRMS es:
Euthecosomata Meisenheimer, 1905
- Cavolinioidea J. E. Gray, 1850 (1815)
- Cavoliniidae J. E. Gray, 1850 (1815)
- Cavoliniinae J. E. Gray, 1850 (1815)
- Cavolinia Abildgaard, 1791
- Diacavolinia van der Spoel, 1987
- Diacriinae Rampal, 2019
- Diacria J. E. Gray, 1840
- Telodiacria Rampal, 2019
- Vaginellinae A. W. Janssen, 2020
- Cliidae Jeffreys, 1869
- Creseidae Rampal, 1973
- Boasia Dall, 1889
- Creseis Rang, 1828
- Styliola Gray, 1847
- Cuvierinidae van der Spoel, 1967
- Hyalocylidae A. W. Janssen, 2020
- Limacinoidea Gray, 1840
- Heliconoididae Rampal, 2019
- Heliconoides A. d’Orbigny, 1835
- Limacinidae Gray, 1840
- Thieleidae Rampal, 2019
Algunas especies de Eutecosomata citadas en el Mediterráneo o en la Península Ibérica:
Bibliografía
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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.
Bibliografía basada en los trabajos de Steve Long, 2006. Bibliography of Opisthobranchia 1554-2000 y Gary McDonald, 2009. Bibliographia Nudibranchia, con actualizaciones posteriores procedentes de otras fuentes.
Styliola subula, un Euthecosomata
