Ontogenetic changes in the spicule formation and their possible role in chromodorid opisthobranchs (Mollusca, Chromodorididae)

Abstract

Many dorid nudibranchs have calcareous spicules in their integument. These structures were lost during their evolution in some dorid lineages, as usually stated for most genera of the family Chromodorididae. Nevertheless, in this article, the presence of calcareous spicules is reported upon for the first time in 12 southern European species belonging to the genera Felimare and Felimida: Felimare villafranca, Felimare picta, Felimare orsinii, Felimare fontandraui, Felimare bilineata, Felimare cantabrica, Felimare tricolor, Felimida luteorosea, Felimida purpurea, Felimida krohni, Felimida luteopunctata and Felimida britoi. The spicules are arranged in the notum, lateral region of the body, rhinophores, gills and feet, and their morphology is very variable, even in the same species. Moreover, changes in size, shape and arrangement between juvenile and adult stages can be observed. Potential biological roles of these structures are discussed.

Key words:

• Felimare
• Felimida
• spicules
• function
• southern Europe

Acknowledgements

We wish to express our gratitude, for their help and support, to our dive companions Carlos Fernández Velasco, Ángel Fernández Gaytán and Raffaele Deliso. We thank Víctor Tóvar Timmermans for the drawings of the spicules and to Alma Sánchez Santos, Manuel J. López-Rodríguez and José Carlos García Gómez for their comments. This research paper has been supported by projects REN2001-1956-C17-02/GLO, CGL2006-05182/BOS and CGL2010-17187/BOS of the Spanish Ministry of Economy and Competitiveness (including the former Ministry of Sciences and Innovation). This is CEI·MAR journal publication 28.

Editorial responsibility: Christiane Todt

Supplementary material

One specimen of Felimida krohni that had been killed by cooling (not freezing) in a fridge and preserved in 70% ethanol was kept and used for the micro-CT study. Before carrying out the scanning, it was dehydrated using the standard procedure of 2 h in each of a series of ethanol solutions (with increasing percentage of concentrations: 80, 90 and 100%). Subsequently, it was immersed for 2 h in hexamethyldisilazane and air-dried overnight. Animals were scanned with the micro-CT SkyScan 1172 C by conducting an oversized scan (number of connected scans=4) with the following parameters: no filter, image pixel size (µm)=3.08, source voltage (kV)=42, and source current (µA)=47. Images were reconstructed with ^©NRecon software, with a Smoothing kernel=2 (Gaussian). Later, reconstructed images were ‘cleaned’ with ^©CT-Analyser by running a Custom Processing Task List (Thresholding, Despeckle, ROI-Shrink-wrap, Reload, Bitwise operations and Save bitmaps), obtaining a new series of reconstructed images. Images from the resulting series were corrected for position with ^©Data viewer, and saved as a definitive series, and finally the volume of reconstruction images were obtained with the new volume-rendering software ^©CTvox.

Felimida krohni: A, Spicules of anterior edge of the notum; B, spicules of rhinophores; C, spicules of the body wall (the high number could constitute a stronger inner armature); D, spicules of the tail.