Comparative morpho-physiological analysis between Ciona robusta and Ciona savignyi

• Significant morphophysiological differences between Ciona robusta and Ciona savignyi.
• In both species, the tunic absorbs more water than the other tissues.
• The differences strongly support the hypothesis of divergent ecological strategies.
• A new interesting functional role for the tunic was proposed.

The seasquirt Ciona robusta and its co-generic Ciona savignyi, are two long-divergent species, sharing the same habitat and competing for the same spaces and resources. Their very similar morphology has been responsible for the misidentification of the two organisms, and, consequently, for the underestimation of their geographic distributions and new areas of co-occurrence. In spite of the large amount of knowledge built up by developmental biologists, few data are available regarding the morpho-physiology of the two sea squirts. The comparison of morphological and physiological features carried out in the present study (i.e. length-body weight correlation, tunic/organ ratio, tissue-specific retained water and metabolic rate), highlighted slight, but significant, differences strongly supporting the hypothesis of different ecological strategies for the two species. More precisely, C. savignyi invests more energy in growing faster and taller, likely to improve the quality and concentration of the filtered food, and sustains a faster metabolic/growth rate. Conversely, C. robusta invests more into tunic thickness, reducing the risk of predation, even if this likely means a slower metabolic/growth rate. In addition it was noted that in both species, the tunic absorbs more water than the other tissues, a peculiarity that may allow them to better counteract sudden fluctuations in environmental salinity.