Ecophysiology of mesohyl creep in the demosponge Chondrosia reniformis (Porifera: Chondrosida)

Chondrosia reniformis is a common marine demosponge that shows striking tissue plasticity and unusual body deformability. This sponge can develop long and slender outgrowths extending from the parental body. According to some authors, this phenomenon, called \u201ccreeping\u201d, can be related to asexual reproduction, atypical mechanisms of \uablocalized\ubb locomotion or passive response to environmental stress. Here we address this phenomenon by means of an interdisciplinary approach consisting of field survey, experimental field studies and experimental laboratory studies. During field survey and field experimental survey we observed that the instability of substratum is an important factor that trigs the beginning of creeping. The sponge size does not seem to be directly involved in the occurrence of the phenomenon. Specimens of Bergeggi (Ligurian Sea, northern Italy) show a high correlation between the creeping phenomenon and the sea temperature; this seems to support the hypothesis that the phenomenon is related to asexual reproduction, which is in its turn seasonally regulated by environmental temperature. In addition, experimental laboratory studies performed in different mechanical conditions on isolated samples of both ectosome and choanosome showed that temperature affects mesohyl mechanical properties: the lower is the temperature the stiffer is the mesohyl. The different physiological states recorded by the laboratory experiments are expressions of the mechanical adaptability of the collagenous mesohyl of C. reniformis and suggest that stiffness variability is under cellular control. On the basis of present results we can infer that C. reniformis can exert some control on the creeping phenomenon and that the primary factors implied in inducing creeping phenomena are the instability of substratum and the temperature. Interestingly the capability to modulate the mechanical properties of the collagenous matrix is an uncommon feature that C. reniformis shares with the mutable collagenous tissue (MCT) of Echinoderms. This close analogy, which is supported by morphological and physiological evidence, is an intriguing point that opens a wide range of evolutionary and functional questions