Microstructure variation in sponges sharing growth form: The encrusting demosponges Dysidea avara and Crambe crambe

The goals of this study are to assess variability of microarchitecture in sponges with a similar growth form and to look for correlates between microstructural organization, biological functions and ecological strategies. A comparison of the microstructure of the encrusting sponges Dysidea avara (Dendroceratida) and Crambe crambe (Poecilosclerida) is performed. The species co‐occur in sublittoral habitats of the western Mediterranean. Histological techniques, image analysis and corrosion castings are used in order to quantify thickness, degree of development of the aquiferous system, relative amount of structural and cellular materials, and size of the choanocyte chambers in both species. The fine structure and cellular types are described and analysed through SEM/TEM. These sponges are known to feature contrasting ecological strategies: D. avara has higher growth and clearance rates than C. crambe , it is more susceptible to predation and uses a more opportunistic strategy of space acquisition and maintenance. C. crambe grows slowly, it is chemically protected from predation and it competes successfully for space with other invertebrates. These differences in biological strategies are here shown to have clear structural correlates: D. avara is structurally simpler, with fewer cell types and very scarce matrix material. Most of its section is occupied by the highly developed aquiferous system. Its construction is much looser than in C. crambe . In the latter species, structural complexity is higher, it produces long‐lasting structures such as the spicules, and it is more compact and thinner. It is concluded that there is wide scope for microarchitectural patterns, even in species with similar growth form, and that structural and cytological characters may prove to be useful descriptors of biological strategies in sponges.