Exoskeletons of Bougainvilliidae and other Hydroidolina (Cnidaria, Hydrozoa): structure and composition

Abstract

The exoskeleton is an important source of characters for the taxonomy of Hydroidolina. It originates as epidermal secretions and, among other functions, protects the coenosarc of the polypoid stage. However, comparative studies on the exoskeletal tissue origin, development, chemical, and structural characteristics, as well as its evolution and homology, are few and fragmented. This study compares the structure and composition of the exoskeleton and underlying coenosarc in members of “Anthoathecata” and some Leptothecata, but does so mainly in bougainvilliid polyps histological analyses. We also studied the development of the exoskeleton under experimental conditions. We identified three types of glandular epidermal cells related to the origin of the exoskeleton and the secretion of its polysaccharides component. The exoskeleton of the species studied is either bilayered (perisarc and exosarc, especially in bougainvilliids) or corneous (perisarc). The exoskeleton varies in chemical composition, structural rigidity, thickness, extension, and coverage in the different regions of the colony. In bilayered exoskeletons, the exosarc is produced first and appears to be a key step in the formation of the rigid exoskeleton. The exoskeleton contains anchoring structures such as desmocytes and “perisarc extensions.”

Phylogeny and morphological evolution of the so‐called bougainvilliids (Hydrozoa, Hydroidolina)

“…Hydroids without gonophores or sufficiently developed medusa buds are difficult to identify. Also, the diagnostic exosarc covering the hydranth is influenced by contraction of the organisms (Mendoza‐Becerril et al., ; Schuchert, ) and environmental conditions (Mendoza‐Becerril et al., ; Vannucci & Rees, ).…”

“…(cf. Mendoza‐Becerril et al., ). AP and GP production increase and GAG production in the MM decreases in Leptothecata, resulting in the development of a rigid ME (albeit with varying levels of cover; Figure ).…”

“…Character 4 was also recoded. We added 12 new morphological characters (7–18) based on well‐defined species descriptions (Calder, ; Cornelius, ,; Mendoza‐Becerril et al., ; Schuchert, , ). Therefore, the final matrix includes 11 binary morphological characters and seven multistate, nonadditive characters (SMA).…”

“…A complex and important, but often overlooked, aspect to the natural history of Medusozoa is the development and evolution of the exoskeleton (Mendoza‐Becerril, Marian, Migotto, & Marques, ; Mendoza‐Becerril et al., ). In this context, some taxa have special relevance, but have yet to be explored in depth.…”

“…However, studies of the histology and development of the exoskeletal system of Hydroidolina, including the hydrotheca, are recent. These studies described the pseudohydrotheca as part of an extensive outer layer (the exosarc) in a bilayered exoskeleton (Mendoza‐Becerril et al., , ). This external layer (the exosarc) and the chitin‐proteic layer (the perisarc) have important implications for both evolution and ecology (Mendoza‐Becerril et al., , ).…”