Cement gland apparatus of the angelfish Pterophyllum scalare (Teleostei, Cichlidae): functional morphology in comparison with adhesive organs of other Chordata

In this report, the developmental stages of P. scalare are described, with particular attention to the functional morphology of the adhesive apparatus, composed of three pairs of glands, two dorsal and one ventral, which serve to attach the larvae to the substrate prior to their becoming free-swimming. The results show that this apparatus is a complex structure of remarkable dimensions. In particular, some of its components are in a decidedly posterior position compared to the larvae of other Chordata. The dorsal adhesive glands, located in the rostral most part of the embryo, begin to secrete before hatching, whereas the ventral ones, located in the anterior most part of the head, have not yet begun to function, despite their larger size. As development proceeds, the ventral glands are located in a more anterior position, optimal for attachment to a substrate. The fact that the glands constituting the adhesive apparatus do not function simultaneously, increases the time for secreting mucus, thus allowing the larva to remain anchored until it reaches an advanced stage of development, with muscles allowing autonomous movements. Chronological differences in development and functionality, therefore, lead to a morpho-functional situation only apparently different from the adhesive structures of Chordata.