Fig. 2

Summary of the main events in sponge reproduction and development. A Amoeboid oocytes (o) in the mesohyl (m) of the demosponge Agelas oroides collected in August 2018 and first described here. B Late-stage oocyte with nucleolated nucleous surrounded by a layer of folicular cells (fc) within the mesohyl of the demosponge Aplysina aerophoba [43]. C Release of egg (eg) masses by the demosponge Xestospongia muta (unpublished, UH). D Release of eggs (eg) in mucous strands by the demosponge Agelas sceptrum (photo by Stephen Frink). E Morules of A. aerophoba developing externally in the water column [43]. F Section of a morula of A. aerophoba similar to those in Fig. 1E, showing that they are surrounded by a collagen envelop (co) and a layer of follicle cells (fc). G A free-swimming parenchymella larva of the demosponge Ircinia felix, showing a posterior tuft of long cilia, which are part of a larval photoreceptor organ [44]. H Non-tufted parenchymella of the demosponge Ectyoplasia ferox, with a translucent posterior pole [45]. I Hand-made section through the body of an individual of demosponge Craniella zetlandica that was collected from a reef of the cold-water coral Lophelia pertusa in the Stjersundet area of a Norwegian fjord in August 2018 (unpublished, MM). This species undergoes a direct development and, therefore, lacks the larval stage. The picture shows smaller embryos (e) being brooded in deepest regions of the body, which migrate towards the surface of the sponge during development to be finally released as miniature sponge (juvenile = j) by squeezing themselves through the cells and spicules of the sponge cortex