Introduction

The form and organization of spicules in the sponge skeleton, sometimes combined with spongin fibers, are crucial for sponge taxonomy, systematics and phylogeny. Spicule shapes and morphology are genetically controlled (Uriz et al., 2003). Spicule evolution has been discussed since long (e.g., Cárdenas et al., 2011, 2012; Dendy, 1925; Dohrmann et al., 2017; Reid, 1968, 1970; Schuster et al., 2015) and, recently, it was confirmed that there is a relatively good agreement between molecular phylogenies and taxonomy based on morphology (including spicules) especially in Hexactinellida and Calcarea (subclass Calcinea) in comparison to other sponge groups (Dohrmann et al., 2017; Klautau et al., 2013). Homoplasies appear in several levels of sponge classification. In Calcarea, some characters of the skeleton architecture are highly homoplastic, as are characters of the aquiferous system (Manuel et al., 2003). The tetractinal symmetry of spicules of Demospongiae and Homoscleromorpha seems to have originated independently (Gazave et al., 2012; Uriz, 2006). Also, within demosponges several sponge groups established (among others), on spicule morphology, turned out to be polyphyletic (e.g., Cárdenas et al., 2011; Erpenbeck et al., 2006). It is especially in Astrophorina that convergent evolution and secondary loss occurred frequently in spicule evolution and took place many times in all major clades, for both megascleres and microscleres (Cárdenas et al., 2011). For example, it is assumed that calthrops, mesocalthrops and mesodichotriaenes have originated independently many times in these groups (Cárdenas et al., 2011). The same is true for phyllotriaenes, which are only known in some lithistid families. They may have evolved independently at least twice in Phymaraphiniidae and Theonella (Cárdenas et al., 2011). Also, sigmas (C‐shaped spicules) evolved more than once, i.e., in Biemnida, Poecilosclerida and Haplosclerida (Morrow et al., 2013). In other cases, close relationships between some sponge groups can be difficult to infer due to the highly autapomorphy morphologies of their spicules, like in the orders Halichondrida and Agelasida (Erpenbeck et al., 2006). Despite these reservations, systematics based on spicules has proven its usefulness in identification, classification, and reconstructions of sponges phylogenetic relationships.

The aim of this catalogue is to gather, define, and illustrate all types of sponge spicules described in various taxonomic studies (e.g., Bergquist, 1978; Borojevic et al., 1968; Boury‐Esnault & Rützler, 1997; Gadea Buisán, 1947; Green, 1991; Hooper, 2003; Hooper & Soest, 2002; Lundbeck, 1902; Reid, 1970; Ridley & Dendy, 1887; Schulze & Lendenfeld, 1889; Sollas, 1888; Reiswig & Kelly, 2011, 2018; Tabachnick & Reiswig, 2002; Vacelet, 2012; de Vos et al., 1991; Wiedenmayer, 1944, 1977 and the references cited therein; van Soest, 2021) using high resolution scanning electron microscope (SEM). Some terms were adopted or modified from the Thesaurus of Sponge Morphology (Boury‐Esnault & Rützler, 1997).

This comprehensive source of information about the occurrence of various spicule types, both in living sponges and in fossil representatives, will allow sponge specialists and non‐specialists, dealing with isolated spicules, to recognize and name them. It will be thus of great interest for taxonomists, ecologists, and paleontologists.