Abstract
The presence of dark pigment spots associated with primary tentacles (or structures derived from them, i.e., rhopalioids) in Staurozoa was recently overlooked in a study on the evolution of cnidarian eyes (defined as a “region made of photoreceptor cells adjacent to pigment cells”, irrespective of image formation, i.e., including all photoreceptive organs). Review of old and recent literature on Staurozoa shows that dark pigment spots are present in virtually all species of Manania, as well as some species of Haliclystus, Stylocoronella, and probably Calvadosia. The known ultrastructure of ocelli seems to be compatible with light perception, but no immediate response to changes in light intensity have been observed in the behavior of staurozoans. Therefore, although further studies addressing photic behavior are required, we discuss an earlier hypothesis that the dark spots in some stauromedusae may be related to synchronous spawning, as well as the possible sensorial function of rhopalioids. Observations summarized here suggest a possible ninth independent origin of eyes in Cnidaria, within a lineage of benthic medusae. Alternatively, documented similarity across medusae of Cubozoa, Scyphozoa, and Staurozoa—with eyes being topologically associated with primary tentacles in each of these taxa—could indicate shared ancestry and a single origin of eyes in this clade known as Acraspeda. Information on Staurozoa, one of the least studied groups within Cnidaria, is often neglected in the literature, but correctly recognizing the characters of this class is crucial for understanding cnidarian evolution.
Highlights
Staurozoa is a cnidarian class currently represented by 50 species classified in 11 genera (Miranda et al, 2016a; Miranda et al, 2018; Fig. 1)
Manania distincta (Kishinouye 1910) The first unequivocal mention on the dark pigment spots in Manania in the literature was provided by Kishinouye (1910) in his description of M. distincta: ‘‘the eight primary tentacles are transformed into small, cylindrical bodies
All dark pigment spots in staurozoans are associated with the primary tentacles, the region where they used to be, e.g., Stylocoronella and Calvadosia, or with the rhopalioids that are derived from primary tentacles (e.g., Manania and Haliclystus)
Summary
Staurozoa is a cnidarian class currently represented by 50 species classified in 11 genera (Miranda et al, 2016a; Miranda et al, 2018; Fig. 1). They are all marine, benthic, and generally reported in shallow temperate waters (Miranda et al, 2018). In Staurozoa, metamorphosis is mainly observed in the apical region (i.e., calyx) and the stauromedusa remains attached to the substrate by a basal peduncle
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