Genus Aequorea Péron & Lesueur, 1810

Aequorea Péron & Lesueur, 1810: 334;

type species Aequorea forskalea Péron & Lesueur, 1810 by subsequent designation by Mayer (1910).

Mesonema Eschscholtz, 1829: 112 .

Stomobrachiota Brandt, 1835: 220; type species Stomobrachiota lenticularis Brandt, 1835 by monotypy.

Stomobrachium Brandt, 1837: 189 .

Mesonema (Zygodactyla) Brandt, 1838: 360; type species Mesonema (Zygodactyla) coerulescens Brandt, 1838 by monotypy.

Crematostoma A. Agassiz, 1862: 360 .

Rhegmatodes A. Agassiz, 1862: 361 .

Aequorea (Aequoranna) Haeckel, 1879: 219; type species Aequorea discus Haeckel, 1879 by monotypy.

Aequorea (Aequorella) Haeckel, 1879: 219 .

Aequorea (Aequoroma) Haeckel, 1879: 220 .

Aequorea (Aequorissa) Haeckel, 1879: 221; type species Aequorea albida Agassiz, 1862 by monotypy.

Rhegmatodes (Regmatella) Haeckel, 1879: 222 .

Rhegmatodes (Regmatissa) Haeckel, 1879: 223 .

Staurobrachium Haeckel, 1879: 224: type species Aequorea stauroglypha Péron & Lesueur, 1810 by monotypy.

Mesonema (Mesonemanna) Haeckel, 1879: 226 .

Mesonema (Mesonemella) Haeckel, 1879: 226 .

Mesonema (Mesonemissa) Haeckel, 1879: 226 .

Diagnosis: Medusa manubrium very wide, circular; no gastric peduncle, but often with jelly cone within stomach; subumbrella without radial rows of gelatinous papillae. With numerous, unbranched radial canals, new radial canals develop centrifugally from stomach base. Gonads on radial canals, separated from manubrium. Marginal tentacles hollow; usually with excretory pores or papillae on adaxial side of base; no marginal or lateral cirri; statocysts closed; no ocelli.

Hydroid where known of ‘campanulinid’ type, small usually lacking species-specific characters (see Bouillon et al., 2006).

Remarks: Aequorea is a difficult genus as can be suspected from the complex synonymy given above. A comprehensive revision of its species is needed (Purcell, 2018). Kramp (1961) plainly expressed “The species are more or less doubtful.” Russell (1953) had similar thoughts. However, both experts contributed much in consolidating and establishing a workable species level taxonomy which is still in use today.

Dawson (2004) and Zheng et al. (2009, 2014) have begun to study species level systematics using DNA sequences, but we are still missing sequence data for many populations and species and their correlation with morphotypes.

Here, we separated the species first using their 16S data and then searched for suitable names that matched their morphology. A few photographs of fully grown medusae that were not sampled could afterwards also be attributed to nominal species. It seems, however, that the sequence data are not really congruent with described nominal species. While the species level diversity is higher than expected, it is also surprising that some have a much wider distribution than thought before [see also Pruski & Miglietta (2019) for A. australis and its occurrence in the Gulf of Mexico].

Some additional species were examined for this study to allow a better comparison and species delimitation. This material is listed above in the section Material and Methods.