Aiptasia Gosse, 1858

Grajales, Alejandro & Rodríguez, Estefanía, 2014, Morphological revision of the genus Aiptasia and the family Aiptasiidae (Cnidaria, Actiniaria, Metridioidea), Zootaxa 3826 (1), pp. 55-100 : 58-59

Genus Aiptasia Gosse, 1858

Diagnosis (after Carlgren 1949). Aiptasiidae with wide and regularly shaped pedal disc. Column usually elongated, smooth, with cinclides in 2–3 longitudinal rows in mid-column; in some species scattered cinclides proximally. Column not distinctly divisible into scapus and capitulum. Mesogleal marginal sphincter relatively strong, short, alveolar. Strong longitudinal ectodermal muscles in distal column. Tentacles long, simple, to 192, always smooth, without projections. Same number of mesenteries distally and proximally or more distally. Six pairs of perfect mesenteries. First and second cycles fertile. Retractor muscles diffuse to restricted. Acontia well developed. Symbiotic with Symbiodinium spp. or Amphidinium spp. Asexual reproduction by longitudinal or transversal fission. Cnidom: spirocysts, basitrichs, microbasic b- mastigophores and p -amastigophores.

Type species. Aiptasia amacha Gosse, 1858 by monotypy (Fautin et al. 2007a). Aiptasia amacha is a junior synonym of A. couchii , but the latter is not the type species of Aiptasia and should not be cited as such per Article 67.1.2 of the International Code of Zoological Nomenclature 1999 (the Code hereafter, ICZN 1999).

Included species. The genus Aiptasia includes 13 species ( Fautin 2013). However, molecular evidence shows that the genus is not monophyletic (Rodríguez et al. 2012, 2014). Similarly, we find two groups that differ morphologically, one corresponding to forms similar to the well-known European species A. mutabilis and the second corresponding to those forms similar to the more widely-distributed species known up to now as A. pallida — for which we erect a new genus, Exaiptasia gen. nov. (see below).

Aiptasia and Exaiptasia gen. nov. are distinguishable by gross external morphology when specimens are alive, but are difficult to distinguish once specimens are preserved. They differ in the number of mesenteries proximally and distally (more numerous distally or equal numbers proximally and distally in Aiptasia whereas this number is equal in Exaiptasia gen. nov.) and the mesogleal marginal sphincter muscle (reticulated and relatively stronger in Exaiptasia gen. nov. than in Aiptasia ). The cnidae of both genera differ particularly in the size and shape of the microbasic b- mastigophores in the column; those of Aiptasia are longer and slimmer (similar to the ones found in other aiptasiid genera) than the ones in Exaiptasia gen. nov., which are shorter and thicker and thus similar to those of Aiptasiogeton ; the microbasic p -amastigophores in the acontia of Exaiptasia gen. nov. are also slightly shorter than those of Aiptasia (see Tables 1–3 and Schmidt 1972). Aiptasia and Exaiptasia gen. nov. also differ in mode of asexual reproduction (transverse or longitudinal fission in Aiptasia but pedal laceration in Exaiptasia gen. nov.) and in habitat ( Exaiptasia gen. nov. is restricted to tropical and subtropical waters whereas species of Aiptasia are restricted to subtropical and temperate waters of the Eastern Atlantic and the Mediterranean Sea (except for A. prima , but see below). In addition, both genera are associated with different species of symbiont; Symbiodinium sp. “A1-med” clade and Amphidinium sp. in Aiptasia compared to Symbiodinium minutum “B1” and Symbiodinium sp. “A4” in Exaiptasia gen. nov. ( Thornhill et al. 2013; Grajales 2014).

Although most of the morphological differences we find are of degree more than of kind (e.g. marginal sphincter muscle), we think that the combination of morphological, cnida, and ecological differences warrant the distinction of Aiptasia and Exaiptasia gen. nov. Thus, after this revision, we restrict the use of the genus Aiptasia to three species (see Taxonomic remarks below). Nevertheless, the identity of A. prima remains unclear (indicated by a question mark in Included species, above) because the available data are not conclusive (see Taxonomic remarks following the treatment of Aiptasia mutabilis , below).