Zyzzyzus Stechow 1921

Zyzzyzus Stechow, 1921

Table 1

Zyzzyzus Stechow, 1921: 249; 1923: 49.— Kramp, 1933: 12, 1949: 198.— Millard, 1975: 38.— Watson, 1978: 302.— Petersen, 1979: 121.— Bouillon, 1985: 112.— Wedler & Larson, 1986: 72.— Migotto & Silveira, 1987: 104.— Calder, 1988: 49.—Hirohito, 1988: 22.— Petersen, 1990: 177.— Migotto, 1996; 25.—Marques & Migotto, 2001: 465.

Zyzzygus;— Neave, 1940: 712 [incorrect subsequent spelling].

Zyzzyzuz;— Bouillon, 1985: 243 [incorrect subsequent spelling].

Description. Hydroid solitary. Hydranth vasiform or oblong. Oral tentacles filiform, short, in one or two whorls, circular in cross-section, adnate to hypostome. Aboral tentacles in one whorl, short and slightly capitate in juvenile polyps, long and filiform in mature ones, flattened in cross-section proximally, circular in cross-section distally. Hydrocaulus long, cylindrical, slightly widening basally, covered by thin perisarc originating from circular groove immediately below hydranth base. Space of coenosarc of hydrocaulus filled by large vacuolated gastrodermal cells, peripheral region with longitudinal endodermal canals or ridges formed by columnar epithelium. Distal portion of endodermal canals not continuously circular but opened in ridges, fusing proximally to form true canals, some wider than others. Anastomoses of canals occurring at any level of hydrocaulus. Basal region of hydrorhiza forming swollen structures of variable morphology: flattened base resembling basal disc, supporting small finger-shaped processes to rounded bulbs or anchoring processes of varied length. Hydranth with male and female gonophores on the same or on separate blastostyles. Gonophores cryptomedusoid, supported by short, dichotomously developed blastostyles. Mature female gonophores containing up to four actinulae.

Type species. Zyzzyzus warreni Calder, 1988, nomen novum for Zyzzyzus solitarius (Warren, 1906); originally described as Tubularia solitaria Warren, 1906 (Millard 1975, Migotto & Silveira 1987, Calder 1988).

Taxonomic history. The genus Zyzzyzus was established by Stechow (1921) for a single species, Tubularia solitaria Warren, 1906. However, that species name is a permanently invalid junior primary homonym of Tubularia solitaria Rapp, 1829 (ICZN 1999; Art. 57.2), and Calder (1988) proposed the binomen Z. warreni as a replacement name.

The genus remained monospecific for many years, exclusively for Z. warreni, but it now includes five nominal species, viz. Zyzzyzus warreni Calder, 1988, the most widespread species; Zyzzyzus spongicolus (von Lendenfeld, 1884); Zyzzyzus calderi Petersen, 1990; Zyzzyzus floridanus Petersen, 1990; and Zyzzyzus robustus Petersen, 1990.

Historically, Watson (1978) transferred the Australian coastal species Tubularia spongicola von Lendenfeld, 1884 to the genus Zyzzyzus, as Z. spongicolus (von Lendenfeld, 1884). Zyzzyzus calderi, a coastal species of Bermuda, was described by Petersen (1990) from part of the material previously described by Calder (1988) as Z. warreni. Zyzzyzus floridanus is also known from the north Atlantic region, but restricted to the southeastern coast of the United States of America (Petersen 1990). Lastly, Z. robustus is the only nominal species of the genus recorded from deep-water (Petersen 1990).

Hirohito (1988: 24) suggested that Corymorpha iyoensis Yamada, 1959 should be included in Zyzzyzus. His proposal has not been adopted in subsequent papers, and we also prefer to exclude it from the genus. Studies of type material of C. iyoensis, or examinations of fertile hydroids of the species from Japan, are needed to clarify its taxonomic affinities.

The systematic position of the genus has also been debated. Initially, Stechow (1923) included Zyzzyzus in the subfamily Corymorphinae Allman, 1872, within the family Tubulariidae. Kramp (1933) regarded Zyzzyzus as a synonym of Corymorpha M. Sars, 1835 at first, but later (Kramp 1949) regarded it as congeneric with Tubularia Linnaeus, 1758, after noting morphological and ecological similarities between the two (viz. association with sponges; perisarc structure; hydrorhiza not homologous with root filaments of Corymorpha; and production of an actinula larva in its life cycle). He therefore returned Zyzzyzus to the Tubulariidae.

Millard (1975) kept Zyzzyzus in the Tubulariidae based mainly on the existence of an actinula. She nevertheless recognized resemblances of the genus with some species of Corymorphidae (viz. coenosarc of the hydrocaulus with longitudinal endodermal canals; basal portion of the hydranth thickened and forming a diaphragm; and tentacular endoderm separated from the hypostomial endoderm by a mesogleal lamella). She also interpreted the hydrorhizal processes of Zyzzyzus as adaptations for living embedded in sponges, and not as being homologous with the filaments of corymorphids. Millard’s arguments were followed by Watson (1978), Petersen (1979), Migotto & Silveira (1987), Hirohito (1988), and Bouillon (1985), noting that “some characters from the Corymorphidae ” are present in the species of Zyzzyzus.

Calder (1988) presented alternative arguments about the taxonomic position of Zyzzyzus. Although acknowledging resemblances to Tubularia and Tubulariidae, he described the existence of a well-developed gastrodermic diaphragm (Calder 1988, fig. 38) between basal and hypostomial portions of the hydranth of Z. warreni which, associated with the solitary condition and both internal and external morphology of the hydrocaulus, were considered reasons for including the genus in the Corymorphidae.

During the 1990s, the taxonomic position of Zyzzyzus was finally discussed within a phylogenetic framework. Petersen (1990) considered both Tubulariidae and Corymorphidae to be referable to different superfamilies, Tubularioidea and Corymorphoidea. He (Petersen 1990: 121–122) proposed that the Corymorphidae had the following synapomorphies differentiating the family from tubulariids: “direct development of polyp through encysted gastrula stage, […] perisarc gelatinous, sheath-like, secreted from just under the aboral tentacles, […] aboral end of hydrocaulus pointed, […] statocysts present in the papillae or rooting filaments developed from the hydrocaulus of the hydroids”. Moreover, he noted that Tubulariidae had the following synapomorphies differentiating the family from corymorphids: “development from egg over sterrogastrula [sic] to actinula, […] hydranth vasiform, with broad base carrying the long aboral tentacles, and slender distal portion with short oral tentacles, […] parenchymatic cushion under the aboral tentacles enclosed by mesolamella”. In common, both superfamilies would have (page 121) the “development from egg to polyp without a planula stage”. This group was later supported in molecular analyses using mitochondrial 16S and 28S rDNA and was named Aplanulata, a putative clade uniting Hydridae with Candelabridae, Corymorphidae, and Tubulariidae (Collins et al. 2005, 2006). However, as noted by Collins et al. (2006: 111), many taxa have yet to be sampled (e.g. Acaulidae, Margelopsidae, Paracorynidae, and Tricyclusidae) to confirm the monophyly of Aplanulata and to understand the relationship among its families and genera (including Zyzzyzus).

Distribution. North and south Atlantic, Indian, Pacific.