Stephanomiidae Huxley, 1859

Pugh, P. R. & Baxter, E. J., 2014, A review of the physonect siphonophore genera Halistemma (Family Agalmatidae) and Stephanomia (Family Stephanomiidae), Zootaxa 3897 (1), pp. 1-111 : 82

Family Stephanomiidae Huxley, 1859

Huxley (1859) established the Family Stephanomiadae to include four genera, namely Halistemma , Forskalia , Stephanomia , and Agalma . He differentiated between the various genera primarily on the presence or absence of an involucrum at the proximal end of the cnidoband of the tentillum. As noted above, he believed that such a structure was absent on the species of both the genera Halistemma and Forskalia , while it was present for the other two. Huxley then distinguished between the latter two genera on the basis of the unicornuate ( Stephanomia ) or tricornuate ( Agalma ) arrangement of the tentillum.

As the recent studies by Dunn et al. (2005a) have shown the grouping of these four genera together into a single family is no longer tenable. Halistemma and Agalma belong to the family Agalmatidae as their nectophores are budded off on the dorsal side of the stem. Although, like these two genera, Forskalia is monoecious and has both ascending and descending mantle canals, its nectophores are borne on the ventral side of the stem and, because of their multiserial arrangement, the genus has long been considered to belong to a distinct family, the Forskaliidae Kölliker, 1853 . The genus Stephanomia stands alone. It belongs to the dioecious group of physonect siphonophores, with only an ascending mantle canal. The studies by Dunn et al. (2005a) required considerable changes to the systematics of physonect siphonophores, and resulted in several genera, previously included in the catch-all family Agalmatidae , having an uncertain systematic position (see Pugh, 2006). Although the data clearly indicated that Stephanomia belonged in the dioecious clade of physonects, the position of certain genera, for instance Marrus , remain uncertain as they were not included in the original analysis. The genus has been included ( Pugh, 2006) within the dioecious clade despite the fact that specimens of one species appear to be monoecious. This has also been found to be the case for certain undescribed Bargmannia species (Pugh, personal information). The taxonomic position of Marrus was further discussed by Dunn et al. (2005b), who noted that there were important variations in some of the more general characters across the known species, including the absence of a muscle-free zone on the nectosac of M. orthocannoides Totton , which was present in the other species. It seems clear, therefore, that some of the species presently included in the genus Marrus may not belong there, as is the case for several undescribed species that have tentatively been associated with that genus. More morphological and genetic studies are required before the systematic position of that genus can be properly established.

However, there are certain characters of Stephanomia amphytridis that clearly distinguish it from all other physonects, and these are deemed of sufficient importance to warrant the re-establishment of the family Stephanomiidae . The very distinctive ridge pattern on the nectophores sets this species apart, as does the course of the lateral radial canals on the nectosac. Almost all of the known dioecious physonect species have straight lateral radial canals; the exception being Pyrostephos vanhoeffeni Moser. But that species has other characters that clearly set it apart. Palpons are present on specimens of S. amphytridis but are often absent on other dioecious species. For many species the cnidoband of the tentillum is straight, e.g. families Erennidae and Pyrostephidae , although for Marrus they, like S. amphytridis , are loosely coiled, without an involucrum and with a single terminal filament.