Eudoxoides mitra

Eudoxoides mitra is a calycophoran siphonophore of the family Diphyidae, accepted in the order Siphonophorae ^[1]. Huxley described it in 1859 under the name Diphyes mitra ^[1]. It lives in the shallow layers of warm seas, from the surface to about 200 m or more, and has been reported between 36° N and 38° S in all seas except the Mediterranean ^[2].

The world taxonomic register lists Diphyes mitra Huxley, 1859 and Diphyopsis mitra as unaccepted names of this species, now called Eudoxoides mitra ^[1]. Mapstone recounts that Huxley introduced the bracts of the eudoxid phase of this tropical diphyid from samples taken on his cruises ^[2].

The Diphyidae, the family it belongs to, comprise 45 species in eight genera. Their polygastric colony has one or two nectophores, and the small, bullet-shaped anterior nectophore provides the key data for identifying the species: somatocyst, mouthplate, ostial teeth, hydroecium and longitudinal ridges ^[3].

That nectophore is made of gelatin and is easily damaged when a sample is fixed; ethanol dehydrates it and shrivels it until identification becomes difficult ^[3]. In a study of 10 diphyids, all specimens were bullet-shaped except Diphyes dispar and Eudoxoides spiralis, which had modified variants, triangular and twisted respectively; Eudoxoides mitra was bullet-shaped ^[3].

In protein spectra obtained by MALDI-ToF, Eudoxoides mitra and Eudoxoides spiralis showed almost identical features in the most marked peaks, although the overall peak pattern did separate them from other species ^[3].

It is an epipelagic species, found from 0 to 200 m or more, reported as common between 36° N and 38° S and present in all seas except the Mediterranean; Mapstone counts it among the warm-water species that live mostly in shallow layers at tropical latitudes ^[2]. It occurs in the western Pacific, in the waters around Taiwan between 21° and 26° N ^[4], and in the equatorial Atlantic off northern Brazil ^[5].

In the study of the Amazon plume, Eudoxoides mitra dominated the epipelagic community but was almost absent from the neuston, the thin surface film of water; the authors infer that it prefers deeper layers, where they observed an oxygen minimum zone ^[6].

As in the rest of the Diphyidae, the colony goes through an alternation of generations: the polygastric phase produces the eudoxid phase by asexual reproduction, and the eudoxid gives rise by sexual reproduction to new polygastric colonies ^[3]. In calycophorans in general, mature cormidia detach from the end of the stem and live free as eudoxids ^[2]; Mapstone does not describe this for this species in particular ^[2].

Mapstone summarises that small, active siphonophores of the epipelagic zone, which reaches down to about 300 m, lie in wait for their prey, mostly copepods, and spread their tentacles quickly to trap them. That description is of the group and not a measurement made on this species ^[2].

In the waters around Taiwan, where the monsoon changes conditions with the season, Eudoxoides mitra was one of the species contributing most to similarity within a group of 45 stations, mostly from February, with the lowest chlorophyll and the lowest siphonophore abundance; together with Abylopsis tetragona and Abylopsis eschscholtzi they added up to more than 27 % ^[4]. In another group, of stations mostly in the southern Taiwan Strait and east of the island, sampled in May, August and November, its mean abundance was 20 individuals per 100 m³ ^[4].

In the equatorial Atlantic off northern Brazil, it was present in 75.6 % of samples, among the most frequent siphonophores. It was one of the representative species of the oceanic stations outside the influence of the Amazon plume, and also among those that were most abundant under it ^[5].