Obelia dichotoma ( Linnaeus, 1758 )

Mendoza-Becerril, María A., Estrada-González, Mariae C., Mazariegos-Villarreal, Alejandra, Restrepo-Avendaño, Luisa, Villar-Beltrán, Rogelio D., Agüero, José & Cunha, Amanda F., 2020, Taxonomy and diversity of Hydrozoa (Cnidaria, Medusozoa) of La Paz Bay, Gulf of California, Zootaxa 4808 (1), pp. 1-37 : 24-26

Obelia dichotoma ( Linnaeus, 1758)

Figs. 9 C–J, 10A

Sertularia dichotoma Linnaeus 1758: 812 .

Obelia plicata Hincks 1868: 159 , pl. 30, fig. 1.— Fraser 1938a: 38.— Fraser, 1938b: 109.— Fraser, 1948: 215.

Obelia dichotoma .— Fraser 1938a: 36; Fraser 1948: 214.

Type locality. South West England ( Cornelius 1975) .

Material examined. Polyp—PB6_5, sampling site 2 (1), mature, on rock; PB7_23a, sampling site 1 (2), mature, 27°C, on Gracilaria crispata Setchell & Gardner and acrylic (artificial substrate); PB7_23b, sampling site 1 (2), mature, 27°C, on Sargassum sp. and unidentified algae; PB7_31, sampling site 2 (1), immature, 27°C, on alga.

Description. Colonies erect, sympodial, in some cases with first-order branches, up to 13 mm high, arising from a creeping hydrorhiza. Main stem monosiphonic, divided in internodes at regular intervals. Internodes 634.3 μm long, each with four annulations at base and one distal apophysis alternately given off the hydrothecal pedicel. Hydrothecae short, conical, 1.1x higher than wider, 290.4 μm long, 262.7 μm wide at margin, and 127.1 μm wide at diaphragm, with margin entire and slightly flared. Hydrothecal diaphragm oblique. Hydrothecal pedicel 280.4 μm long, with 6–12 annulations along its whole length. Gonothecae conical, 652.4 μm long and 301.1 μm in maxi- mum diameter, with a short distal neck, arising from the base of hydrothecal pedicel or the axis of main stem and branches. Gonothecae pedicels short (104.5 μm long), with 2–3 annulations throughout. Main stem of some colo- nies with dark red perisarc.

Nematocysts (length x diam.). Microbasic b-mastigophore type A (4.7–5.3 x 1.4–1.9 µm).

More detailed description in Calder (1991) and Cornelius (1995).

Taxonomic status. Accepted. AphiaID 117386.

Remarks. Colonies associated with diatoms between the hydranth and the hydrotheca and on the stem. Fraser (1938a) reported Obelia hyalina Clarke, 1879 from Baja California Sur, and although this species has been considered a synonym of O. dichotoma ( Linnaeus, 1758) ( Cornelius 1975; Calder 1991a), new evidence from morphology, genetics, and distribution ( Calder 2013; Cunha et al. 2020) support its validity. Therefore, the record of Fraser (1938a) is valid as well. The same is true for Obelia griffini Calkins, 1899 , which was reported from Cedros Island by Fraser (1948), and it is now supported as valid ( Calder et al. 2014; Cunha et al. 2020). Similarly, the status of Obelia plicata Hincks, 1868 has been a source of disagreement between taxonomists (see Calder 1991a; Cornelius 1990), but evidence from nematocyst structure and isoenzyme patterns suggests O. plicata is conspecific with O. dichotoma ( Linnaeus, 1758) ( Östman 1982a, b; Cornelius 1990). We follow this consensus and consider the record of Fraser (1938a) as O. dichotoma ( Linnaeus, 1758) . It is now widely accepted and supported that the traditional concept of O. dichotoma (cf. Cornelius 1995) comprises multiple cryptic lineages ( Calder 2013; Calder et al. 2014, 2019; Cunha et al. 2017, 2020). However, as discussed by Calder et al. (2019), affinities between local populations in the eastern Pacific still need to be determined, and molecular studies will be necessary for this determination.

Distribution. The species is traditionally considered to be cosmopolitan ( Cornelius 1995), but this should be reassessed, considering it is a species complex. In Mexican Pacific it has been reported from Baja California (Ángel de la Guarda Islands and Todos Santos Island) ( Fraser 1938b; Fraser 1948); Baja California Sur (Bahía de los Angeles, Espíritu Santo Island, Natividad Island, Santa Maria Bay) ( Fraser 1938a; Fraser 1938b; Fraser 1948; Aguirre & Bücle 1992); Sonora (Tiburón Island) ( Fraser 1948), Colima (Clarion Island) ( Fraser 1938a; Fraser 1948, as O. plicata ), Jalisco (Navidad Head) ( Fraser 1938a), Guerrero (White Friars) ( Fraser 1938a), and Oaxaca (Chacahua Bay, Corralero Bay, Mazunte, and Tangola-Tangola Bay) ( Fraser 1938a; Fraser 1948; Humara-Gil & Cruz-Gómez 2018).

Figures

FIGURE 9. A–B Clytia simplex (Browne, 1902): A, statocyst; B, nematocysts microbasic b-mastigophore type A. C–J Obelia dichotoma (Linnaeus, 1758): C–E, colony; F, central stem and hydrothecal pedicels; G–H, hydrotheca and annulations in pedicels; I–J gonothecae. Scale equals A—50 µm; B—20 µm; C—1.5 mm, D—1 mm; E—500 µm; F, G, H, J—200 µm; I—100 µm.