Monotheca gibbosa, Calder & Faucci, 2021

Calder, Dale R. & Faucci, Anuschka, 2021, Shallow water hydroids (Cnidaria, Hydrozoa) from the 2002 NOWRAMP cruise to the Northwestern Hawaiian Islands, Zootaxa 5085 (1), pp. 1-73 : 38-40

Monotheca gibbosa , sp. nov.

Figs 11b, c , 12

Plumularia margaretta .— Cooke, 1977: 100, fig. 27 (not Monotheca margaretta Nutting, 1900 ).

Type locality. USA: Hawaii, Laysan Island , at SCUBA depth, on calcareous rubble .

Etymology. The specific name is derived from the Latin adjective gibbus, meaning “bulging”, “protuberant”, or “hunpbacked”, in allusion to the prominent hump at the base of the hydrotheca.

Voucher material. HOLOTYPE: Laysan Island , on a small piece of calcareous rubble, 18.ix.2002, three fragments of a colony, to 2 mm high, without gonothecae, coll. A. Faucci, ROMIZ B5448 .

PARATYPE: Laysan Island , on algae, 18.ix.2002. two colonies or colony fragments, to 2 mm high, without gonothecae, coll. A. Faucci, ROMIZ B5492 .

Description. Colonies minuscule, inconspicuous, with erect cormoids reaching 2 mm high, arising from a creeping hydrorhiza having occasional internal perisarcal spurs in its lumen. Hydrocaulus monosiphonic, strongly geniculate distal to a straight pedicellate base of varied length, unbranched, divided at regular intervals into long, slender, ahydrothecate internodes by distinct, transverse nodes; cauline internodes relatively straight, 180–230 µm long beyond basal region, 19–25 µm wide at nodes, with firm perisarc and internal perisarcal ridges at proximal and distal ends, each internode with one nematotheca in axil of a distal apophysis and one about midway on internode on side opposite apophysis. Apophyses with a mamelon and a distal perisarcal ridge, given off alternately from opposite sides of hydrocaulus, bearing short, unbranched hydrocladia each with a single distal hydrotheca. Hydrocladia 140–190 µm long, with one proximal athecate internode and a longer distal thecate internode; nodes nearly straight; athecate internodes short, lacking nematothecae, 35–40 µm long, narrowest proximally, widest distally, with a distal perisarcal ridge; thecate internodes 120–150 µm long, saddle-shaped in lateral view, each with a hydrotheca, a median inferior nematotheca, two lateral nematothecae, and usually with three internal perisarcal ridges beneath hydrotheca, terminating in a blunt and short central point between lateral nematothecae, ending proximal to adaxial wall of hydrothecal orifice. Nematothecae bithalamic, movable, conical; cauline nematothecae 45 µm long; median nematothecae 30–40 µm long, not reaching base of hydrotheca, rim sloping only slightly downwards from abaxial to adaxial wall; lateral nematothecae 35–45 µm long, arising from slight lateral bulges at distal end of internode, not reaching hydrothecal orifice. Hydrothecae prostrate, with base elevated on raised subhydrothecal chamber of internode, creating a prominent basal hump, proportionately deep, with maximum depth 125–145 µm, slender and cylindrical but with a pronounced U-shaped bend, perisarc firm relative to thecal size; adaxial wall concave, adnate to internode except free at distal end; abaxial wall strongly convex except becoming strongly concave below margin; rim nearly circular, entire, facing obliquely upwards, aperture diameter 65–67 µm; base with a ring of desmocytes; intrathecal ridge lacking.

Gonothecae not seen.

Remarks. Monotheca gibbosa , sp. nov., is immediately distinguishable from all other species currently assigned to Monotheca Nutting, 1900 in the distinctive shape of its hydrothecae. These are proportionately elongate, slender, and bent in the shape of a curved cylinder, with the abaxial wall having a prominent basal hump. In this, they somewhat resemble an inchworm (caterpillar of a geometrid moth) in motion. By contrast, hydrothecae in other species of the genus, namely M. obliqua ( Johnston, 1847) , M. australis ( Kirchenpauer, 1876) , M. hyalina ( Bale, 1881) , M. pulchella ( Bale, 1881) , M. spinulosa ( Bale, 1881) , M. flexuosa ( Bale, 1894) , M. margaretta Nutting, 1900 , M. posidoniae Picard, 1951 , M. togata Watson, 1973 , M. amphibola Watson, 2011a , M. bergstadi Gil & Ramil, 2021 , are shaped like a cup, bowl, scoop, cowl, or bonnet. Also differing in the same way are other species that have been assigned at times to Monotheca , including Plumularia excavata ( Mulder & Trebilcock, 1911) , P. crateriformis ( Mulder & Trebilcock, 1911) , P. vervoorti ( Leloup, 1971) , P. epibracteolosa Watson 1973 , and P. meretricia Watson, 1973 . More closely resembling hydrothecae of M. gibbosa are those of Plumularia goldsteini Bale, 1881 . However, each hydrocladium in that species bears 3–4 hydrothecae rather than one, and the aperture of the hydrotheca is vertical rather than oblique in orientation.

Other noteworthy characters also set M. gibbosa apart from its congeners. For example, there are no abaxial and adaxial flanges on the hydrothecae, as in M. amphibola . Thecate internodes of the species end in an inconspicuous point, while those of M. spinulosa terminate in a prominent spine ( Bale 1881; Watson 2011a). Hydrothecae lack an intrathecal ridge, such as that present in both M. australis and M. spinulosa . Nematothecae are not adaxially excavated like those in M. spinulosa and M. togata , and median nematothecae are neither adaxially excavated nor immovable as in M. australis and M. amphibola ( Watson 2011a) . The hydrothecal rim is entire rather than being indented by a median adaxial notch as in M. margaretta . Characters of the gonothecae are also of value in distinguishing species of Monotheca , but those of M. gibbosa are as yet unknown.

Although described here as a new species, this is the second report of M. gibbosa from Hawaii. Cooke (1977) reported it, as Plumularia margaretta , from red algae ( Amansia sp. ) on the Kahe Point Reef, Oahu. As noted above, the Atlantic M. margaretta differs in having hydrothecae that are cup-shaped and shallow rather than being elongate and in the form of a curved cylinder.

Hydroid colonies of M gibbosa are exceptionally minuscule and inconspicuous. Of all the species in a genus having diminutive cormoids, it is perhaps the smallest. Colonies described here were only 2 mm in height, and specimens reported from Kahe Point Reef, Oahi, by Cooke (1977, as M. margaretta ) were likewise 1–2 mm high. Other species of Monotheca with colonies less than 5 mm high include M. togata and M. amphibola ( Watson 2011a) . The largest colonies of the genus appear to be those of M. obliqua , which can reach 4 cm high ( Gravili et al. 2015).

Although most species of Monotheca are believed to be epiphytic ( Watson 2011a), hydroids of M. gibbosa examined here occurred on a fragment of calcareous rubble as well as on algae. Of 11 species recognized in the genus in this work ( M. obliqua , M. australis , M. hyalina , M. spinulosa , M. pulchella , M. flexuosa , M. margaretta , M. posidoniae , M. togata , M. amphibola , and M. bergstadi ), only four appear to be substrate generalists. The predominantly European M. obliqua has been reported from algae, sponges, hydroids, bryozoans, cirripedes, and rocks ( Cornelius 1995b; Gravili et al. 2015). In the Pacific, M. flexuosa has been found on red and green algae, mussels, ascidians, and polychaete tubes ( Watson 2011a). The common Atlantic hydroid M. margaretta has most often been observed on algae (e.g., Sargassum , Turbinaria ) and seagrasses ( Thalassia ), but it also occurs on invertebrates and inanimate substrates ( Galea 2010, as Plumularia margaretta ; Mendoza-Becerril et al. 2018; Calder 2019). The recently described M. bergstadi was found on algae and a bryozoan ( Gil & Ramil 2021).

Reported Distribution. Hawaiian archipelago. Oahu: Kahe Point Reef ( Cooke 1977, as Plumularia margaretta ).

Figures

FIGURE 11. Plumulariidae. a, Monotheca flexuosa (Bale, 1894), part of hydrocaulus with two hydrocladia, each with a hydrotheca, Kure Atoll, ROMIZ B5447. Scale equals 0.1 mm. b, Monotheca gibbosa, sp. nov., holotype, part of hydrocaulus with hydrocladia and three hydrothecae, Laysan Island, ROMIZ B5448. Scale equals 0.1 mm. c, Monotheca gibbosa, sp. nov., holotype, part of hydrocaulus, and a hydrocladium with one hydrotheca, Laysan Island, ROMIZ B5448. Scale equals 0.05 mm. d, Plumularia floridana (Nutting 1900), part of hydrocaulus with bases of two hydrocladia, one with two hydrothecae, Gardner Pinnacles, ROMIZ B5449. Scale equals 0.1 mm. e, Plumularia strictocarpa Pictet, 1893, part of hydrocaulus with bases of two hydrocladia, one with two hydrothecae, Kure Atoll, ROMIZ B5453. Scale equals 0.1 mm. f, Plumularia strictocarpa Pictet, 1893, part of hydrorhiza, and proximal end of hydrocaulus with a gonotheca, Kure Atoll, ROMIZ B5453. Scale equals 0.2 mm.

FIGURE 12. Plumulariidae. Monotheca gibbosa, sp. nov., holotype, Laysan Island, ROMIZ B5448. a, part of colony, with hydrorhiza, hydrocaulus, six hydrocladia, and six hydrothecae. b, distal end of hydrocaulus, with four hydrocladia and four hydrothecae. c, distal end of hydrocaulus, with hydrocladium and a hydrotheca.