Diphasia digitalis ( Busk, 1852 )

Gil, Marta & Ramil, Fran, 2017, The genus Diphasia L. Agassiz, 1862 (Cnidaria, Hydrozoa) in Northwest Africa, Zootaxa 4363 (3), pp. 301-349 : 332-335

Diphasia digitalis ( Busk, 1852)

( Fig. 16 , 22B ; Table 9)

Sertularia digitalis Busk, 1852: 393 .

Diphasia digitalis: Bale, 1884: 101 , pl. IX, figs. 3–5; Nutting, 1904: 110 –111, pl. XXX, figs. 2–7; Buchanan, 1957: 365, fig.

20; Vervoort, 1959: 254–255, fig. 22; Vervoort, 1968: 37–38, fig. 17; Millard, 1975: 257– 258, fig. 85E; Schuchert, 2003: 166, fig. 25; Calder, 2013: 24–26, fig. 8a.

Not Diphasia digitalis: Vervoort, 1972: 99 –101, fig. 29.

Material examined. Guinea Bissau. BISSAU-0810, stn BS191, 10º18´25"–10º19´20"N, 16º08´47"–16º08´47"W, 24–25 m, 3-XI-2008: 984 colonies 10–140 mm high, 41 colonies growing on Idiellana pristis , 18 on worm tubes, 12 on bryozoans, five on crustaceans, four on bivalves, three on Sertularia marginata , one on Eudendrium ramosum and one on Halecium lankesteri ; 22 colonies with gonothecae.

BISSAU-0810, stn BS192, 10º19´30"–10º20´15"N, 16º10´27"–16º10´59"W, 24 m, 3-XI-2008: 78 colonies 15– 167 mm high, one of them growing on Idiellana pristis and another one on a bryozoan; 16 colonies with gonothecae.

BISSAU-0810, stn BS193, 10º21´13"–10º21´51"N, 16º10´17"–16º10´56"W, 21– 20 m, 3-XI-2008: twelve colonies 49–103 mm high, one of them with gonothecae.

BISSAU-0810, stn BS194, 10º21´59"–10º22´36"N, 16º13´23"–16º14´03"W, 21–24 m, 3-XI-2008: 22 colonies 39–126 mm high, no gonothecae.

BISSAU-0810, stn BS195, 10º22´44"–10º23´22"N, 16º15´37"–16º16´18"W, 29 m, 3-XI-2008: twelve colonies 48–120 mm high, one colony attached to Idiellana pristis , one colony with gonothecae.

BISSAU-0810, stn BS200, 10º18´55"N, 16º25´07"W, 79 m, 4-XI-2008: one colony 62 mm high, no gonothecae.

Guinea. CCLME-1205, stn BT283, 9º16´25"– 9º15´44"N, 14º18´39" –14º17´53"W, 45–49 m, 10-V-2012: two colonies 89–105 mm high, no gonothecae.

CCLME-1205, stn BT294, 9º46´00"–9º45´10"N, 14º46´03"–14º44´37"W, 34– 33 m, 12-V-2012: several colonies attached to shell fragment no gonothecae.

CCLME-1205, stn BT303, 10º03´44"–10º02´51"N, 15º28´30"–15º27´50"W, 28–29 m, 14-VI-2012: seven colonies 27–200 mm high, two colonies attached to gorgonians, one colony attached to Halopteris carinata , two colonies with gonothecae.

Biology. Diphasia digitalis has been found growing on different substrates like mud, sand and shell fragments and on the hydroid Idiellana pristis (Lamouroux, 1816) ( Ritchie 1910; Vervoort 1968). We found colonies growing on shell fragments, gorgonians, bryozoans, bivalves, crustaceans, worm tubes and hydroids.

Fertile material collected in April, July, August and December ( Bale 1884; Nutting 1904; Vervoort 1959; Schuchert 2003). In our material, gonothecae were found in June and November.

Distribution. Species with circumglobal distribution in tropical and subtropical waters ( Vervoort 1959; Schuchert 2003; Calder 2013). Recently reported in the eastern Mediterranean Sea as an invasive species ( Morri et al. 2009; Gravili et al. 2013). In Northwest Africa, D. digitalis has been reported in Guinea-Bissau ( Billard 1931b), Guinea ( Vervoort 1959) and Ivory Coast ( Redier 1965). The present material was obtained from Guinea Bissau and Guinea.

The bathymetric distribution ranges between 5 and 383 m ( Nutting 1904; Vervoort 1968). The present material was collected from a depth of 20– 79 m. The species was also reported in an intertidal area (Nagale & Apte 2014) and at a depth of 1431–1750 m ( Vervoort 1972); however, we consider both records doubtful (see Remarks).

Description. Hydrorhiza composed of a network of perisarcal tubules attached to hard substrates or a mass of intertwining tubules adhering to sandy or muddy sediments for anchoring the colony in soft bottoms. Stem unbranched and monosiphonic, with two rows of hydrothecae disposed in opposite pairs. In the proximal part of the stem, a pair of hydrothecae is placed on lateral surfaces and separated frontally by a conspicuous gap. In the distal region, hydrothecal pairs contiguous and placed on the anterior surface (fig. 16C). Hydrocladia pinnately arranged left and right in the same plane, with two pairs of hydrothecae separating two contiguous hydrocladia; they arise from the posterolateral surface of the main stem between two consecutive hydrothecae. Members of the same hydrothecal pair contiguous along the entire length of the hydrocladia. Stem and hydrocladia divided into regular internodes by transverse nodes that are only visible distally on the stem; each internode carries a hydrothecal pair (fig. 16B).

Hydrothecae in opposite pairs, with members of consecutive pairs almost touching or overlapping. Hydrothecae long, tubular, adnate for 3/4 of its entire length and with the distal part curved slightly away; hydrothecal cross-section rounded at the basal part but distinctly pentagonal distally, with 3–5 ridges visible in the frontal view as longitudinal striae. Hydrothecal margin sinuous but not toothed, with a broad and deep adcauline sinus; aperture directed upwards and closed by a single and large operculum attached to the adcauline sinus, forming an arched cap over the aperture.

Gonothecae on stem arranged in two very close rows, spindle shaped and with numerous short, stout spines disposed in 10–12 longitudinal rows with 10–12 spines per row; aperture terminal at the end of a small tubular prolongation (fig. 16D–E).

Remarks. Diphasia digitalis is easily recognizable by having hydrothecal pairs adnate for almost their entire length, by the pentagonal aperture of the hydrotheca and by having longitudinal striae on the hydrothecal walls. Another diagnostic characteristic is the color of the colonies, which varies between grey and black because of the presence of blackish pigment granules in the coenosarc ( Schuchert 2003) (fig. 22B).

The material described by Vervoort (1972) as D. digitalis from the Western Indian Ocean shows some differences related to the morphology of the hydrothecae, with a longer free part of the adcauline wall, aperture with “two rounded and fairly indistinct lateral teeth,” and the absence of longitudinal carinae on the walls. Vervoort (1972) interpreted this material as a young specimen; however, in our opinion, it could represent a different species. Similarly, the record of Nagale & Apte (2014) from an intertidal area in the Gulf of Kutch, India, needs to be confirmed with new data because the authors pointed out that the axial and hydrocladial hydrothecae were different in shape, while in D. digitalis , both types of hydrothecae are identical.

CCLME-1205

Stn BT303

Hydrotheca, length adnate part adcauline wall 670–850

length free part adcauline wall 190–310

length abcauline wall 770–990

diameter at rim 130–-390 Gonotheca, total length 1375–2200

maximal diameter 450–800

Figures

FIGURE 16. Diphasia digitalis (Busk, 1852). A–C, stn BS192; A, colony; B, hydrocladium, lateral view; C, part of axis with insertions of hydrocladia.D–E, stn BT303, gonothecae.Scales: a, 3 cm; b–e, 1 mm.

FIGURE 22. A, Diphasia nigra (Pallas, 1766), part of hydrocaulus with carina, abfrontal view. B, Diphasia digitalis (Busk, 1852), part of hydrocaulus with blackish pigment granules.C, Diphasia sp., frontal ramification.