Hydroids (Cnidaria, Hydrozoa) of the Danish expedition to

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168 gonothecae were triangular with a wide distal aperture, thus differing from the ones repeatedly observed in D. mutulata (Fig. 26D), which are egg-shaped and have their opening on a distinct neck. Watson (2000), assumed that Millard’s gonothecae were immature. Although possible, I somewhat doubt this, because Millard could identify the sex of her specimen and she had several gonothecae at hand. New fertile material from the type locality of D. heurteli must be examined before further conclusions can be drawn. Therefore, following Millard (1975), Diphasia heurteli is here kept separate from D. mutulata, although they could prove to be conspecific. The hydrothecae in the present material also showed some variation within the same stem (Fig. 26A, C), but two distinguishable morphs were not discernible. Thornely’s (1904) material identified as D. mutulata had alternate hydrothecae and Ritchie (1909) referred it to Diphasia thornelyi Ritchie, 1909. The alternate hydrothecae seem to be the only reliable diagnostic character to distinguish D. thornelyi from D. mutulata. However, Ritchie (1909) and Billard (1925b) mention that occasionally some hydrothecae can be paired in D. thornelyi. Jäderholm (1919) identified material from Japan as D. thornelyi despite that it had opposite hydrothecae throughout. In one of the colonies examined for the present study, opposite and alternate arrangements occurred within the same stem. I therefore suspect that D. thornelyi is only a form of D. mutulata. I refrained from synonymizing them definitively because I have not seen the type material of D. thornelyi. Besides the two mentioned species, there are at least seven more Diphasia species known from Indonesia: D. cauloatheca Billard, 1920b; D. cristata Billard, 1920b; D. densa (Stechow, 1923); D. minuta Billard, 1920b; D. orientalis Billard, 1920b; Diphasia mutulata; and D. scalariformis. The latter two species were also found in the present material. Diphasia mutulata is easily distinguishable from D. scalariformis. Diphasia scalariformis has overlapping, quite straight, rectangular hydrothecae with sharp edges that are distinctly displaced towards one side of the branch surface. D. mutulata has its hydrothecae on the lateral P. SCHUCHERT sides of the branch, they are mostly not contiguous, they are more curved and their distal end is tubular without sharp edges. Diphasia digitalis is distinguishable from D. mutulata through its pentagonal cross-section of the distal part of the hydrothecae, the much larger hydrothecae, the absence of intrathecal processes, the branched colonies, and the larger gonothecae with numerous spines. Diphasia cauloatheca forms large, pinnate colonies and has stems without hydrothecae. Diphasia cristata is unbranched and has a characteristic pattern of sharp ridges on its hydrothecae and also on the rear side of the stem. Diphasia orientalis resembles D. mutulata, but its female gonothecae have complicated processes and the hydrotheca has a median ridge flanked by two channel-like concavities of the hydrothecal wall. Diphasia densa is not sufficiently well known. Diphasia minuta forms small (5 mm) shoots and has smooth gonothecae. Otherwise it closely resembles D. mutulata and it may in fact be only a form of this species Distribution Northern Australia, Indonesia, Andaman Sea, Red Sea. Type locality: Prince of Wales Channel, Torres Strait, Australia, 16 m. Diphasia scalariformis Kirkpatrick, 1890 Fig. 27. Diphasia scalariformis Kirkpatrick, 1890: 609, pl. 15: fig. 3. – Jäderholm 1903: 287. – Billard 1925b: 216, figs 55–56. Nigellastrum mutulatum. – Stechow & Müller 1823: 468. – Billard 1925b: 216, footnote. Material examined: Kei Islands Expedetion stations: 15. – 16. – 18. – 26. – 30. – Kei Island Expedition, Sulawesi, Ujungpandang, Samalon Island, 5 m, 28 Jun 1922; colonies of all stations with gonothecae and growing on Lytocarpia angulosa (Lamarck, 1816). Description Colonies growing on Lytocarpia angulosa, shoots straight, without hydrocladia or with some irregular side-branches, 2–5 cm high. Hydrothecae in opposite pairs, each pair contiguous on one side of shoot, separate on rear, thus hydrothecae shifted to anterior side of
HYDROIDS OF THE DANISH EXPEDITION TO THE KEI ISLANDS Fig. 27. Diphasia scalariformis Kirkpatrick, 1890; A–C, E, station 30; D, station 18. A. Two pairs of hydrothecae in anterior view, shown opaque. B. Hydrothecae in posterior view, shown transparent. C. Hydrocladium with female gonothecae having short spines, side view. D. Variant with longer and more recurved hydrothecae, left hydrotheca in side view. E. Female gonotheca with long spines. – Scale: A–E = 0.2 mm. 169 hydrocladium, the two rows of hydrothecae forming an angle smaller then 180°. Successive hydrothecae overlapping slightly. Hydrotheca quadrangular with sharp edges, distal half slightly curved outward, basal part straight, anterior and lateral sides of hydrotheca nearly plane, abcauline side 0.5–0.6 mm, adcauline side 0.35– 0.45 mm, adcauline side adnate for most of its length, opening of hydrotheca oblique, lateral margin S-shaped, anterior margin slightly depressed and thus forming two lateral marginal cusps, edges of hydrotheca reinforced by perisarc thickenings, on abcauline side slightly above middle an intrathecal shelf or flap projecting upwards into hydrotheca. Stem and branches without nodes. Gonothecae in a single row along anterior side of stem, very numerous, arising below hydrothecal pairs. Gonotheca egg-shaped, 0.45 mm long, diameter 0.3 mm, lateral and abcauline side with up to 12 spines, some colonies have blunt and shallow spines, some pointed and long ones, aperture of gonotheca at distal end on short neck. Female gonothecae with more than 50 small eggs, diameter of egg about 25 µm. Hydranths small, about 12 tentacles, without abcauline blind-sac. Remarks The tightly set quadrangular hydrothecae and the small gonothecae make this species immediately recognizable. Furthermore, in Indonesia this species only grows on the hydroid Lytocarpia angulosa (Lamarck, 1816). Jäderholm reported this species from southern Japan, growing on L. secundus (Kirchenpauer, 1872), a subjective synonym of Monoserius pennarius (Linnaeus, 1758) (see Ritchie 1910a). Diphasia digitalis is readily distinguishable from D. scalariformis through its pentagonal cross-section of the distal part of the hydrothecae, the much larger hydrothecae, the absence of intrathecal processes, the pinnately branched colonies, and the larger gonothecae with numerous spines. The differences from D. mutulata are given under the latter species. Billard (1925b) found that the male gonothecae of D. scalariformis has more pointed spines. Perhaps this correlation was due to con-
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