Hydroids (Cnidaria, Hydrozoa) of the Danish expedition to

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