Hydroids (Cnidaria, Hydrozoa) of the Danish expedition to

HYDROIDS OF THE DANISH EXPEDITION TO THE KEI ISLANDS
unbranched material identified by Pictet as S.
compressa to D. cornicina. However, Calder
(1991) showed that D. cornicina is not unambiguously
identifiable and McCrady’s material
could have been either Dynamena disticha
(Bosc, 1802) or Sertularia distans (Lamouroux,
1816) (syn. Dynamena distans Lamouroux, 1816
or Tridentata distans; see Calder 1991). Calder
(1991), Migotto (1996), Medel & Vervoort
(1998) and others subsequently regarded D. cornicina
as conspecific with Dynamena disticha
(Bosc, 1802), a view also shared by the present
author. However, comparison of Mediterranean
material of Dynamena disticha (Fig. 30) with
Dynamena moluccana (Fig. 29) revealed that
both species are distinguishable. The shoots of
Dynamena disticha (see Calder 1991 for description
and synonyms) are usually simple (unbranched).
Only rarely, some shoots can be
branched once, as was found in material from the
Mediterranean (Fig. 30A). This contrasts with D.
moluccana, which is often pinnately branched,
but colonies with unbranched stems also occur.
Comparing the stems of both branched forms, it
is evident that they are different. The stem of
branched D. disticha has opposite hydrothecae
and it does not differ in structure from the one of
the side-branches (Fig. 30A). In D. moluccana,
the stem differs profoundly from the hydrocladia:
there are three hydrothecae per segment,
the distal pair is subopposite to almost alternate,
and the most proximal hydrotheca is almost completely
free and strongly recurved (Fig. 29A).
This structure of the stem segments has repeatedly
been observed in material coming from
different localities (e.g., Thornely 1904 (as Desmoscyphus
palkensis); Billard 1925b, Vervoort
1941, Cooke 1975, Millard 1975, Gibbons &
Ryland 1989 (all as D. cornicina); Hirohito 1995
(as D. exigua)). The hydrocladia of D. moluccana
and D. disticha resemble each other very
closely and it is understandable that both have
been synonymized. It seems, however, that D.
moluccana has slightly larger hydrothecae (abcauline
side 0.40–0.45 mm versus 0.25–0.35
mm). The gonothecae are also slightly different.
Those of D. moluccana are more elongate, those
of D. disticha characteristically spherical. The
gonothecae of D. disticha are also often strongly
annulated, sometimes even provided with trans-
173
Fig. 30. Dynamena disticha (Bosc, 1802); Mediterranean
material. A. Stem with rare branching point. B. Gonotheca. –
Scales: A–B = 0.2 mm.
verse ribs, while the ones of D. moluccana are
smooth, moderately annulated, or with transverse
ribs (Cooke 1975, Millard 1975). The variability
of the annulation precludes somewhat that
this character can serve as a reliable diagnostic
trait. The best characters to positively identify D.
mollucana are thus the pinnate shoots and the
stem morphology. Unbranched colonies of this
species are difficult to separate from unbranched
D. disticha, but a more oblong and smoother
gonotheca as well as larger hydrothecae are characteristic
for D. mollucana. Using these characters
to distinguish the species, it is evident that
Hirohito (1995) probably found both species in
sympatry. Hirohito’s D. exigua is clearly recognizable
as D. moluccana, while his D. cornicina
is obviously referable to D. disticha. This sympatric
occurrence of both morphotypes strongly
argues in favour of two separate species being
present and not geographic variants only (subspecies).
It was noted that in the branched shoots of D.
moluccana there is often a patch of larger nematocysts
in the epidermal lining of the hydrotheca.
This thickened patch, frequently found in many
Sertulariidae (Schuchert 2001), is usually found
at the curvature of the abcauline side of the