Bull. Natl. Mus. Nat. Sci., Ser. A, 47(4), pp. 157–161, November 22, 2021
DOI: 10.50826/bnmnszool.47.4_157

The Hydroid Stage of the Medusa Koellikerina bouilloni(Cnidaria, Hydrozoa)
Hiroshi Namikawa1,* and Mariko Kawamura2
1

Department of Zoology, National Museum of Nature and Science,
4–1–1 Amakubo, Tsukuba, Ibaraki 305–0005, Japan
2
Seto Marine Biological Laboratory, Field Science Education and Research Center, Kyoto University,Shirahama, Nishimuro, Wakayama 649–2211, Japan*E-mail: namikawa@kahaku.go.jp(Received 24 August 2021; accepted 22 September 2021)

Abstract The polyps of Koellikerina bouilloni Kawamura and Kubota, 2005 were discovered
growing on the shell of the gastropod Nassarius festivus (Powys, 1835) collected from Isozaki, Hitachinaka, Ibaraki, Japan. This hydrozoan species was so far known only from the medusae collected
in Tanabe Bay, Wakayama, Japan (type locality) and Papua New Guinea. The discovery of the polypand the rearing of the medusae permitted us to complement the description of this species.
Key words: Koellikerina bouilloni, polyp, hydroid colony, nematocyst, host gastropod, Nassarius
festivus.

IntroductionMaterials and Methods

The author (HN) had the opportunity to survey
the marine Hydrozoa of Ibaraki Prefecture, Japan
as part of the monitoring program “Comprehensive surveys of Plants, Animals, and Geology in
Ibaraki Prefecture by the Ibaraki Nature
Museum—3rd period—.” In this faunal survey of2018, some bushy colonies of anthoathecatehydroid with medusa buds were found on theshells of living Nassarius festivus (Powys, 1835),
a gastropod collected from seagrass bed in
Isozaki, Hitachinaka, Ibaraki, Japan. Thesehydroids could subsequently be identiﬁed asKoellikerina bouilloni Kawamura and Kubota,
2005 based on the morphology of the mature
medusae obtained by rearing the animals in thelaboratory. Koellikerina bouilloni has been, however, only described based on the mature medusa
morphology in the original description (Kawamuraand Kubota, 2005). Therefore, the species description is here augmented and supplemented by theinformation of the polyp stage and its cnidome.

Eight hydroid colonies on the shells of living
Nassarius festivus (3 specimens collected on 1stJuly 2018 and 5 specimens on 11 August 2018)
were used for the taxonomic study. The host gastropods were gathered by baited traps from theseagrass bed in Isozaki, Hitachinaka, Ibaraki
Prefecture, Honshu, Japan. All hydroid colonies
growing on the gastropods were taken alive to theDepartment of Zoology, National Museum ofNature and Science, Tsukuba, and maintained in
the laboratory in culture containers (6 cm in diameter, 3 cm in height) ﬁlled with artiﬁcial seawater(Marine Art SF-1: Osakayakken. Co. Ltd, Osaka)
at 23–24°C in order to obtain the mature medusae,
because knowledge of the medusa morphology isrequired the species within this family.
The polyps on the gastropod shells were fed
once per week with nauplii of Artemia sp. andtheir host gastropods with ﬂakes used to feedornamental ﬁsh. The seawater in the culture containers was renewed almost daily. The released
medusae were cultured until maturity under thesame conditions as the polyps. Living polyp and© 2021 National Museum of Nature and Science

158

H. Namikawa and M. Kawamura

medusa morphologies were observed under abinocular stereo-microscope. Sizes of eachnematocyst type observed in one mature medusa
and some polyps were measured with a compound microscope.
After observation, the specimens were ﬁxed
with 37% formaldehyde solution diluted 1 : 10with sea water and then deposited as voucher
specimens of the above-mentioned survey program in the Ibaraki Nature Museum with theaccession numbers INM-1-96232–96239 (thepolyps on the host gastropods) and 96240–96243(the subadult medusae).
Description
Family Bougainvilliidae Lütken, 1850
Genus Koellikerina Kramp, 1939
Koellikerina bouilloni
Kawamura & Kubota, 2005
(Figs. 1–2)

PolypsHydroid colonial (Fig. 1A), only found on theold, eroded shells of living gastropods identiﬁed

as Nassarius festivus (Powys, 1835). They were
not found on fresh shells with well deﬁned
grooves and sculptures, and other substrata suchas rocks, seagrass, and the other collected gastropod species. All colonies were covered withdetritus and algal overgrowth. Stolons grow randomly on the eroded surfaces of the shell.
Hydranths on unbranched hydrocauli (height up
to 6 mm), the latter arising randomly distributedon the hydrorhiza. Hydranth body fusiform, withup to 13 ﬁliform tentacles in a single whorl
below a conical hypostome. Hydrorhiza andhydrocauli covered with thin periderm, continued
onto hydranth body as ﬁlmy pseudohydrotheca
but not enveloping tentacle bases. Medusa buds
develop on the lower part of the cauli (Fig.
1A–B), ellipsoid shape, length about 0.4 mm,stalks shorter than bud. Nematocysts: desmonemes and microbasic euryteles (Table 2).
Medusae
Medusae (Fig. 2) were only obtained from thereared hydroid colonies in the laboratory, butwere not found in the plankton. The meristic values of the newly liberated (within 24 hours after
liberation from the hydroids), 2 weeks old, and 1

Fig. 1. The polyps of Koellikerina bouilloni Kawamura and Kubota, 2005 collected from Isozaki, Hitachinaka,Ibaraki, Japan. A. hydroid colony growing on the shell of Nassarius festivus. This photo was taken after thecolony was cleared from detritus and algal overgrowth, B. a polyp with a medusa bud (m) and a hydranth (h).
Scales＝5 mm (A), 0.5 mm (B).

The Hydroid Stage of Koellikerina bouilloni

159

Fig. 2. The Medusae of Koellikerina bouilloni Kawamura and Kubota, 2005 released form the rearing polyps inthe laboratory. A. a newly released medusa; B. a 2-weeks old medusa; C. a 2-months old male medusa withwell visible gonads; D. gonads of C; E. a batch of marginal tentacles of C. Scales＝0.5 mm (A), 1 mm (B),
2 mm (C).

month old medusae with gonads are given in
Table 1. The measured characters of Table 1 correspond to the ones of table 1 in Kawamura andKubota (2005). The medusae had a spherical orbarrel-shaped umbrella with four radial canals

and a ring canal. The radial canals end in tentacle
bulbs. Manubrium initially cylindrical with asimple mouth at its tip dangling from apex of
sub-umbrella, mature with pronounced cruciform
base (Fig. 2A, D). Number of marginal tentacles

H. Namikawa and M. Kawamura——＋, adaxial——
perradial——
2–3

marginal bulb orange
marginal bulb orange
marginal bulb orange

per bulb increasing during development from one
to up to seven. One ocellus on the basal part ofeach marginal tentacle in medusae having gonads(Fig. 2E). In most advanced stages observed four
gonad groups at base of manubrium, developing
close to perradial and adradial positions (Fig.
2D), resembling irregular groups of bulbous outgrowths or lappets. Examined animals were presumably all males. Three types of nematocysts:
desmonemes, microbasic euryteles and basitrichous isorhizas (Table 2).
Discussion

2095
Newly liberated*2
2-weeks old
1-months old*1: Items of this table followed those of table 1 in Kawamura and Kubota (2005).
*2: “Newly liberated” means the medusae within 24 hrs. after liberation from hydroid colonies.

—＋＋———
0
2–3
5–6
11
4–7
13
4–7
0.8–1.0
1.3–2.0
4.0–4.3

perradial interradialwidthheight

No. ofexamined
Stage of medusae

0.8–1.0
1.3–2.0
4.0–4.3

Projection orconstriction
Ocelli and
Peduncleof umbrella
its position
apex
No. ofbranchingof oraltentacles
No. of tentacles/
marginal bulb
Size of umbrella(mm)

Table 1.

Morphology of medusae of K. bouilloni obtained in the laboratory.*1No. of
lateralPositionfolds ofof gonadgonad(pairs)

Color except forocelli

160

The hydroids examined in this study could beidentiﬁed as Koellikerina bouilloni because thesubadult medusae obtained from the hydroid colonies agree well with the original description
given by Kawamura and Kubota (2005), notably
the size of umbrella, the arrangement of gonads,and number of tentacles per marginal bulb. Thishydroid shows a strong substrate preference,
being found exclusively on old, eroded shells ofliving Nassarius festivus.
Kawamura and Kubota (2005) stated that thisspecies occurs mainly in tropical waters. The
Ibaraki Prefecture (the new locality of our specimens in the present study) is, however, north of thelocalities given in the original description of thisspecies and inﬂuenced by a cold current (Oyashiocurrent). This ﬁnding suggests that K. bouilloni
may be more widespread across the tropical totemperate zones of the western Paciﬁc Ocean. Thisis also compatible with the wide distribution of
Nassarius festivus, the speciﬁc host gastropod of
this hydroid, from the Philippines to southern Hokkaido, Japan (see Tsuchiya, 2017). Therefore, inthe future, the hydrozoan species growing on theshells of N. festivus must be veriﬁed in the various
localities to conﬁrm the true distribution area of K.
bouilloni, because the real distribution of a hydrozoan species should be restricted to the sedentary
polyps, but not in the medusae which may be
spread by currents to a much wider area.
The hydroid of K. bouilloni is relatively simple
and corresponds to hydroids found in several otherThe Hydroid Stage of Koellikerina bouilloni
Table 2.

161

Dimensions (mean±S.D., range) of each type of nematocysts of Koellikerina bouilloni.

PolypsDesmonemes
Microbasic euryteles
Medusae
Oral tentacles
Desmonemes
Microbasic euryteles
Marginal tentacles
Desmonemes
Microbasic euryteles
Umbrella
Basitrichous isorhizas

nLength

Width

30
30

5.7±0.2 (5.4–5.8)
9.2±0.3 (8.3–9.7)

3.5±0.3 (3.2–4.2)
4.3±0.2 (3.7–4.6)3030

5.6±0.3 (5.0–5.8)
11.5±0.3 (10.8–11.6)

3.1±0.2 (2.9–3.7)
4.3±0.2 (4.2–4.6)3030

7.3±0.3 (6.6–7.5)
7.5±0.4 (6.6–8.3)

4.5±0.2 (4.2–5.0)
3.4±0.3 (3.3–4.2)

30

11.7±0.3 (11.6–12.5)

9.2±0.7 (8.3–10.0)

species of the Bougainvilliidae and Pandeidae(comp. Schuchert, 2007). Like many hydroid
stages in these two families, it cannot be identiﬁed
to species level without knowledge of the adultmedusa. In the genus Koellikerina, the polyps wereso far only known for K. fasciculata (Péron andLesueur, 1810) (Bouillon et al., 2006; Schuchert,2007). The hydroid colonies of Koellikerina species appear to be regularly covered by detritus
judging from the conditions of colonies observedin K. fasciculata (in Schuchert, 2007) and K.
bouilloni (in this study). The polyps of K. bouilloni can be distinguished from those of K. fasciculata by the host animals (speciﬁc gastropod species versus polychaetes and spines of cidaroid seaurchins), and the position of medusa buds (onbasal part of hydrocaulus versus upper part).
The cnidome of the K. fasciculata medusa comprises according to Schuchert (2007) desmonemes
and microbasic euryteles. In K. bouilloni medusae
we found additionally basitrichous isorhizas.
Acknowledgements
We wish to express our sincere gratitude to Dr.

Peter Schuchert (Museum dʼhistoire naturelle,Switzerland) for his critical review of our manuscript. We are also grateful to Mr. Yoshinori
Kobayashi of the Fisheries of Cooperative Association of Isozaki, Ibaraki Prefecture and themembers of “Association for the Study of Sea
Animals from Ibaraki”, especially Dr. Hiroshi
Morino (the Head of this association) and Ms.
Hiromi Ikezawa (the Secretariat), for their generous help in collecting specimens.
References
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