Monobrachium parasitum, a One-Tentacled Hydroid, Collected 
at Vancouver Island 
Richard D. Campbell 1 
The colonial hydroid Monobrachium para- 
situm (Mereschowsky) is of interest to system- 
atic and developmental biologists because of its 
peculiar form and its commensal habitat. Each 
feeding polyp has only a single tentacle, and 
the colony grows and develops a characteristic 
polymorphic pattern on living bivalves (Wag- 
ner, 1890; Hand, 1957). This note reports the 
rapid collection of this unusual hydroid with- 
out heavy equipment. 
On August 16, 1964, 13 colonies of M. para- 
situm were collected at Fraser’s (1918) original 
locality in Nanoose Bay, Vancouver Island, 
British Columbia. Nanoose Bay is about 1 km 
wide and 3 km long, and opens eastward. Water 
turnover is extensive during tidal changes. Tidal 
currents are located predominantly along the 
northern shore. Accordingly, the bottom of the 
northern third of the bay is composed of sand 
and light gravel, with remains of broken shells. 
The southern two-thirds presents a graded bot- 
tom from fine sand centrally to mud along the 
shallow southern shore. The bivalve Axinopsis 
was collected with a 1-mm mesh conical net 2 
inches in diameter, towed for 10 minutes be- 
hind a small boat. Monobrachium colonies were 
found in three out of seven tows made in the 
central third of the bay (124° 9.2' W, 49° 
1 Friday Harbor Laboratories, San Juan Island, 
Washington. Present address: Department of Orga- 
nismic Biology, University of California, Irvine, Cali- 
fornia 92664. Manuscript received July 23, 1966. 
15.8' N, 16-17 fathoms), but not in either the 
deeper northern or the shallower southern sides. 
Axinopsis with Monobrachium which were 
brought into the laboratory showed rapid and 
extensive burrowing through fine sand. Gen- 
erally the hydroids were completely under the 
surface of the sand. No colonies showed any 
evidence of reproductive polyps or of medusa 
buds. 
Hand (1957) found Monobrachium in a 
number of dredge loads from the California 
and Baja California coast. Besides Hand, only 
Fraser (1918) had reported Monobrachium 
from the West Coast of North America. Re- 
peated dredgings in the San Juan Archipelago, 
Washington, failed to reveal any specimens, 
even in habitats similar to those prevailing on 
the east coast of Vancouver Island. 
REFERENCES 
Fraser, C. McLean. 1918. Monobrachium 
parasitum and other west coast hydroids. 
Trans. Roy. Soc. Can. 12:131-138. 
Hand, Cadet. 1957. The systematics, affinities, 
and hosts of the one-tentacled commensal 
hydroid Monobrachium, with new distribu- 
tional records. J. Washington Acad. Sci. 
47(3) : 84-88. 
Wagner, Jules. 1890. Recherches sur l’organi- 
sation de Monobrachium parasiticum. Arch. 
Biol. 10:273-309. 
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