Host Specificity, Settling, and Metamorphosis of the 
Two-ten tacled Hydroid Proboscidactyla flavicirrata 
Richard D. Campbell 1 
The colonial hydroid Proboscidactyla has 
been found only on the leathery tubes of marine 
sabellid worms (Uchida and Okuda, 1941:433; 
Hand, 1954; Brinckmann and Vannucci, 1965: 
367). Nothing is known about the means by 
which this specific commensal association arises. 
In this paper I present observations on planula 
settling and metamorphosis, which indicate that 
larvae are caught in the tentacles of the sabellid 
worm and transferred to the rim of its tube. 
methods and materials 
Medusae of P. flavicirrata (Brandt) were 
dredged in East Sound, Orcas Island, Washing- 
ton at a depth of 15 meters, on August 16-18, 
1964. Most specimens contained ripe gametes. 
Medusae kept in glass dishes without feeding 
shed and fertilized many eggs for three or four 
days at about 5 am. Developing larvae were kept 
in sea water at 17°C, changed every 24 hours. 
To test the influence of substrate on settling, 
five glass dishes of sea water were prepared with 
the following: (1) 3 sabellid worm tubes from 
which the worms had been removed; (2) 3 
tubes with worms; (3) 3 worms in glass tubes; 
(4) some perisarc of obelia, and (5) sea water 
only. About 50 one-week-old planulae were 
pipetted into each dish. Observations were made 
during the following 8 hours, and at intervals 
over the next 8 days. 
Studies on metamorphosis were made on 
planulae which had settled on a tube (see be- 
low), and which were transferred to a micro- 
scope slide where they completed metamor- 
phosis. 
OBSERVATIONS 
Planula settling and metamorphosis took 
place only on tubes containing sabellid worms, 
1 Friday Harbor Laboratories, University of Wash- 
ington. Present address: Department of Organismic 
Biology, University of California at Irvine, Irvine, 
California 92664. Manuscript received May 11, 1967. 
regardless of whether the tubes were natural or j 
artificial. No settling occurred on tubes without j 
worms, or on other surfaces. 
Several hundred planulae were kept in clear 
glass dishes for 18 days. During this period 
none metamorphosed or settled. 
Observations on the behavior of the planulae 
and sabellid worms during the settling process !j 
indicated the role of the worm. Initiation of 
the settling process began when a planula was I 
caught in the ciliary currents of the sabellid’s 
radioles (tentacles). In the vicinity of the 
radicles these currents are much swifter than 
the planula’ s swimming movement; therefore, 
the planula must be considered as a passive 
participant in initiating this association. How- [ 
ever, when the planula does contact the worm, jj 
nematocysts discharge and anchor the planula j; 
(Fig. 1). The physical attachment is clearly 
indicated when a single planula binds to several |; 
adjacent pinnules, clumping them. 
The next stage in the settling process involves ! 
the transfer of the planulae from the radioles to 
the rim of the tube. This is mediated through 
retractions of the worm into its tube, which 
Fig. 1 . Proboscidatyla flavicirrata. Planula (ar- i 
row ) attached to two pinnules (p) of a sabellid ||| 
worm. Printed from 16 mm film. Scale: 0.5 mm. 
