Variation in Form, Orientation and Mode of Attachment of the Cirriped, Octolasmis stella (Ann.), Symbiotic on the Gills of Lobster

Abstract

The pedunculate cirriped, Octolasmis stella (Annandale), was found to occur in large numbers on the gills of spiny lobster, Puerulus sewelli Ramadan, caught at depths of 200-300 m off the Kerala coast. Even among the cirripeds attached to a single gill of Puerulus, variation in the form of the capitulum was observed (Dinamani 1964). The nature of this variation is examined in this paper, together with certain observations on the mode of attachment and orientation of the cirriped. The distribution of cirripeds on a typical podobranch of Puerulus is shown in Fig. 1, and also, in separate frames, the form of the capitulum of the cirriped (labelled A-D) occurring in different regions of the gill. In those forms occurring on the base of the gill (form A), the tergum is a somewhat irregular plate with shallow notches and projections, one of the notches (scutal margin) accommodating the rounded tip of the occludent arm of the scutum, the scutal and carinal valves are robust plates with small primordia in each. In forms occurring immediately below the region of the gill bearing filaments (form B), the tergal notch has become deeper and is separated from the tip of the scutal arm by a gap. The projections of the tergal plate are also more in the form of rays, while the basal arm of the scutum and the forks of the carina are not as well developed as in form A. In Octolasmis occurring on the gill proper between the gill filaments (form C), the tergal notch is turned away from the scutum and towards the aperture, and the rays of the tergal valve, particularly the median ray, are elongated. Variation in the same direction results in form D growing near the tip of the gill, where the tergal notch has rotated further towards the aperture and the elongated median ray makes contact with the tip of the occludent scutal arm. Also in forms C and D, the valves are generally thin and delicate, particularly the basal arm of the scutum and the tines of the carinal fork. The valves also have larger primordia and are only faintly calcified. The angle which the occludent arm of the scutum makes with the basal arm increases from form A to form D, becoming nearly vertical in forms C and D at the tip of the gill. The variation in the capitular valves thus shows two features: (i) a gradual reduction in the mass/area of the plates as the cirripeds move inwards into the branchial chamber of the lobster, and, correspondingly, (ii) a change in the pattern of the skeletal support of the capitulum, from a ring of closely fitting thick plates to one forming a delicate support of the peripheral regions of the capitulum. The change in the pattern of the capitular plates is clearly indicated in the shifting of the scutal margin of the tergum and its median ray. These regions of the tergum face anteriorly in form A, but gradually shift ventrally (forms B and C), and eventually face the aperture (form D). However, both the above features are concomitant changes, and, it is apparent, arise from the need for more protection by way of shell armament in those more exposed (as in cirripeds occurring near