Graduate Thesis Or Dissertation

 

Ecology and reproductive biology of Tonicella lineata (Wood, 1815) (Mollusca - Polyplacophora) Public Deposited

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  • Along the central and southern Oregon coast and on San Juan Island, Washington, the lined chiton Tonicella lineata is very abundant in the lower intertidal levels on rocky shores. It is usually found on encrusting coralline algae which, with epiphytic diatoms, make up the major portion of its diet. On the central and southern Oregon coast, T. lineata are closely associated with the purple urchin Strongylocentrotus purpuratus and are often found in the burrow of the urchin. This association, together with algal cover, protects T. lineata from desiccation during periods of tidal exposure, whereas on San Juan Island the lack of an intertidal urchin reduces the diversity of habitats available to Tonicella. The asteroids Pisaster ochraceous and Leptasterias hexactis are the most common predators of T. lineata. The average growth in length of twelve T. lineata studied for ten and one-half months was 0.40 cm (range 0.88 to 0.06 cm) with great variation among animals 2.0 to 3.0 cm in body length. Some T. lineata exhibit a homing behavior. The reproductive cycle was studied by determination of a monthly gonad index and examination of gonad histology. In 1968 and 1969 Tonicella lineata showed a distinct annual reproductive cycle along the central Oregon coast, spawning between 1 April and 15 April in 1968 and during the middle of April in 1969. Southern Oregon coast data is inconclusive, but spawning in 1968 and 1969 probably occurred In February or March. In 1968 and 1969 spawning took place during May and June on San Juan Island. Tonicella lineata thus shows a latitudinal difference in the timing of its annual reproductive cycle. Histological examination of the gonads revealed a close correlation of gamete buildup and release with gonad index changes. Gamete production starts immediately after spawning. The gonads of T. lineata do not go through a resting period as has beenobserved for those of the chitons Katharina tunicata, Mopalia hindsii and Cryptochiton stelleri. Comparison of gonad index and histological data with sea surface temperature data shows that gametogenic activity increases while the water temperatures are dropping. Spawning behavior is described and data are presented which suggest that there may be a correlation between the time of spawning and the lunar phase. The development of Tonicella lineata is described from the time of fertilization through metamorphosis. The features described follow closely those which have been described for other chitons except for the description of previously unnoted structures on the pretrochal region of the trochophore larva. In laboratory cultures the development of the trochophore stops between 110 and 160 hours post-fertilization, depending upon the temperature. Further development will take place only when the larvae are presented with the proper settling substrate of encrusting coralline algae. In substrate choice experiments, T. lineata larvae settle only on encrusting coralline algae and on pieces of tile previously soaked in an encrusting coralline algae extract. After settling, the trochophore larvae go through a drastic metamorphosis within twelve hours, losing the apical tuft and prototroch, taking on the shape of a small chiton, and developing seven shell plates. When 30 days old the young chitons have a fully developed radula and feed on encrusting coralline algae.
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