The Salinity Tolerance of Polychaete Larvae

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Abstract
The salinity tolerances of the larvae of Pomatoceros triqueter, Phyllodoce maculata, Scoloplos armiger, Nereis diversicolor, and a capitellid (probably Notomastus latericeus, were investigated. Two types of experiments were performed. In both types larvae were placed in sea-water of abnormal salinity. In the former experiments samples of larvae were returned to normal sea-water and the percentage recovery noted, in the latter they were observed while continuously immersed in water of abnormal salinity and percentage showing activity noted. The results show that the most tolerant larvae were those of Nereis, followed by Scoloplos, Pomatoceros, the caoitellid and finally Phyllodoce. The order of adult distribution down an estuary was similar, except that Pomatoceros was usually confined to the mouth of the estuary with Phyllodoce. From a comparison of the tolerance of the larva with that of the adult, as inferred from published accounts of distribution, it is concluded that the distribution of Pomatoceros may be limited by the relatively greater sensitivity of the larvae. In Phyllodoce, the tolerances of larva and adult are similar. It is possible that the estuarine penetration both of Scoloplos and of the capitellid Notomastus sp. is limited by adult salinity tolerance or by substrate selection. Information from several sources suggests that in Nereis the young worms are more tolerant than the adults and the adults more so than the larvae. Experiments at different temperatures showed that Pomatoceros larvae had an optimum temperature of about 14[degree]C at which the maximum salinity tolerance was displayed. Trochophore larvae placed in dilute sea-water tended to swell and become inactive. After a time they not only either returned to their original size or shrank less rapidly, but also showed some resumption of movement. In concentrated sea-water they shrank at first, then stabilized and similarly showed partial recovery of activity. Phyllodoce larvae were photopositive and geo-negative. In a column of sea-water, dilute at the top, concentrated at the bottom, they tended to collect at about the 12%o level.

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