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Factors regulating the reproductive cycles of some West Coast invertebrates Himmelman, John H.

Abstract

Annual reproductive cycles are found in many marine invertebrates. There is a vast literature on the subject, but the mechanisms involved have seldom been demonstrated. In the present study, 8 species of chitons and one sea urchin were studied for 3-5 years in southwestern British Columbia, at Vancouver and Porteau in the Strait of Georgia estuary, and at Botanical Beach on the outer coast of Vancouver Island. Reproductive condition was assessed mainly by the gonadal index method (percentage gonadal weight). There was a distinct annual cycle in the mean gonadal index of the urchin, Strongylocentrot-us droebachiensis Müller, and the chitons, Tonicella lineata Wood, Tonicella insignis Reeve, Mopalia hindsii Reeve, Mopalia laevior Pilsbry, Mopalia ciliata Sowerby, and Katharina tunicata Wood. In S. droebachiensis, T. lineata, T. insignis, M. laevior, and M. ciliata an abrupt spawning occurred in the spring, usually in April, and in M. hindsii there was usually an earlier spawning. K. tunicata sometimes spawned in April but the main spawning period was June. In M. laevior, K. tunicata, and probably M. hindsii, the gonads remained small during the summer and rapid gonadal growth occurred in the autumn and winter. In contrast, in S. droebachiensis, T. lineata, T, insignis, and M. ciliata gonadal growth started shortly after spawning. The data on reproduction in Mopalia lignose Gould were less clear. Mature animals were found in several seasons and drops in the mean gonadal index occurred in late winter-spring as well as in the summer. In Mopalia muscosa Gould animals in ripe and spent condition were found throughout the year. Consideration was given to the possible factors controlling gonadal growth. In a number of species, particularly species of warm water origin, it has been clearly demonstrated that gonadal development in the spring and summer is stimulated by increased temperatures. If temperature affects gonadal development in the species in the present study, it must act in several ways, since gonadal growth occurs through 2-3 periods of steadily increasing or decreasing temperature. The initiation of gonadal growth in K. tunicata and T. lineata in California and Oregon occurred at the same time as in the present study, although temperatures in the southern localities were fluctuating due to upwelling, in contrast to the regular temperature changes which occurred in British Columbia. This would suggest that temperature was not important, at least during the early stages of gonadal growth in K. tunicata and T. lineata. There are distinct annual photoperiod changes throughout the geographical ranges of the species in the present study, and in S. droebachiensis, T. lineata, T. insignis, and M. ciliata most gonadal growth occurred during the period of decreasing day length. Food conditions are known to affect the number of gametes produced in a number of species, including S. droebachiensis and K. tunicata, but there is no evidence that the timing of gonadal growth in the species in my study is controlled by a change in food conditions. The importance of temperature in stimulating spawning has been stressed by many authors, but I know of no instance where it has been demonstrated that a temperature change, sufficient to induce animals to spawn in the laboratory, actually occurred at the time of natural spawning. At First Narrows, there was usually a major spawning when the temperature reached 7-8 °C in the spring. However, in 1971, S. droebachiensis spawned when the temperature was about 6.3 °C, and temperature differences would not account for an abrupt spawning in 1973 In: "Perspectives in Marine Biology", A. A. Buzzati-Traverso (Ed.), University of California Press, Berkeley, pp. 67-36. Compared to the prolonged spawning in 1974. At Porteau, water temperatures showed a slow rise of only 0.8 °C during a two week period in which there was a complete spawning in Tonicella lineata, Tonicella insignis, and Mopalia laevior. At Botanical Beach, temperatures were a few °C warmer than at First Narrows when T. lineata, S. droebachiensis, and M. hindsii spawned, and the temperature at the time of spawning of T. lineata and K. tunicata varied several °C in different years. These observations suggest that spawning did not occur in response to a physiological threshold temperature, or to a sudden change in temperature. In 1973, S. droebachiensis and T. lineata were collected at First Narrows in late March, prior to spawning, and maintained under various temperature and light conditions: at 5.5 and 14 °C in darkness, and at 5.5 and 14 °C in light conditions similar to those in the field. These animals did not spawn when spawning occurred in the field. Similarly, S. droebachiensis, T. lineata, and T. insignis collected prior to spawning in 1974 and maintained in the laboratory did not spawn. However, animals returned to the field from the laboratory did spawn. This suggested that some condition in the field, which was not present in the laboratory, stimulated spawning, and this factor did not appear to be light or temperature. An abrupt spawning at First Narrows and Porteau in 1973 occurred at the time of the spring phytoplankton outburst, but in 1974 spawning at First Narrows was less abrupt corresponding to the slow development of the phytoplankton bloom in that year. In the laboratory, a large proportion of S. droebachiensis, T. lineata and T. insignis spawned when they were exposed to natural phytoplankton collected during the bloom with a 50 μ mesh net. This suggested that some substance bound to or released by phytoplankton stimulated spawning. For species with planktotrophic larvae the synchronization of spawning with the phytoplankton bloom increases the probability of both favourable food and temperature conditions for development or eggs, larvae, and juveniles. Gonadal growth during the coldest part of the year and spawning at the time of the spring phytoplankton bloom was found in S. droebachiensis, T. lineata, T. insignis, M. ciliata, and probably K. tunicata. This pattern is characteristic of marine invertebrates with pelagic larvae living in cold waters.

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