Light Cycles and Gametogenesis in Three Temperate Ascidian Species

Abstract

Many temperate ascidians reproduce seasonally but the factors controlling their gametogenic cycles have not been experimentally demonstrated. I tested the effects of daylength on gametogenesis in three ascidians (Boltenia villosa, Styela gibbsii, and Chelyosoma productum) by exposing animals to normal or reversed light cycles and measuring gonad and oocyte sizes. Under a normal cycle, all species followed patterns reported for field populations. B. villosa showed evidence of continuous reproduction; S. gibbsii and C. productum spawned primarily in the spring (though a secondary mid-winter spawn may also have occurred in S. gibbsii). Altering the light cycle demonstrated that the gametogenic cues differ in these three species. Gametogenesis in B. villosa and S. gibbsii was largely unaffected by a reversed light treatment but the cycle of C. productum was dramatically changed. Even in C. productum, however, the gametogenic cycle was not completely reset in the 12-month experiment. This suggests that other factors (e.g., temperature, endogenous rhythms) contribute to gametogenic control. Longer-term experiments are needed to establish the relative importance of light cycle in these species. Additional key words: tunicates, reproduction, seasonality, synchrony, daylength Synchronous spawning is critical for marine invertebrates that release gametes into the water column and rely on currents for fertilization. Water temperature, light, phytoplankton abundance, lunar cycle, and presence of conspecifics all may cue populations to synchronize spawning and maximize fertilization (reviewed by Giese & Kanatani 1987; see also Starr et al. 1990, 1991; Tyler et al. 1992). For such cues to be effective, however, some earlier signal must entrain gametogenic cycles so that gonads contain viable sperms and full-grown oocytes or ova when the spawning cue arrives. Laboratory experiments have shown that gonad growth and maturation cycles in some marine invertebrates are affected by temperature or food abundance (reviewed by Giese & Pearse 1974; see also Babcock et al. 1986; Franz 1986; Tyler 1988; Tyler & Young 1992; Tyler et al. 1992; Beauchamp 1993). Some of the clearest evidence for extrinsic control of gametogenesis, however, comes from experimental studies with light. For example, gametogenic cycles of polychaetes (Olive & Pillai 1983; Fong & Pearse 1992a,b), a cephalopod (Richard 1967), seastars (Pearse & Eernisse 1982; Pearse & Beauchamp 1986; Pearse & Walker 1986; Pearse et al. 1986a; Xu & Barker 1990), sea urchins (Pearse et al. 1986b; McClintock & Watts 1990), and a cephalochordate (Fang et al. 1989) have been altered or completely reset by simple modifications of the light cycle. Giese & Kanatani (1987) suggest that daylength may be a fairly general gametogenic cue, but to date, only a few species in a few phyla have been studied. In this paper, I report on the first test for such effects in ascidians (phylum Chordata, subphylum Urochordata). Millar (1952, 1958) and Berrill (1975) suggested that temperature controls gametogenesis in ascidians. At temperate latitudes where annual temperature fluctuations are large and predictable, gametogenic cycles could cue to temperature changes. In tropical waters where temperatures remain quite constant, absence of the temperature cue could lead to non-seasonal reproduction. Consistent with this hypothesis, temperate ascidian species often show highly seasonal reproduction (reviewed by Millar 1971; Berrill 1975) while continuous reproduction has been demonstrated for some tropical species (Goodbody 1961, 1963). However, temperature control of ascidian gametogenesis has never been tested experimentally, and Svane & Young (1989) suggest that daylength should also be considered. The hypothesis that light controls reproductive seasonality can accommodate continuously reproducing tropical species since annual changes in daylength This content downloaded from 207.46.13.129 on Sun, 26 Jun 2016 07:01:17 UTC All use subject to http://about.jstor.org/terms