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less abundant), and/or increase the rate of larval development due to increased water temperature. Nevertheless, larvae will metamorphose later in the year and may not attain a juvenile size capable of surviving the less favourable environmental conditions in winter.

Seasonal prevaihng environmental conditions and/or prey availability (or quality) would further compound the problems facing a larva metamorphosing late in the season. If an organism is required to attain a minimum size prior to reproduction, a seasonal

environment may restrict juvenile growth giving rise to an adult of a size incapable of producing sufficient gametes for successful reproduction. To achieve maximal fitness, a small adult should grow during the following season attaining reproductive size and/or maturity later in the season. If this delay became progressively later, finally, a release of gametes very late in the season may result in total reproductive failure which equates to zero fitness for a semelparous organism. In this instance, selection ought to favour delaying reproduction until the following year - a biennial life cycle. In the plant kingdom, it has been recognised that an individual must attain a minimum size prior to reproduction (Werner 1975). Also in the plant kingdom, there are many documented cases where environmental conditions result in a delay in reproduction: Lacey (1986) concluded that in the wild carrot Daucus carota, nutrient supply determines the year of flowering. Thus a seasonal environment can result in directional selection towards an extended life cycle.

But the need to attain large body size and, by inference, a high fecundity may not be the determining selective force for a biennial life cycle. Reproductive success requires an individual to replace itself in subsequent generations, and to that end, a parent should maximize the probability of larval and juvenile survival. Halickondria panicea has seasonal variations in its biochemical composition and therefore its perceived prey value.

For Archidoris pseudoargus, the timing of reproduction and/or its generation time may assume greater importance than absolute fecundity. Hodgkin & Barnes (1991)

demonstrated that generation time had a greater selective value over fecundity for a nematode species. Adult A. pseudoargus attain a body size capable of high reproductive output by the late summer/autumn of their second year but delay reproduction (at risk of

metamorphose in late spring/early summer, thus maximising the time available for growth against a seasonally variable prey. Delaying reproduction will result in an increase in body size and by inference fecundity, but it is not high fecundity per se that provides the primary selective pressure for the delay.

There are few documented cases of biennialism in the marine environment.

Somerton & Macintosh (1985) found that the blue king crab Paralithoides platypus has a biennial reproductive strategy and concluded that this was an adaptive strategy to enable individuals to expend more energy per brood and reduce the risks associated with an annual moult. P. platypus has a longer life cycle, in comparison to its’ congener the red king crab, P. camptschatica, which has annual reproduction, to offset less frequent reproduction. Jenson & Armstrong (1989) considered P. platypus in more detail and proposed that a biennial reproductive strategy could be an adaptive feature (as proposed by Somerton & Macintosh), or a limitation caused by physiological constraints. Jenson & Armstrong (1989) concluded that the evolution of a biennial reproductive strategy was a consequence of physiological and energetic constraints incurred by species in a ‘harsh’ environment. Similarly, the chaetognath Sagitta elegans has biennial life cycle in arctic waters, but an annual life cycle in a nearly land-locked iQord with an elevated water temperature on Baffin Island, Newfoundland (McLaren 1966). Sardà (1991) recorded biennial spawnmg and moult synchronism in Mediterranean populations of the Norway

Idbstsr Nephrops norvégiens, but was unable to satisfactorily determine the primary cause. Sardà (1991) concluded that biennial behaviour could be a consequence of endogenous natural variation and/or exogenous variation in food quality, temperature or even commercial fishing pressure.

MiUer-Way & Way (1989) recorded semelparous biennial life histories in

populations of the freshwater pleurocerid gastropod Leptoxis dilatata, and concluded that food quality and quantity were the primary cause of an extended life cycle. Similarly,

5 : Reproduction 116 Payne (1979) attributed semelparous biennial (& triennial) life histories in the pleurocerid gastropod Goniobasis livescens to variations in food quality. These data suggest that a biennial reproductive strategy is favoured when the growing season is short (= reduced food supply) and/or prevailing environmental conditions are stressful.

What selective factors may have led to Archidoris pseudoargus, Tritonia hombergi

and Cadlina laevis evolving an extended life history strategy? These organisms are subject to similar physical environmental conditions to other north Atlantic nudibranch species which have annual life cycles. It would appear unlikely that physical environmental conditions alone could have such a marked evolutionary effect, as this implies that the internal physiology of these longer lived species is very different. Nudibranchs have evolved to feed on a wide variety of prey and perhaps this is the key factor determining the life cycle. Reproduction requires energy to fuel the process of gametogenesis, and any reduction or seasonal variation in the energy supply may preclude successful

reproduction. If the supply of energy is seasonal, or perhaps there are initial prey handling problems for juveniles, it may be selectively advantageous to maximize body size during the first growing season and thereby increase fecundity in the second season. A seasonal energy supply may provide the selective pressure for the evolution of a biennial or perennial life history strategy.

CHAPTER 6