BROADCASTING FABLES - IS EXTERNAL FERTILIZATION REALLY PRIMITIVE - SEX, SIZE, AND LARVAE IN SABELLID POLYCHAETES

Traditionally, broadcast spawning and planktonic larvae have been cons idered the plesiomorphic 'ground plan' for the Polychaeta and other me tazoan groups. To assess whether this reproductive mode is in fact 'pr imitive', the study of monophyletic groups with various reproductive m odes should be informative. A large range of body sizes would allow te sting the ideas that aspects of reproductive mode may be functionally constrained. The family Sabellidae is one such group, with sexual repr oductive modes ranging from broadcast spawning to intratubular broodin g to ovovivi-parity, and a body size range over more than five orders of magnitude. Sabellids have previously been the subject of detailed c ladistic analyses (Fitzhugh 1989, 1991); here we introduce several new characters based on morphology of reproductive structures. Larval dev elopment in four brooding sabellid species is also described with the aim of introducing new characters for future systematic analyses. Our cladistic analysis of sabellid genera suggests that gonochorism and br ooding of direct-developing larvae are plesiomorphic in the Sabellidae , with external fertilization and swimming larvae limited to apomorphi c clades in the subfamily Sabellinae. The presence of sperm with elong ate heads may be correlated with the presence of intratubular brooding , though an adequate causal explanation for this relationship can not yet be presented. The concept that 'modified' sperm must be derived fr om 'primitive' sperm is shown to be false, with 'modified' sperm being plesiomorphic for the Sabellidae, from which 'primitive' sperm is der ived in apomorphic Sabellinae. All sabellids have lecithotrophic devel opment and appear to be phylogenetically constrained in this regard. D ata gathered on body size and reproductive variables in the Sabellidae suggests the following (when phylogenetic effects are not controlled) : (1) egg number and total egg volume are significantly correlated wit h body size, with small animals having fewer, larger eggs than large a nimals; (2) individual egg volume is not correlated with body size; (3 ) reproductive mode is significantly correlated with body size; intrat ubular brooders tend to be small-bodied, whereas broadcast spawners ar e large. However when the effect of body size is controlled for, then (4) egg number, egg volume and total egg volume all vary significantly with reproductive mode. Broadcast spawners expel a large number of sm all eggs for a high total egg volume. Intratubular brooders have a few relatively large eggs for a small total egg volume. When statistics a re performed using phylogenetically independent contrasts there is a s ignificant correlation between total egg volume and body size but not for egg number and body size. The effect of non-independence (due to p hylogeny) of our data needs to be more fully controlled in future anal yses but methods of incorporating continuous data into cladistic analy ses should also be investigated. We show that some predictions can be made about reproductive mode based on body size but ad hoc patterns of reproductive character-state transformation should not be made indepe ndent of empirical hypotheses of phylogenetic relationship. Further st udies of this kind throughout the Annelida are needed to determine the plesiomorphic reproductive mode for the phylum.