Broadcasting fables: Is external fertilization really primitive? Sex, size, and larvae in sabellid polychaetes

Abstract

Traditionally, broadcast spawning and planktonic larvae have been considered the plesiomorphic ‘ground plan’ for the Polychaeta and other metazoan groups. To assess whether this reproductive mode is in fact ‘primitive’, the study of monophyletic groups with various reproductive modes should be informative. A large range of body sizes would allow testing the ideas that aspects of reproductive mode may be functionally constrained. The family Sabellidac is one such group, with sexual reproductive modes ranging from broadcast spawning to intratubular brooding to ovovivi‐parity, and a body size range over more than five orders of magnitude. Sabellids have previously been the subject of detailed cladistic analyses (Fitzhugh 1989, 1991); here we introduce several new characters based on morphology of reproductive structures. Larval development 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 brooding of direct‐developing larvae are plesiomorphic in the Sabellidae, with external fertilization and swimming larvae limited to apomorphie clades in the subfamily Sabellinae. The presence of sperm with elongate 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 from ‘primitive’ sperm is shown to be false, with ‘modified’ sperm being plesiomorphic for the Sabellidae, from which ‘primitive’ sperm is derived in apomorphic Sabellinae. All sabellids have lecithotrophic development and appear to be phylogenetically constrained in this regard. Data gathered on body size and reproductive variables in the Sabellidac suggests the following (when phylogenetic effects are not controlled): (1) egg number and total egg volume are significantly correlated with body size, with small animals having fewer, larger eggs than large animals; (2) individual egg volume is not correlated with body size; (3) reproductive mode is significantly correlated with body size; intratubular brooders tend to be small‐bodied, whereas broadcast spawners are 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 small eggs for a high total egg volurne. Intratubular brooders have a few relatively large eggs for a small total egg volume. When statistics arc performed using phylogenetically independent contrasts there is a significant correlation between total egg volume and body size but not for egg number and body size. The effect of non‐independence (due to phylogeny) of our data needs to be more fully controlled in future analyses but methods of incorporating continuous data into cladistic analyses should also be investigated. We show that some predictions can be made about reproductive mode based on body size but ad ho...

Polychaete systematics: Past and present

“…The palps can also be replaced directly in situ in many taxa (Dales 1955;Holthe 1986) and in some terebellids. In the sabelHds and serpulids, the grooved palps form a prostomial branchial crown (Orrhage 1980;Fitzhugh 1989;Rouse & Fitzhugh 1994). The prostomial derivation is visible in developing juveniles in which the buds for the crown appear in front of the prototroch (Rouse 1993).…”

“…Nereididae, Fitzhugh 1987, Sabellidae Fitzhugh 1989and Rouse & Fitzhugh 1994Phyllodocidae, Pleijel 1991;superfamily Nereididacea, Glasby 1993;Pilargidae, Lieber & Westheide 1994; Alciopidae, Wu & Lu 1994;Terebellidae, McHugh 1995); however, an overall analysis has yet to be performed on the group. This paper is the second of three papers exploring the relations within Annelida and Polychaeta.…”

“…In other taxa, the joint structure may be modified to form a tentacular crown, and the prostomium proper is no longer identifiable as a separate entity (e.g. sabellids and serpulids, Fitzhugh 1989;Rouse & Fitzhugh 1994).…”

Copulation in antiarch placoderms and the origin of gnathostome internal fertilization

“…The implied loss of bony claspers and implied reversion to external fertilisation in crown gnathostomes appears heterodox: loss of internal fertilization and acquisition of external fertilization is not widely accepted, at least in vertebrates 1,11 although it could have happened multiple times in invertebrates 24 . The shared, unique morphology and post-pelvic position of claspers in all placoderms is more consistent with a single origin, and thus represents a potential synapomorphy supporting placoderm monophyly 25 .…”

Ultrastructural study of spermatogenesis in Phoronopsis harmeri (Lophophorata, Phoronida)

“…However, the recent findings of modified sperms in the basal groups of some taxa suggest that internal fertilization and modified sperms are plesiomorphic in early Bilateria (see Jamieson 1991;Rouse and Fitzhugh 1994;Buckland-Nicks and Scheltema 1995).…”