It is currently Sex Week at Deep-Sea News, and this post is written in honour of that event. Sex, after all, is a big part of biology (hur, hur, hurr).
Marine worms of the family Syllidae are small polychaetes, usually less than two centimetres in length and about a millimetre in width. Many syllids are interstitial (living buried in sand), others live in association with corals or sponges (on which they may feed). The main feature of syllids that has captured people's attention, however, is their extremely multifarious sex lives (Franke, 1999).
Syllids are one of a number of polychaete families exhibiting what is called epitoky, a significant metamorphosis between the juvenile or atokous and sexually mature or epitokous stages. The originally benthic worm grows long, extended parapodia, and the eyes and other sensory organs become greatly enlarged. More significantly, the reproductive tissue expands to fill almost the entire body, and other organs such as the digestive system degenerate, so the final epitokous worm is basically a highly sensitive pelagic gonad. In other worms such as members of the family Nereididae, the mature worm will swim up into the water column to meet up with other pelagic gonads, after which the entire mass will explode in a cloud of gametes.
Syllids, however, do things a little differently. Seemingly not as keen on ending life with a bang, they have evolved a number of ways to continue on with their life after maturity. The original syllid mode of reproduction involved metamorphosis to an epitoke as in other related polychaete families, but without the degeneration of the digestive system. And nereidids release their gametes by fatally rupturing the body wall, syllid epitokes release theirs through modified nephridia. Afterwards, the syllid epitoke is able to return to the ocean floor and partially revert back to its original atokous form, ready to reproduce another year. This mode of reproduction, called epigamy, remains the one used in two of the four syllid subfamilies, Eusyllinae and Exogoninae, as well as part of the subfamily Autolytinae.
Two other syllid lineages, the remainder of Autolytinae and the subfamily Syllinae, developed a mode of reproduction called schizogamy - the joy of budding. In schizogamous syllids, instead of the whole worm developing into an epitoke, a separate epitoke buds off the original atoke. In the most basic form of schizogamy, it is the posterior part of the worm that metamorphoses into the epitoke, in most cases growing its own separate, fully-developed head before breaking away from the anterior "parent" part (some Syllinae have headless epitokes). In some syllids, another epitoke may begin developing in front of the original epitoke before it breaks away, and possibly even more, so that the animal turns into a chain of developing worms. Others produce a number of epitokes growing in a bunch. After the epitoke(s) break off, the remaining atoke will regenerate any losses. Indeed, the atoke may begin regenerating even before the epitoke breaks off - the left and right sides of the new posterior part grow on either side of the epitoke, and once the epitoke is gone they fuse together down the middle.
Very few syllids have developed true asexual reproduction, where they fragment to give rise to new atokes instead of epitokes. But no survey of syllid budding would be complete without mention of the most bizarre of all syllids, the deep-sea sponge-dwelling Syllis ramosa. In this species (shown above in a drawing from here), buds develop laterally but don't detach from the parent worm. As these lateral buds grow, they start growing their own lateral buds, so that over time the worm develops into a branched network, spreading through the channels of its hexactinellid host.
Franke, H.-D. 1999. Reproduction of the Syllidae (Annelida: Polychaeta). Hydrobiologia 402: 39-55.