The Pisionidae are tiny (about a centimetre or less in length), transparent, mostly marine annelid worms (a single species has been described from fresh water - San Martín et al., 1998). They are usually counted as members of the meiofauna, the community of minute animals (and other heterotrophic eukaryotes) that live in the spaces between sand grains, though Rouse and Pleijel (2001) quibbled with this assignation on the grounds that pisionids are too large relative to average sand grain size. Nevertheless, they are probably predators of meiofaunal organisms that they capture with their venomous jaws. About forty species of Pisionidae are currently recognised; however, in light of their ease of being overlooked, it would not be surprising if many species remain undescribed. Most pisionids are found in shallower and intertidal waters but some have been recorded at depths of up to 1000 m. The life histories of few pisionids have been studied but fertilisation is direct with males possessing paired ventral "penes" on varying numbers of segments that transfer the aflagellate sperm to the females' seminal receptacles. Larvae of the species Pisione remota are planktic feeders, capturing food by means of a mucus net.
Phylogenetic studies of Pisione remota have provided strong support for a position of pisionids among the scale worms, a clade of annelids normally characterised by the possession of paired chitinous elytra (Wiklund et al., 2005). This is supported both by molecular analysis and by similarities in the venom glands. The study of Wiklund et al. (2005) went so far as to find P. remota nested among species of the elytra-bearing family Sigalionidae and suggested that 'pisionids' may in fact be derived sigalionids (however, only a single pisionid and three sigalionids were included in the analysis so further more extensive studies are required to confirm the families' relative positions). The implied loss of the elytra in pisionids is usually suggested to be an adaptation to their infaunal habitat and small size. However, other infaunal scale worms have not lost their elytra so this may not be the entire story.
Rouse, G. W., & F. Pleijel. 2001. Polychaetes. Oxford University Press.
San Martín, G., E. López & A. I. Camacho. 1998. First record of a freshwater Pisionidae (Polychaeta): description of a new species from Panama with a key to the species of Pisione. Journal of Natural History 32 (8): 1115-1127.
Wiklund, H., A. Nygren, F. Pleijel & P. Sundberg. 2005. Phylogeny of Aphroditiformia (Polychaeta) based on molecular and morphological data. Molecular Phylogenetics and Evolution 37 (2): 494-502.