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Pisionidae
Description
The prostomium is small and located between the large,
anteriorly-directly first parapodia in Pisione; it is conical in
Pisionidens. The peristomium is reduced to lips. Paired
frontal antennae are present (Pisionidens indica) or absent
(Pisione remota); a single median antenna occurs in
Pisionella hancocki. The palps are fused to the ventral
surfaces of the first parapodia in Pisione, and
emerge ventrally on the conical head in Pisionidens. Nuchal
organs are absent, at least in Pisione. The longitudinal muscles are grouped in
bundles, and segmentation is present. The first segment is
folded around the prostomium and bears parapodia in Pisione;
it is indistinct and bears two pairs of tentacular cirri in
Pisionidens. The neuropodia are well developed; the
notopodia comprise short cirri with an internal acicula. Dorsal
and ventral cirri are present. Branchiae and epidermal papillae
are absent, and lateral organs and dorsal cirrus organs have
not been observed. One pair of pygidial cirri is present. The
buccal organ is an axial, muscular eversible pharynx with
terminal papillae and may bear two pairs of dorso-ventral
jaws. A gular membrane is absent, and the gut is straight. The
segmental organs are protonephromixia and occur in
numerous segments. Ova may be present in up to 40 segments
in Pisione, but gonads are restricted to a few segments in
Pisionidens. A circulatory system and heart body are lacking.
Aciculae are present. All chaetae are neuropodial and include
compound chaetae with distally falcate appendages attached
to shafts by single ligaments. Capillary chaetae are present
and some other chaetae are spine-like, but are no
thicker than capillaries. Pisionidens has aciculae only.
The above definition is taken from Hutchings (2000), which is based on that of Fauchald & Rouse (1997).
Identification tips
Recognising the family
Pisionids are small slender interstitial worms, typically occurring in sandy sediments. The morphology of the anterior end varies considerably between genera of Pisionidae, however the family should be readily recognised by the pair of buccal aciculae that converge anteriorly to the mouth. Though the cuticle is often thick and the buccal aciculae internal, they are easily observed with transmitted light in these small, semi-transparent polychaetes.
Distinguishing species
Characters useful in distinguishing genera and species of pisionids include: presence or absence of a median antennae, the number of cephalic appendages and whether the pharynx has jaws or is unarmed.
Description | Identification
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