The prostomium is distinct, and bears large ommatophores
anteriorly in some genera. The peristomium is represented
only by lips around the mouth. Lateral and median antennae
are present. The ventral, tapering palps lack
articulations; they are fused to the medial side of the first pair
of parapodia. Nuchal organs are present.
The longitudinal muscles comprise four bands.
The first segment curves around the prostomium; its
parapodia are structurally similar to others but they are fused
to the base of the prostomium. Two pairs of tentacular cirri are
present. The neuropodia are distinctly longer than the
notopodia. Elytra replace some of the dorsal cirri along the
body; ventral cirri are present. In some acoetids, finger-like
epidermal branchiae extend into the space between the elytra
and the body wall. Lateral organs and
dorsal cirrus organs have not been observed, and epidermal
papillae are lacking. The pygidium bears one pair of cirri. The
eversible, muscular axial pharynx bears two pairs of dorsoventrally
oriented jaws, and
terminal papillae are present around the mouth. There is no
gular membrane; the gut has lateral branches.
As in all scale-worms, metanephridia are present. No other information is available on the structure of
segmental organs. The circulatory system is unknown,
although presumed to be closed; the presence of a heart body
has not been confirmed. Aciculae are present, as are variously
ornamented capillaries and spines. Fine silk-like notochaetae are released and woven to
form tubes. The microscopic structure of the acoetid ‘branchiae’ is
unknown; thus, it is unknown whether or not they function as
‘true’ branchiae in the sense of the term used here.
The above definition is taken from Hutchings (2000),
which in turn is based on that of Fauchald & Rouse (1997).
This family was previously known as Polyodontidae, but that name is correctly applied to a family of fishes. Pettibone (1989) resolved the conflict by resurrecting the generic name Acoetes, thereby making the family name Acoetidae available for this taxon. (Previously Polyodontes was the type genus for the family.)
Identification tips
Recognising the family
Acoetids are large, long-bodied scale worms that are recognised by spinning glands found in the notopodia. Spinning glands are located on the ventral side of the notopodium, commencing at chaetiger 8 or 9. The family of long-bodied scale worms most likely to be confused with Acoetidae is the Sigalionidae. However, in Sigalionidae elytrae are present on alternate segments anteriorly but are present on every segment posteriorly whereas in Acoetidae elytrae occur on alternate segments throughout the body.
Acoetids are most likely to be confused with one of the other families that also have dorsal rows of scales. The quick guide to scale worms should help separate these 6 families.
Distinguishing species
Identification of this group requires that a selection of parapodia along the body need to be mounted under the compound microscope to document the type of noto and neurochaetae present as well as the development of anteroventral bracts on the neuropodia and the presence of parapodial branchiae. In specimens without an extruded pharynx this must be dissected out to investigate the number of pharyngeal papillae present around the opening and the number of lateral teeth present on the jaws.
Characters used to separate genera include the shape of the prostomium, presence or absence of medial antennae and whether eyes sessile or on ocular peduncles and the presence or absence of parapodial branchiae. Within a genus species are distinguished largely on the ornamentation and type of chaetae present and their distribution along the body.
