Longosomatidae Hartman, 1944

Blake, James A. & Maciolek, Nancy J., 2023, New species and records of Heterospio (Annelida, Longosomatidae) from continental shelf, slope and abyssal depths of the Atlantic Ocean, Pacific Ocean, Indian Ocean and adjacent seas, Zootaxa 5260 (1), pp. 1-74 : 6

Family Longosomatidae Hartman, 1944

Type genus: Heterospio Ehlers, 1874 .

Synonym: Heterospionidae Hartman, 1963 . Fide Petersen 1992: 80, Borowski 1994.

Generic Diagnosis: Same as genus Heterospio (see below).

Remarks: Longosomatids are easily recognized by the unusually long segments of the middle body region; this character was considered a synapomorphy by Fauchald & Rouse (1997). Another unusual aspect of longosomatid segmentation is that the setal fascicles emerge on the anterior end of each of the short anterior and elongated middle segments, rather than from the middle of the segment as is typical of most polychaetes. The enlarged bulbous posterior end with pairs of curved spines likely serves as an anchor, which probably accounts for most specimens being fragmented when removed from sediment.

Ehlers (1874, 1875) originally placed his new genus in the Spionidae and even after being referred to a new family ( Hartman 1944, 1965) it was considered a spioniform ( Fauchald 1977, Rouse & Fauchald 1997, Blake & Arnofsky 1999; Read & Fauchald 2022). However, recent interpretation of morphology and newly obtained molecular data supports a close affiliation of Longosomatidae with Cirratulidae ( Blake & Maciolek 2019; Rouse et al. 2022); this position was also taken by Wilson (2000) and Grosse et al. (2021). Longosomatids are similar to cirratulids in having long filamentous branchiae that differ from the broad flattened branchiae of spionids, a pair of long dorsal tentacles and in most species, the arrangement of abdominal setae into cinctures that mostly surround the body.