Therochaeta Chamberlin, 1919

Salazar-Vallejo, Sergio I., 2013, Revision of Therochaeta Chamberlin, 1919 (Polychaeta: Flabelligeridae), Zoosystema 35 (2), pp. 227-263 : 231-233

Genus Therochaeta Chamberlin, 1919

Therochaeta Chamberlin, 1919: 397 (key, etymol.). — Hartman 1965: 179-180 (emended). — Fauchald 1977: 117.

TYPE SPECIES. — Stylarioides collarifer Ehlers, 1887 , by original designation.

GENDER. — Feminine ( ICZN 1999, Art. 30.1.1).

DISTRIBUTION. — Species of the genus are found in tropical and subtropical regions, especially in shallow water but have been found down to about 1000 m depths, in mixed bottoms.

DIAGNOSIS (EMENDED). — Body swollen or tapered, often constricted between chaetigers 2 and 3. Chaetiger 2 elongated; chaetiger 3 with a transverse band of long papillae. Anterior margin of chaetiger 1 with a dorsal lobe. Cephalic cage made by chaetigers 1-2. Tunic generally covered with sediment grains. Chaetigers 5-7 with pseudocompound neurohooks; posterior neurohooks blunt. Four or many pairs of cirriform branchiae, sessile on a low branchial plate.

REMARKS

Therochaeta has been regarded as resembling Pherusa because of the branchial arrangement and because some species in the latter have been described as having pseudocompound neurochaetae along few anterior chaetigers. This would explain why these two genera were regarded as synonyms, but there are consistent differences between them. Thus, although the species in both genera have distal branchiae arranged as four filaments which are not separated by the caruncle, the proximal filaments have different patterns. In Therochaeta the filaments are often arranged in a spiral, usually with more than four filaments per side, whereas in Pherusa there are only two filaments per side. Further, regarding the presence of pseudocompound or transition neurohooks, as herein restricted, the species belonging to Pherusa do not have any such chaetae and most of their neurochaetae are anchylosed and falcate, whereas in the species belonging in Therochaeta , there are pseudocompound or transition neurohooks, at least along a few anterior chaetigers.

These two genera also differ in the sediment cover on the tunic and relative length of chaetiger 2. The species of Therochaeta have a sediment cover with closely packed grains, generally fused, forming a continuous cortex along the whole body, or at least over some anterior chaetigers, whereas in Pherusa sediment grains, even if they are abundant, are rather loosely organized and never fuse to each other to form a crust. In addition, Therochaeta has a marked difference in the relative length along chaetigers 1-3, with chaetiger 2 being the longest and often with a constriction in its posterior margin.This feature, however,may be misleading because the interparapodial body width is more or less the same along subsequent chaetigers.The most notorious feature is thus the elongated second chaetiger, and the fact that it is anteriorly expanded, its chaetae contributing to the cephalic cage, and the chaetiger becomes narrower posteriorly.

Therochaeta resembles Semiodera Chamberlin, 1919 as revised elsewhere ( Salazar-Vallejo 2012b) because both genera include species having a shield made of cemented sediment particles, and pseudocompound neurohooks in some anterior chaetigers. They differ because in Therochaeta papillae often incorporate sediment particles and frequently form sediment tubercles, whereas in Semiodera papillae are usually clean, without sediment particles, and never form sediment tubercles. Their species also differ in their habits as Therochaeta sp. live in soft substrates, not forming calcareous structures, whereas Semiodera species bore in consolidated sediments or calcareous substrates and often produce calcareous layers inside them.

There are eight species currently assigned to Therochaeta : Therochaeta antoni Kirkegaard, 1996 from the Bay of Bengal, Indian Ocean, T. caudata n. comb. from the Eastern Pacific Ocean, T. collarifera ( Ehlers, 1887) from the Grand Caribbean Sea, T.coronata ( Ehlers, 1908) from two localities in the Indian Ocean, T. flabellata (Sars in Sars, 1872) from the Norwegian Sea, T. pacifica Fauchald, 1972 from the Eastern Pacific Ocean, Therochaeta scutigera ( Ehlers, 1887) from the Grand Caribbean Sea, and T. scutigeroides ( Augener, 1918) from the Eastern Atlantic Ocean. These species can be arranged into two different body patterns, especially indicated by the relative development of body papillae, and by the presence of pseudocompound neurohooks. The first group contains species with large body papillae or with thin papillae forming large sediment tubercles, and having pseudocompound hooks, whereas the second group has smaller body papillae, often with a sediment cover forming a thin layer over few anterior chaetigers, and lacking pseudocompound hooks. The first group is herein restricted to Therochaeta and includes T. caudata n. comb., T. collarifera ( Ehlers, 1887) , T. fauchaldi n. sp., T. flabellata , T. pacifica , and Therochaeta sp. The second group is herein recognized as a different genus, Paratherochaeta n. gen. with T. antoni as its type species, it also includes Paratherochaeta africana ( Rullier, 1965) n. comb., n. stat. is moreover removed from Stylarioides delle Chiaje, 1831, Paratherochaeta augeneri n. sp., P. corona ( Ehlers, 1908) n. comb., Paratherochaeta scutigera ( Ehlers, 1887) n. comb., and P. scutigeroides ( Augener, 1918) n. comb.

KEY TO SPECIES OF THEROCHAETA CHAMBERLIN, 1919 , RESTRICTED

1. Anterior shield present, sometimes continued throughout the body ........................... 2

— Anterior shield missing; body papillae with fine sediment particles ............................. 5

2. Sediment particles concentrated in the anterior region or forming sediment tubercles in some anterior chaetigers; a marked constriction between chaetigers 2 and 3 ............... 3

— Sediment particles homogeneously distributed throughout the body; no marked constriction between chaetigers 2 and 3 ......................................................................................... 4

3. Sediment tubercles in chaetigers 3-4 elevated, laterally fused forming complete collars; sediment tubercles in following chaetigers large, projected, but not fused laterally ......... ............................................................................................ T. collarifera ( Ehlers, 1887)

— Sediment tubercles in chaetigers 3-4 low, barely fused laterally, not forming complete collars .................................................................................. T. pacifica Fauchald, 1972

4. Papillae adhering fine sediment particles; grayish... T. flabellata (Sars in Sars, 1872) Norway

— Papillae adhering large sediment particles or forams; whitish ......................................... ................................................................................. T. cf. flabellata Mediterranean Sea

5. Papillae few per segment, dorsal ones larger ............. T. caudata ( Rioja, 1963) n. comb.

— Papillae abundant, all of about the same size .................................... T. fauchaldi n. sp.