Aphelochaeta Blake 1991

Genus Aphelochaeta Blake, 1991

Type species: Tharyx monilaris Hartman, 1960. Original designation by Blake (1991).

Diagnosis. (emended, Dean & Blake 2016). Prostomium conical to rounded; peristomium elongate with pair of grooved dorsal tentacles arising either on or anterior to setiger 1. Anterior segments often expanded, crowded or uncrowded; abdominal segments sometimes beaded or moniliform in appearance; setae simple capillaries lacking distinct serrations using light microscopy but distinct fibrils may be visible using SEM; posterior end frequently expanded, tapering to a simple pygidial lobe.

Remarks. Blake (1991) assigned bitentaculate cirratulid species having only simple, non-serrated capillary setae to the genus Aphelochaeta. The species belonging to Aphelochaeta are among the most enigmatic and difficult to identify of all cirratulids because of the lack of obvious variability in setal morphology. However, there is considerable variation in body shape, details of the prostomium and peristomium, placement of the dorsal tentacles and anterior branchiae, presence or absence of dorsal and ventral grooves and ridges, form of the posterior end and pygidium, and Methyl Green staining patterns. Sometimes the capillaries do differ in thickness and length among species, but this is difficult to quantify and depict in a manner that enables readers of descriptions to distinguish one species from another. Additionally, the capillaries often exhibit numerous fibrils along the shaft, but these are best seen with SEM and difficult to observe in light microscopy. Despite the specific nature of different combinations of these characters, closely related species are usually difficult to distinguish from one another. Blake (1996) pointed out that it is usually only at the local or regional level that keys to species have any utility.

Hartman (1966, 1967) referred all bitentaculate cirratulids having only capillary setae and previously reported from Antarctica to Tharyx according to definitions then in use. Types of all the Antarctic species and most of the western South American species have been examined. Tharyx fusiformis Monro, 1939, originally understood to have only capillary setae has been found to have spines in posterior setigers and is referred to a new genus Chaetocirratulus n. gen. and to synonymy with Heterocirrus andersenensis Augener, 1932. The syntypes of Tharyx epitoca Monro, 1930, were examined and also found to have spines in far posterior parapodia; the species is also referred to Chaetocirratulus n. gen. The holotype of Heterocirrus cincinnatus Ehlers, 1908, has been examined. This species was originally described from the sub-Antarctic Kerguelen Islands in the Indian Ocean, and has been reported widely from Antarctica by numerous authors (usually as Tharyx cincinnatus or Aphelochaeta cincinnata). However, it appears that this species is limited to subantarctic locations and the records from the high Antarctic belong to other species, mostly described herein. Due to errors in the original descriptions or misinterpretation of the published descriptions, each of the three species cited above have been misconstrued in the Antarctic ecological literature. As such, the published records of these species most certainly refer to other species described in the present study. These issues illustrate some of the problems with interpreting previous records of Antarctic bitentaculate cirratulids.

In the present study, 11 species of Aphelochaeta from South America and Antarctica are treated; eight are new to science. Other undescribed species are also present in the South American and Antarctic collections but the specimens are few, incomplete, and not in sufficiently good condition to be described.

All seven species of Aphelochaeta from Antarctica reported herein are new to science and have proven to be especially difficult to separate from one another despite the availability of large numbers of specimens. Most specimens are incomplete, have lost tentacles and branchiae, and the morphology of the anterior and posterior ends has proven to be highly variable largely due to different methods of collection, handling, and damage or contraction during preservation. Contraction of the peristomium compresses the region where the dorsal tentacles and first branchiae are located, making these structures sometimes difficult to locate and to correctly define as being on the peristomium or first setiger. Shirlastain A or SEM is needed to better highlight the scars or stumps of these structures and careful study of the specimens is time consuming. The shape and size of the posterior end is often diagnostic, but this part of the body is usually missing. The presence of separated posterior ends in the samples is helpful, however. Only three species, A. brandtae n. sp., A. hormosa n. sp., and A. spectabilis n. sp., have a prominent and distinctive MG staining pattern on the pre-setiger region and/or the venter of anterior segments. Aphelochaeta dearborni n. sp. and A. palmeri n. sp. each have a weak, diffuse staining pattern on the pre-setiger region, but other species have little or no staining pattern. Methyl Green does stain the thoracic segments of several species to differing degrees, but except for the prominent bands on the venter of A. brandtae n. sp., this is of itself usually only a supporting character. Finally, the lack of distinctive setal morphology in species of Aphelochaeta presents challenges in this genus and adds to the difficulty of distinguishing between the various Antarctic species in particular.