Aphelochaeta petila, Choi & Jung & Yoon, 2018

Choi, Hyun Ki, Jung, Tae Won & Yoon, Seong Myeong, 2018, Two new species of Aphelochaeta (Annelida: Polychaeta: Cirratulidae) from Korean waters, Zootaxa 4504 (4), pp. 586-594 : 587-590

Aphelochaeta petila new species

Figures 1 A–B, 2A–F

Material examined. Type locality: South Korea, Gyeongsangnam-do Province: Namhae-gun County, Changseonmyeon , Danghang-ri , 128°28′24″E, 34°47′33″N, 31 Jul. 2014, intertidal soft bottom. Holotype: complete specimen ( NIBRIV0000325701 ). Paratypes: one complete specimen (MABIKNA00155285); one complete specimen (MABIKNA00155286); one complete specimen (MABIKNA00155287); one complete specimen (MABIKNA00155288); one complete specimen (MABIKNA00155289) on stub for SEM observation. Non-type material: 6 specimens, collection details same as type materials (MABIKNA00155290) .

Description. Holotype. Complete; body slender and elongate, 14.5 mm long and 0.8 mm maximum wide; thoracic region slender, not expanded, gently swollen dorsally, rounded ventrally, composed of 26 crowded segments, and without ventral and dorsal groove; abdominal region less crowded than thoracic with nonmoniliform segments; posterior end not expanded, rounded dorsally, slightly flattened ventrally, with shallow ventral groove; pygidium rounded and with anus opening terminally ( Figs. 1 A–B, 2A–C).

Prostomium conical, pointed anteriorly, slightly longer than wide, eyespots absent, paired nuchal organs at posterior edge. Peristomium slightly broad, with three annulations, posterior-most annulation longest, dorsal crest lacking. Paired dorsal tentacles grooved, arising from posterior margin of third peristomial annulation. Branchiae distinctly annulated in alcohol; first pair immediately lateral to dorsal tentacles on posterior margin of third peristomial annulation, second pair dorsal to notochaetal tori at posterior margin of chaetiger 1, and subsequent branchiae arising in similar location throughout. Thoracic parapodia well developed, fleshy dorso-lateral lobes; notopodial lobes slightly swollen, but not elevated over dorsum. Abdominal parapodia poorly developed ( Figs. 1 A–B, 2D).

Chaetae all capillaries, emerging from lateral body wall, long, simple, with short fibrillar extension along one side of blade; thoracic notochaetae numbering 9–11 and 7–9 neurochaetae; abdominal noto- and neurochaetae with 6–9 capillaries, and notochaetae slightly longer than neurochaetae ( Fig. 2 E–F).

Methyl green staining pattern (MGSP). Venter of the thoracic region with distinct transverse dark blue strips on middle region of segment from chaetigers 6–16 ( Fig. 2A ).

Etymology. The epithet of the specific name, petila , is derived from the Latin petilus, meaning “slender”. This name refers to the slender body showing non-expanded anterior and posterior ends.

Habitat/Distribution. This species was collected from sandy mud sediments in the intertidal zone at Namhaegun in the southern coast of Korea.

Remarks. Aphelochaeta petila n. sp. is very similar to Aphelochaeta monilaris , the type species of the genus, in the following characteristics: the prostomium is pointed distally; the peristomium has three annulations; both the dorsal tentacles and the first branchiae are arising from the third peristomial annulation; the dorsal crest is indistinct; and the thoracic region lacks a ventral groove ( Blake 1996; Dean & Blake 2016). However, the new species can be clearly distinguished from the latter by the following morphological features: the body of the new species is not expanded anteriorly and posteriorly as it is in A. monilaris ; the abdominal segments are nonmoniliform in the new species and moniliform in A. monilaris ( Blake 1996; Dean & Blake 2016). Moreover, the methyl green staining pattern (MGSP) of A. petila n. sp. differs from those of A. monilaris by having ventral transverse stripes on the thoracic region that do not extend laterally to the parapodia while in A. monilaris , they extend to the anterior edge of the notopodia. The tip of the prostomium remains unstained in A. petila n. sp. while it is stained in A. monilaris ( Blake 1996) .

Aphelochaeta petila n. sp. has a very distinct MGSP on the venter of the thoracic region, with transverse dark blue stripes on chaetigers 6–16. A. petila n. sp. is closely related to A. elongata Blake, 1996, A. striata Dean & Blake, 2016, and A. tigrina Blake, 1996 in that all these species have distinct MGSP forming transverse stripes on the venter of the thoracic region ( Blake 1996; Dean & Blake 2016). There are some differences among the new species and A. striata and A. tigrina in the MGSP: the thoracic ventral bands of A. petila n. sp. are found on chaetigers 6–16 but only on chaetigers 4–8 in A. striata; the posterior end is unstaining in the new species but staining intensively in A. tigrina ( Blake 1996; Doner & Blake 2009; Dean & Blake 2016).

Aphelochaeta petila n. sp. is readily distinguishable from A. elongata, A. striata, and A. tigrina based on several characteristics. The median dorsal mounds on thoracic chaetigers are indistinct in the new species while they are distinct in A. elongata. The peristomium has three annulations while annulations are indistinguishable in A. elongata and it has only two annulations in A. striata. In addition, the peristomium is short in A. petila n. sp. while it is elongate in A. elongata and A. striata. The first branchiae arise from the peristomium in A. petila n. sp., but in A. elongata and A. striata they arise from chaetiger 1; chaetiger 1 has a single pair of branchiae in A. petila n. sp. while it has two pairs in A. striata. The dorsal crest on the peristomium is indistinct in the new species when compared to A. tigrina which has a distinct dorsal crest; the thoracic region is not expanded while it is broadly expanded in A. tigrina ( Blake 1996; Doner & Blake 2009; Dean & Blake 2016).

Figures

FIGURE 1. Aphelochaeta petila n. sp., A, B, holotype (NIBRIV0000325701); Aphelochaeta intinctoria n. sp., C, D, holotype (NIBRIV0000325699). A, anterior end, dorsal view; B, anterior end, lateral view; C, anterior end, dorsal view; D, anterior end, lateral view. Scale bars: A–D=0.25 mm. Abbreviations: branchia (br), dorsal crest (dc), dorsal tentacle (dt), nuchal organ (nuO), peristomium (per), prostomium (pr).

FIGURE 2. Aphelochaeta petila n. sp., A, B, holotype (NIBRIV0000325701); C–F, SEM observation in paratype (MABIKNA00155289). A, MGSP of thoracic region, ventro-lateral view; B, posterior end, lateral view; C, thoracic region, lateral view; D, anterior end, lateral view; E, basal region of thoracic capillary chaeta; F, distal region of thoracic capillary chaeta. Scale bars: A–C=0.5 mm, D=0.25 mm, E=0.01 mm, F=0.005 mm. Abbreviations: branchia (br), dorsal tentacle (dt), nuchal organ (nuO), peristomium (per), prostomium (pr).