Timarete posteria, Choi, Hyun Ki, Kim, Hana & Yoon, Seong Myeong, 2018

Choi, Hyun Ki, Kim, Hana & Yoon, Seong Myeong, 2018, Timareteposteria, a new cirratulid species from Korea (Annelida, Polychaeta, Cirratulidae), ZooKeys 806, pp. 1-15 : 4-11

Timarete posteria sp. n. Figs 1, 2, 3, 4

Material examined.

Type locality: South Korea, Gyeongsangbuk-do Province: Pohang-si County, Heunghae-eup, Odo-ri, 36°09 ’17” N, 129°24 ’02” E, 13 July 2017, intertidal rocky bottom. Holotype: complete specimen (NIBRIV0000829700). Paratypes: one complete specimen (MABIKNA00146231); one complete specimen (MABIKNA00146236); one complete specimen (MABIKNA00146238); one complete specimen (MABIKNA00146239); one complete specimen (MABIKNA00146245). Non-type material: 16 specimens (13 complete and 3 incomplete specimens), collection details same as type materials; 11 specimens (all complete), South Korea, Gangwon-do Province: Goseong-gun County, Jugwang-myeon, Munamjin-ri, 35°18'41"N, 129°32'33"E, 10 April 2017, intertidal rocky bottom.

Diagnosis.

Body with deep ventral groove and distinct segments. Prostomium triangular, without eyespots. Peristomium evenly divided into three annulations. Branchial filaments one pair per segment, beginning from third peristomial annulus, and gradually shifting to mid-dorsum between chaetigers 30-78; completely shifted branchiae at about one-third distance between notopodium and dorsal midline. Grooved tentacular finalents arising from chaetigers 5-6 and occasionally 6-7 or 7-8. Chaetae including capillaries and acicular spines; notopodial spines 1-4, pale brown in color, from chaetigers 16-45; neuropodial spines 2-4, curved distally, thicker than notoacicular spines, dark brown in color, from chaetiger 24-69.

Description.

Holotype: complete, 5.5 cm in length (4.8-13.2 cm in paratypes) and 5.7 mm in maximum width (4.0-6.0 mm in paratypes), with approximately 261 segments.

Body elongated, rounded dorsally, flattened ventrally, with distinct ventral groove throughout and tapering posterior end. All segments distinct, narrow, crowded throughout body with distinct lateral shoulders. Body color in alcohol pale grey to dark grey, branchiae and tentacular finalents yellowish grey; live specimens with body dark red and branchiae and tentacular finalents light orange. No separate pigmentation on body (Fig. 1).

Prostomium short, triangular, blunt distally, and as long as three anterior chaetigers. Nuchal organs round, present on posterior-lateral prostomial region. Eyespots absent (Figs 2A, B, 3A).

Peristomium with three annulations nearly equal in length, longer than prostomium and as long as four anterior chaetigers; second and third annulations with 2-3 lateral wrinkles (Figs 2A, B, 3A).

Branchial filaments one pair per segment, from posterior margin of third peristomial annulus, continuing on most segments except about last ten segments; branchial finalents located just above notopodial ridges in anterior 42 chaetigers (29-77 chaetigers in all specimens examined); then shifting gradually to mid-dorsum forming lateral bulge over notopodia from chaetiger 43 (30-78 in all specimens examined) to near posterior end; fully shifted branchiae located about one-third distance between notopodium and dorsal midline. (Figs 2 A–C, 3B).

Tentacular filaments formed two transverse groups separated by median gap and arising on dorsum of chaetigers 5-6 (6-7 or 7-8 in some specimens examined); each group with about 18-21 finalents arranged in 2-3 transverse rows (Figs 2A, B, 3A).

Parapodia, notopodia forming lateral shoulders dorsally; noto- and neuropodium widely separated throughout (Figs 2 A–C, 3A, B).

Chaetae including capillaries with serrated edge observed under light microscopy (400x) and SEM observation and acicular spines. Capillary chaetae about 8-10 capillaries arranged in two longitudinal rows in anterior parapodia. Notopodial spines nearly straight, pale brown in color, present from chaetiger 40 (16-45 in all specimens examined); 1-3 spines per segment accompanied by 1-3 companion capillaries from chaetiger 40 to posterior end. Neuropodial spines curved distally, slightly thicker than notopodial spines, dark brown in color, from chaetiger 30 (24-69 in all specimens examined); 2-3 spines per segment with 1-2 companion capillaries from chaetiger 30 to very posterior end (Figs 3C, D, 4 A–C).

Pygidium with terminal anus (Fig. 4D).

Methyl green staining pattern (MGSP).

Body stained with transverse bands on posterior half of each segment forming complete rings. Branchial and tentacular finalents not stained. Prostomium , peristomium, and dorsum of first 3 or 4 chaetigers intensely stained with dark green. Noto- and neuropodial ridges not stained (Fig. 3A, B).

Variations.

Several morphological characters in cirratulids are highly variable ontogenetically and a few of them are clearly considered size-dependent in Timarete species ( Blake 1996, Magalhães and Bailey-Brock 2010, Magalhães et al. 2014). We examined the relationships between the ontogenetic characteristics including the segmental origin of noto- and neuropodial spines and the shift of branchial finalents, and the total number of chaetigers in the new species according to the correlation analyses based on 31 complete specimens (Fig. 5). In Timarete posteria sp. n., the segmental origin of neuropodial spines ranged from chaetigers 16 to 45, strongly size-dependent (N = 31, r = 0.81). The segmental origin of notopodial spines varied from chaetigers 24 to 69, with weak size-dependent characteristics if compared with those of neuropodial spines (N = 31, r = 0.67). The dorsal shift of branchial finalents occurred between chaetigers 30 and 78 regardless of the total number of chaetigers in the new species (N = 31, r = 0.40) (Fig. 5). It is known that the appearance of tentacular finalents is generally variable in a few Timarete species ( Magalhães et al. 2014). In T. posteria sp. n., the tentacular filaments always originated in the dorsum of two consecutive chaetigers although their locations were variable: usually on chaetigers 5-6 (in 28 specimens) and occasionally 6-7 (in two specimens) or 7-8 (in four specimens) in all the 34 specimens examined. This variation appears to be somewhat related to body size because the tentacular finalents on chaetigers 7-8 occur in specimens containing more than 300 segments (almost similar to specimens with less than 300 segments on chaetigers 5-6). Further studies with a larger population and more temporal samples are needed to determine a more accurate relationship between the segmental origin of the tentacular finalents and the body size.

Etymology.

The epithet of the specific name, posteria , is derived from the Latin posterior, meaning ‘hind’. This name refers to the shift in the appearance of the branchial finalents from relatively posterior chaetigers. The gender of the genus name, Timarete , is feminine and the specific name of this new species is designated as feminine.

Habitat and distribution.

This species is a common inhabitant of seagrass beds in the intertidal rocky bottoms and distributed in the East Sea (or the Sea of Japan) of South Korea.

Molecular information.

In the present study, partial COI sequences each measuring 658 bp in size from five specimens and partial 16S sequence of 519 bp in size from a single specimen were obtained for the genetic analysis of Timarete posteria sp. n. They were deposited in GenBank under the accession number MH708229-MH708233 (COI) and MH822840 (16S). The intra-specific genetic distance between five COI sequences was measured according to the Kimura-2-parameter (K2P) model and ranged from 0 to 0.4 %. We carried out the genetic comparison of the new species with three Timarete species available, including T. caribous (Grube and Ørsted in Grube, 1859), T. ceciliae Magalhães, Seixas, Paiva, and Elias, 2014, and T. punctata (Grube, 1859) from the Brazilian coast, with COI and 16S sequences previously announced from GenBank ( Magalhães et al. 2014). Based on entire genetic data uploaded in GenBank, the inter-specific genetic distances of COI and 16S sequences between the new species and other Timarete species were 23.7-26.2 % and 22.2-26.5 %, respectively (K2P distance). We examined the molecular phylogenetic relationship based on the Maximum likelihood (ML) tree using the genetic data available from GenBank on several cirratulids belonging to the multi-tentaculate genera, Cirriformia , Cirratulus , and Timarete , with the new species ( Rousset et al. 2007, Hardy et al. 2011, Magalhães et al. 2014, Weidhase et al. 2014, Lobo et al. 2016, Weidhase et al. 2016). The GenBank accession numbers of them are represented on Table 1. In ML tree (Fig. 6), all cirratulid species showed the specific validity by the molecular data of the present study. In generic level, the Timarete species including T. posteria sp. n. formed a clade with two Cirriformia species, C. chicoi Magalhães, Seixas, Paiva, & Elias, 2014 and C. tentaculata (Montagu, 1808), showing a similar result to the phylogenetic tree of Magalhães et al. (2017). This result suggests that both of the genera Timarete and Cirriformia are not monophyletic and they are closely related to each other. However, the reality of phylogenetic relationship between Timarete and Cirriformia still merits further study with more morphological and molecular information of the multi-tentaculate genera.

Remarks.

The major characteristics of Timarete posteria sp. n. are mostly similar to those of Timarete luxuriosa (Moore, 1904), originally described from Southern California ( Blake 1996). Both species share the following morphological features: 1) branchial finalents a single pair per segment, gradually shifting toward mid-dorsum from relatively posterior chaetigers compared to its congeners (the dorsal shift of branchial finalents occurred at about chaetiger 35 when the body with 350 segments in T. luxuriosa and occurred at chaetigers 38-78 with more than 300 segments and at chaetigers 30-76 with less than 300 segments in the new species, while that occurred at chaetigers 7-26 in other Timarete species); 2) tentacular finalents arising from the dorsum of chaetigers 5-6 (usually 5-6 and sometimes 6-7 or 7-8 in the new species); 3) notopodial spines originating from chaetiger 50 (24-69 in the new species) and pale brown in color; and 4) neuropodial spines originating from chaetiger 31 (16-45 in the new species) and dark brown in color ( Blake 1996). However, T. posteria sp. n. clearly differs from T. luxuriosa in peristomium, notopodial spines, and shifting branchial finalents as follows: 1) the peristomium is evenly divided into three annulations in T. posteria sp. n., while that of T. luxuriosa comprises one large and three smaller annulations; 2) T. posteria sp. n. bears 2-4 neuropodial spines accompanied by a few capillaries in the posterior chaetigers, however, T. luxuriosa contains a single neuropodial spine without capillaries in the posterior chaetigers after around chaetiger 90; 3) completely shifted branchial finalents are located at about one-third distance between notopodium and dorsal midline in T. posteria sp. n., whereas those of T. luxuriosa are positioned at about two-thirds distance ( Blake 1996).

The methyl green staining pattern (MGSP), which may be of diagnostic value, is unavailable for many Timarete species except for a few species recently described ( Imajima and Hartman 1964, Blake 1996, Çinar 2007, Magalhães and Bailey-Brock 2010, Magalhães et al. 2014). Nevertheless, the MGSP of the new species is distinct from the previously described patterns from five Timarete species, including T. caribous (Grube, 1859), T. ceciliae Magalhães, Seixas, Paiva & Elias, 2014, T. hawaiensis (Hartman, 1956), T. oculata (Treadwell, 1932), and T. punctata (Grube, 1859), by a combination of the following features: 1) intense staining of the prostomium, peristomium, and dorsum of the first three chaetigers; 2) the presence of transverse bands forming complete rings in the posterior half of each segment ( Magalhães and Bailey-Brock 2010, Magalhães et al. 2014). MGSP is a useful diagnostic feature in Timarete species, and additional MGSP information for several Timarete species is still required.