Mysta Malmgren, 1865

Kato, Tetsuya, Pleijel, Fredrik & Mawatari, Shunsuke F., 2001, A new species of Mysta (Annelida, Polychaeta, Phyllodocidae) from Japan, Zoosystema 23 (1), pp. 19-27 : 21-27

Genus Mysta Malmgren, 1865

Mysta – Wilson 1988.

TYPE SPECIES. — Mysta barbata Malmgren, 1865 , by monotypy.

Mysta ctena n. sp.

( Figs 1-3 )

MATERIAL EXAMINED. — Koajiro Bay . Misaki, Honshu, Japan, 10 m, shell sand, 21.V.1996, coll. TK, holotype ( ZIHU 1372 ), 6 paratypes ( ZIHU 1373-1377 ), 5 paratypes ( MNHN-POLY 59 ) ; 9 m, fine sand, coll. TK, 21.V.1996, 1 specimen.

Off Bentenbana. Misaki , Honshu, Japan, 8 m, mud, 21.V.1996, coll. TK, 3 specimens ; 6 m, sand, 21.V.1996, coll. TK, 10 specimens.

Abratsubo. Misaki , Honshu, Japan, intertidal, mud, 19.V.1996, coll. TK, 2 specimens ; 20.V.1996, 2 specimens.

ETYMOLOGY. — Named for the comb-like appearance of the papillose dorsal tentacular cirri when seen in anterior or posterior view, “ ctena ” being Greek for comb.

DISTRIBUTION. — Known only from Misaki, Japan. From intertidal to 10 m of depth, in mud, sand, and shell sand.

DESCRIPTION

Holotype complete, 21 mm long and 1 mm wide at middle of body (including parapodia but excluding chaetae), for 112 segments. Largest examined specimen 43 mm long and 1.7 mm wide at middle of body, for 140 segments (lacking posterior end, ZIHU 1374). See Fig. 4 for other specimens.

Body slender, dorso-ventrally flattened, anteriorly and posteriorly tapered. Prostomium rounded, about as long as wide, with pair of distinct eyes with lenses ( Fig. 1A, B ). Nuchal papilla large, may be poorly delineated and difficult to distinguish. Proboscis with single row of large, discoidal papillae on each side; proximal and distal-most sides without papillae ( Fig. 1D ). Proboscis dorsally with two types of thorny papillae ( Fig. 3B ); small rounded papillae on mid-dorsal part ( Fig. 3C ), and large elongated on dorso-lateral parts ( Fig. 3D ). Terminal proboscis ring with large number of papillae; dorsal and ventral part with small conical papillae, lateral part with large rounded papillae. Internal side of ring densely covered by minute conical papillae ( Fig. 3E ), and with pair of large lateral papillae.

First visible segment (segment 2) with two pairs of tentacular cirri. Dorsal tentacular cirri as long as ventral ones, with single row of rounded papillae on dorsal side; each row with 7-13 papillae ( Figs 1A, B ; 2A ; 3A ), indistinct in some specimens. Ventral tentacular cirri without row of papillae. Neuropodia of segment 3 with c. 10 chaetae and ventral cirri of similar size and shape as those of following segments ( Fig. 2B ). Dorsal cirri of median segments oval, slightly longer than wide ( Fig. 2 C-E); dorsal cirri of posterior segments approaching triangular shape ( Fig. 2F, G ). Neuropodial lobes with 10-15 chaetae. Rostrum of chaetal shaft asymmetrical with single main tooth on anterior side, and two (rarely one) smaller main teeth on posterior side ( Fig. 3F, G ). Blades long and slender. Ventral cirri narrow, as long as neuropodial lobes ( Fig. 2E, G ). Pygidial cirri twice as long as wide, with rounded ends ( Fig. 1C ). Pygidial papilla absent.

Colour

Preserved specimens white with irregular reddish brown pigmentation on dorsum, on dorsal cirrophores and on dorsal cirri. Dorsal pigmentation from segment 4 in small specimens, from prostomium in larger ones. Venter unpigmented. Eyes blackish.

CHARACTER VARIATION WITHIN MYSTA

The new species belongs to the Eteone -group (including Eteone Savigny in Lamarck, 1820, Hypereteone Bergström, 1914 and Mysta ), as seen from the presence of four antennae, a nuchal papilla, two pairs of tentacular cirri on the first visible segment (segment 2), and the absence of tentacular or dorsal cirri on segment 3 ( Pleijel 1991). It is assigned to Mysta based on the presence of thorny papillae and lateral rows of papillae on the proboscis, and the presence of short pygidial cirri with rounded ends.

A comparison of characters between M. ctena n. sp. and other species of Mysta is complicated by the situation that only limited information is available from the literature (e.g., Wilson 1988; Pleijel 1993), most notably relating to characters of the proboscis. For this reason, we re-examined the known species of Mysta for the presence and distribution of a number of characters (summarised in Table 1).

Whereas thorny papillae ( Fig. 3B, C ) are unique within Mysta (absent from M. platycephala [ Augener, 1913]; see below), three different types may be distinguished within the group: 1) small rounded papillae ( Fig. 3C ); 2) large elongated papillae ( Fig. 3D ); 3) single spine papillae:

1. the small rounded type is 12-18 µm in diameter in M. ctena n. sp., 12-16 µm in M. barbata Malmgren, 1865 , and has a large number of teeth distributed over the surface ( Fig. 3C ). These papillae were observed in M. ctena n. sp., M. barbata , M. picta Quatrefages, 1866 and M. syphodonta (Delle Chiaje, 1822) . There are some differences present between the taxa, both in distribution and morphology of the papillae. In M. ctena n. sp., M. barbata and M. syphodonta they are present only dorsally on the proboscis; in M. picta they occur also on the proximal part of the ventral side. Further, in M.barbata and M.syphodonta the papillae are fully rounded and the teeth are radiating, whereas in M. ctena n. sp. and M. picta some papillae are slightly elongated, and the teeth are directed posteriorly on the proboscis in everted state; 2. the large elongated type is 60-100 µm long in M. ctena n. sp., and has, like the previous type, a large number of thorns distributed over the surface ( Fig. 3D ). These papillae are present in M. ctena n. sp. and M. maculata Treadwell, 1920 , and appear in two to three indistinctly separated longitudinal rows on each dorso-lateral side of the proboscis. Our observations from the holotype and only known specimen of M. maculata (USNM 18940) on the distribution of the papillae deviate from those of Wilson (1988). He described the dorsal side of the proboscis in M. maculata as having a dense band of minute denticulate papillae in transverse rows, but whereas we could confirm the transverse folds, we were unable to detect any denticulate papillae; 3. the single spine type is 12-18 µm in diameter, lacks small teeth, and is provided with a single triangular spine. It occurs only in M. tchangsii Uschakov & Wu, 1959 , and the papillae appear dorsally, either individually or in small clusters of two to five on the proboscis.

A

50

40

)

mm

(30

length 20

Body 10

0

0 25 50 75 100 125 150 175 No. segments

B

2

) 1.5

mm

(

width 1

Body 0.5

0

0 25 50 75 100 125 150 175 No. segments

C

2

) 1.5

mm

(

width 1

Body 0.5

0

0 10 20 30 40 50 Body length (mm)

The papillae in the lateral, longitudinal rows in Mysta are large and fleshy, they may be conical, discoidal, or rounded, and they can be arranged in single or several rows. Mysta syphodonta and M. barbata both have conical papillae, but differ in that M. syphodonta has a single row, and M. barbata 2-3 (although poorly delineated) rows. Discoidal papillae are present in M. ctena n. sp., M. maculata and M. platycephala , all of which have the papillae in single rows, and rounded papillae were observed in M. picta only, where they cover the proboscis both laterally and ventrally. These papillae in Mysta tchangsii also have rounded ends, but are irregularly shaped and arranged in single rows.

The presence of small conical papillae on the inside of the proboscis ( Fig. 3E ) is a previously unnoticed character within Phyllodocidae . Whereas these papillae are large in M. ctena n. sp. and M. tchangsii , they are very small and difficult to observe in M. barbata and M. picta . Due to uncertain observations we are unable to confirm their occurrence in M. maculata and M. syphodonta . Their presence or absence in M. ornata and M. platycephala is unknown, and further investigation is needed to assess the variation and distribution of this character.

The presence of a row of papillae on the dorsal tentacular cirri ( Fig. 3A ) in M. ctena n. sp. represents another new character. The papillae, which are situated on the dorsal side of the dorsal tentacular cirri, are small and almost transparent, and examination of dissected cirri at high resolution in a compound microscope is necessary for verification. The papillae appear to be absent from all other species of Mysta , although their presence or absence is unknown for M. ornata . Furthermore, due to the poor condition of some specimens of the examined taxa, the distribution of this character warrants further investigation.

The shape of the dorsal cirrophores in posterior segments is characteristic in larger specimens of M. maculata , M. syphodonta and M. tchangsii , which all have cirrophores that are prominent and elongated. This differentiation between more anterior and posterior cirrophores is absent from all other species of Mysta , including M. ctena n. sp., which have short cirrophores along the entire body.

Wilson (1988) re-examined a syntype (HZM V- 7928) of M. platycephala and additional specimens from Australia. The proboscis was described as having dorsal bands of small thorny papillae in transverse rows, and 10-15 lateral triangular papillae, and with the rostra of the chaetal shafts as carrying one large tooth on each side. Our re-examination of the syntype instead indicates that it lacks thorny papillae, that there are five lateral papillae which are discoid in shape, and that the rostra of the chaetal shafts have a single large tooth. Either these characters are variable, or several species were present in Wilson’s material; the matter warrants further examination. Comparison with Mysta ornata ( Grube, 1878) , described from northern Japan Sea, is problematic in that we have been unable to locate any types or reliably identified specimens, and we lack information relating to the thorny papillae and several other characters in this taxon. Although M. ornata at present must be considered a nomen dubium, the brief original description indicates that it differs from M. ctena n. sp. in having small eyes and three dorsal longitudinal violet bands.

Mysta ctena n. sp. is unique within Mysta in having both small rounded and large elongated thorny papillae; all other known members are provided with a single kind of thorny papillae only. Although it shares similarities in the shape and distribution of the lateral proboscis papillae with M. maculata , M. ctena n. sp. is also unique in having rows of papillae on dorsal tentacular cirri (although the absence of this character requires confirmation in several species of Mysta ). Although it is obvious that M. ctena n. sp. represents a new member of Mysta , we are currently unable to specify any phylogenetic relationships within the group.

Figures

FIG. 1. — Mysta ctena n. sp.; A, anterior end of holotype, dorsal view; B, anterior end of large paratype, dorsal view; C, posterior end of holotype, ventral view; D, dissected proboscis of paratype, ventral view (median area corresponds to dorsal part). Scale bars: 500 µm.

FIG. 2.— Mysta ctena n. sp.; A, tentacular cirri, segment 2, anterior view; B, parapodium of segment 3, anterior view; C, parapodium of segment 4, anterior view; D, parapodium of segment 28, anterior view; E, same, posterior view; F, parapodium of segment 83, anterior view; G, same, posterior view. Scale bar: 500 µm.

FIG. 3. — SEM micrographs of Mysta ctena n. sp.; A, dorsal tentacular cirrus, dorsal view; B, dorsal proboscis papillae; C, small type of dorsal proboscis papillae; D, large type of dorsal proboscis papillae; E, internal proboscis papillae; F, rostrum of setal shaft, medi- an parapodium, anterior view; G, same, posterior view. Scale bars: A, 20 µm; B, 50 µm; C, F, G, 5 µm; D, E, 10 µm.

FIG. 4. — Mysta ctena n. sp.; relationships between A, number of segments and body length; B, number of segments and body width; C, body length and body width.