Myxicola glacialis Bush, 1905

Putignano, Matteo, Langeneck, Joachim & Giangrande, Adriana, 2024, The forgotten diversity of the genus Myxicola (Polychaeta: Sabellidae) in North America: redescription of historical taxa and description of two new species, Journal of Natural History 58 (37 - 40), pp. 1393-1427 : 1415-1419

Myxicola glacialis Bush, 1905

( Figures 11 , 12 )

Myxicola glacialis Bush, 1905: p. 218 –219 pl. XXII, fig. 1; pl. XXV, figs. 1, 2; pl. XXVI, fig. 4b; pl. XXXVIII, figs. 12, 23, 25–32.

Material examined

Syntype YPM IZ 106540 : collected by W. R. Coe , Harriman Alaska Expedition , 1899, Pacific Ocean , North America , USA, Alaska, Aleutians East Borough , Unalaska Island; preserved in 70% ethanol . Syntype YPM IZ 106560 : collected by W. R. Coe , Harriman Alaska Expedition , 1899, Pacific Ocean , North America , USA, Alaska, Aleutians East Borough , Unalaska Island, Dutch Harbour, preserved in 70% ethanol .

Redescription (YPM IZ 106540)

Specimen complete, body orange yellowish ( Figure 11A ); photos from original description ( Bush 1905) show distinctly darker anterior end ( Bush 1905, fig. 13A1, A2); the original description reports ‘[…] the body of usual cream color, the thoracic region and the branchiae colored with deep purple having a tinge of brown’ and ‘In life ‘white or yellowish with brown purple branchiae’ ( Bush 1905: p. 218–219). Body dorso-ventrally flattened, with distinctly narrower and elongated posterior abdomen. Crown slightly darker than body ( Figure 11A ); photos from original description ( Bush 1905) show distinctly darker crown ( Bush 1905, fig. 13A1, A2). Body 3.32 cm long, 3 thoracic and 68 abdominal chaetigers; maximum width 3 mm; thorax longer than wide. Crown 5.16 mm long, with 11 pairs of radioles; ratio body/crown length 6.43. Peristomium 1.07 mm long; anterior and posterior peristomial ring separated latero-ventrally by a narrow rut; mid-ventral lobe triangular, low with rounded tip; peristomium laterally high, covering junction between body and crown; mid-lateral incision narrow, with rounded edges ( Figure 11B, C ). Dorsal and ventral lips and related structures indistinct, as small, flattened discoidal structure or absent ( Figure 11D ). Outer radiolar surface with yellowish rachis, connected by lighter palmate membrane extending into wide radiolar flanges progressively narrowing on distal halves of radiole tips, strap-like, distally rounded. Radiolar tip <1/4 of total radiole length, same colour as radioles ( Figure 11E ); pinnules thick, distally rounded, yellowish, up to 1/3 of total radiole length, alternating along radioles; longest pinnules distally, slightly shortening to beginning of radiolar tips; radiolar skeleton composed of numerous small cells, apparently arranged randomly, surrounded by a hyaline matrix; graft cells for pinnular skeleton squared and big, undivided, ventrally contiguous to latter ( Figure 11G ). Radiolar eyes and peristomial eyes not visible. Glandular girdle near the posterior margin of chaetiger 2. Lateral eyes not visible. Pygidium papilliform, distinctly smaller than the last abdominal chaetigers ( Figure 11F ); pygidial eyes not visible. Numerous narrowly hooded thoracic notochaetae with distinctly long and tapering tips, arranged circularly to form big tufts, decreasing in number of chaetae per fascicle through thorax; peristomial notochaetae with shorter tips; edge of the hood showing marked serration ( Figure 12A, B ). Thoracic uncini curved, with thick main fang and several small teeth, not reaching half of main fang length ( Figures 12D, F , 16C ); in dorsal view secondary teeth seem to form a row on top of main fang, with larger teeth apically, appearing in more than one row in lateral view ( Figure 12E ); thoracic tori inconspicuous. Numerous abdominal neurochaetae narrowly hooded, similar to thoracic notochaetae but shorter ( Figure 12C ), forming less conspicuous tufts than on thorax, significantly decreasing in number of chaetae per fascicle along abdomen ( Figure 12C ). Abdominal uncini with large tooth, about half of main fang length; second tooth visible in dorsal view, aligned with the latter ( Figure 12I ); breast squared; main fang shorter than breast; handle vestigial; uncini longer than high ( Figures 12G–I , 16D ).

Remarks

These specimens were examined to check the validity of M. glacialis Bush, 1905 . The comparison with the original description is mostly consistent, including measurements. Although Bush (1905) reported the presence of longer and shorter forms ( Bush 1905, fig. 13A1–B), they share a slender appearance, due to the progressively tapering abdomen. Differences with respect to the original description concern especially radiolar structures: radiolar tips were described as ‘[…] rather short […]’, even though the morphology reported (both in the text and in the illustration) is consistent with our findings ( Figure 13E ), while pinnules, reported as ‘[…] well-separated […]’ ( Bush 1905: p. 219), can be explained by their remarkable thickness. Lastly, the author reports the presence of abdominal tori forming a complete girdle around the body from the 12th abdominal segment, which was not confirmed herein.

Original illustrations of thoracic uncini show a ‘paleate’ morphology, with teeth, including main fang, all absent. However, this can be explained by frontal observation of an uncinus, similarly to what we noticed in the present study (compare Figure 12F with fig. 14I from Bush (1905)). Abdominal uncini are consistent with our findings; also, we found some variability with longer and shorter breast forms, as reported by Bush (1905, fig. 14J1, J2), but no pattern of distribution between these forms along the tori was observed. As in M. conjuncta, Bush (1905) reported the presence of ‘spear-shaped thoracic chaetae’, not found in the current analysis. Interestingly, a marked serration on the hood of chaetae seems to be present in the drawings, similarly to our findings ( Bush 1905, fig. 14F1–G2).

Scale bars: 0.02 mm.

Although World Register of Marine Species (2023) reports this taxon as synonymous with Myxicola aesthetica ( Claparède 1870) , such synonymy has never been formally proposed. In fact, Berkeley (1930) only cited the possibility of synonymy between M. glacialis and M. aesthetica , referring to McIntosh (1923), who, however, only pointed to the fact that Bush (1905) did not compare M. glacialis with M. aesthetica , notwithstanding the similarities between members of these taxa ( McIntosh 1923).

Specimens of M. aesthetica and M. glacialis have similar biometric features, such as the number of thoracic (3) and abdominal chaetigers (up to 50 in individuals of M. aesthetica ), body length (up to 40 mm) and number of radioles (5 to 10 pairs). However, important differences can be noticed in the shape of thoracic and abdominal uncini (both showing only a single apical tooth in individuals of M. aesthetica ), in the arrangement of the radiolar skeleton (2 skeletal cells in individuals of M. aesthetica ), and in the presence of peristomial dark eyes and otoliths (not found in M. glacialis ), that clearly separate members of these two species.

Distribution and ecology

Dutch Harbour, Unalaska Island. The only information concerning the ecology of these animals reported in the original description ( Bush 1905: p. 219) is ‘[…] embedded in much mucus, under and between stones on shelly sand’.

Figures

Figure 11. Myxicola glacialis (YPM IZ 106540). A. entire worm; B, C. peristomial rings, ventral and lateral views, respectively; D. ventral/dorsal lips complex, hardly distinguishable (arrows); E. radiolar tips; F. pygidium; G. scheme of radiolar cross section.

Figure 12. Myxicola glacialis (YPM IZ 106560). A. peristomium notochaeta; B. 3rd chaetiger thoracic notochaeta; C. 24th chaetiger abdominal neurochaeta; D–F. 3rd chaetiger thoracic uncinus, lateral, dorsal and front views, respectively; G–I. 24th chaetiger abdominal uncini.

Figure 13. Original photos of Myxicola specimens analysed by Bush (1905): A1, A2, M. glacialis (‘short form’); B, M. glacialis (‘long form’); C, M. conjuncta; D. radiole of M. conjuncta; E. radiole of M. glacialis. A1 and C, specimens YPM IZ 106540 and YPM IZ 002773, respectively. Photos from Bush 1905.

Figure 16. Photographs of thoracic and abdominal uncini, respectively, of American Myxicola species: A, B. M. affinis; C, D. M. glacialis; E, F. M. pacifica.