Myxicola affinis 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 : 1408-1415

Myxicola affinis Bush, 1905

( Figures. 7–10 )

Myxicola affinis Bush, 1905: p. 218 , pl. XXXVIII, figs. 17–20 Myxicola monacis Chamberlin, 1919: p. 20

Material examined

Myxicola affinis : Holotype, collected by W. R. Coe in 1901, Pacific Ocean , North America, California ( USA); Monterey County, Pacific Grove. Specimen preserved in 70% ethanol; YPM IZ 002772 .AN; Myxicola monacis : Holotype, deposited by R. V. Chamberlin 1919, Pacific Ocean , California ( USA), Laguna Beach. Specimen fixed in 4% formaldehyde and preserved in 70% ethanol; MCZ: IZ: ANNb-2170 .

Redescription (YPM IZ 002772.AN)

Specimen complete, body beige to yellow, a ‘[…] decided yellow […]’ colouration is reported in the original description ( Bush 1905: p. 218), cylindrical. Crown slightly darker ( Figure 7A ); a slightly greenish colouration is reported at the original description ( Bush 1905). Body 4 cm long, 8 thoracic and 50 abdominal chaetigers; maximum width 5 mm; thorax longer than wide. Crown 1.2 cm long, holding 20 and 21 radioles on left and right lobe, respectively; ratio body/crown length 3.33. Peristomium 2.44 mm long (distinctly longer than thoracic segments), anterior and posterior peristomial rings indistinctly separated; anterior peristomial ring distinctly lighter in colour ( Figure 7A–C ). Mid-ventral lobe of anterior peristomial ring with thin membranous lateral edges and rounded, elongated tip; peristomium lower laterally, with a narrow mid-lateral incision; junction between crown and body not visible ( Figure 7B, C ). Dorsal lips with brownish long filiform radiolar appendages, with thin longitudinal groove and tapering, distally rounded tips ( Figure 7D ); pinnular appendages and ventral radiolar appendages both absent; ventral lips high and tubular, distally enlarged, distinctly yellowish colour on inner side, extending dorsoventrally along inner surface of base of radiolar lobes and connecting ventrally to the radiolar lobes; parallel lamellae and ventral sacs both absent ( Figure 7D ); outer radioles surface translucent, connected by high semi-transparent palmate membrane; radiolar tips <1/5 of total radioles length, same colour as radioles, lanceolate, distally tapered, with blunt tips ( Figure 7E ); blunt pinnules, up to <1/3 of total radioles length, same colour as radioles, in alternating arrangement along radiole; longest pinnules in medial position, gradually shortening to radiolar tips; radiolar skeleton with 8 vacuolated skeletal cells, roughly squared, disposed in two lateral groups of 4 cells surrounded by hyaline matrix; graft cells of pinnular skeleton small, undivided, ventrally contiguous to the latter ( Figure 7G ). Radiolar eyes and peristomial eyes not visible. Glandular girdle near the posterior margin of chaetiger 2, hardly visible. Lateral eyes present from chaetiger 4; thoracic eyespots posterior to chaetal tufts and tori as single eyespots, shifting to more dorsal position in the abdomen and increasing in number (up to 2–3 units each side of chaetigers). Pygidium flattened; pygidial eyes not visible ( Figure 7F ). Numerous narrowly hooded thoracic notochaetae with distinctly narrow wing extending from hood along the shaft, arranged circularly in tufts, decreasing in number of chaetae per fascicle along thorax; peristomial notochaetae slightly longer ( Figure 9A, B ). Chaetigers 2–8 with curved acicular uncini, main fang surmounted by 3–4 small teeth shorter than half main fang length ( Figures 9D, E , 16A ); frontal tooth distinctly longer than others, in apical position in frontal view ( Figure 9F ); thoracic tori inconspicuous. Numerous narrowly hooded abdominal neurochaetae, similar to thoracic notochaetae, forming less conspicuous tufts, which significantly decrease in number of chaetae per fascicle through abdomen ( Figure 9C ). Abdominal uncini with apical tooth slightly longer than half main fang length ( Figure 9G, H ), distinct notch on apical tooth underlines the presence of an additional tooth, attached to this latter, of same length ( Figure 9I ); breast rounded; main fang slightly longer than breast; handle vestigial; uncini as long as high ( Figures 9F, G , 16B ).

Remarks

The holotype of M. monacis (MCZ: IZ: ANNb-2170) is complete but had evidently been desiccated and rehydrated at some point. Body grey-yellowish; sub-cylindrical, flattened dorso-ventrally; crown clearer in colour ( Figure 8A ). Body 3.6 cm long; 8 thoracic chaetiger and 65 abdominal segments; maximum width 3.5 mm; thorax longer than wide. Crown, with 22 pairs of radioles. Peristomium 1.3 mm long; anterior and posterior peristomial ring indistinctly separated; mid-ventral lobe filiform and rounded ( Figure 8B ); peristomium low, with narrow mid-lateral incision; junction between body and crown visible all along peristomium ( Figure 8C ). Dorsal lips with long filiform radiolar appendages; pinnular appendages and ventral radiolar appendages both absent; ventral lips high and tubular, distally enlarged, extending dorsoventrally along inner surface of base of radiolar lobes, connecting ventrally to the radiolar lobes; parallel lamellae and ventral sacs both absent ( Figure 8D ). Radioles connected by high semi-transparent palmate membrane. Radiolar tips tongue shaped ( Figure 8E, F ). Radiolar and peristomial eyes not visible. Radiolar skeleton not visible. Glandular girdle on posterior margin of chaetiger 2. Lateral eyes, thoracic eyespots posterior to chaetal tufts and tori as double eyespots, shifting dorsally and increasing in number in the abdomen (up to 3 units on each side of posterior abdomen). Pygidium flattened; pygidial eyes dark, numerous laterally ( Figure 8G ). Numerous broadly hooded thoracic notochaetae, arranged circularly in tufts ( Figure 10A ). Peristomial notochaetae, thoracic notochaetae and thoracic uncini not available. Thoracic tori inconspicuous. Abdominal neurochaetae narrowly hooded, with tapering tips. Abdominal uncini with apical tooth slightly longer than half main fang length ( Figure 10B, C ), distinct notch on apical tooth underlines the presence of an additional tooth, attached to this latter, of same length or shorter ( Figure 10D, E ); breast rounded; main fang slightly longer than breast; handle vestigial; uncini as long as high ( Figure 10B, C ).

These specimens were examined to check the validity of M. affinis Bush, 1905 and M. monacis Chamberlin, 1919 . Our findings concerning M. affinis are consistent with the original description by Bush (1905): the differences refer to morphology of radiolar tips, reported as ‘[…] long, slender and tapered […]’ ( Bush 1905: p. 218) and measuring 3 mm, and the length of the body and of the peristomium, both measuring 4.5 mm; both these latter can be explained by a double typing error, while the measure of the radiolar tips remains unexplained.Illustrations from the original description are consistent with the observed characters; the only difference regards the abdominal uncini, where one of them shows a main fang clearly shorter than breast and both do not show the distinctive notch on the apical tooth, nor the additional tooth above it ( Bush 1905, fig. 14O1, O2).However, it should be noted that even with today’s instruments these characters are barely visible.

A comparison with the original description of M. monacis ( Chamberlin, 1919) is impossible, as that description was structured as a point-by-point comparison with M. pacifica and does not describe the majority of characters nowadays considered informative. However, biometry and presence of a ‘[…] ventral median process from the first segment drawn out into a slender entire tip […]’, probably referring to lips and radiolar appendages, are consistent with the examined material.

The holotypes of M. affinis and M. monacis show important similarities. In addition to a similar general appearance, biometry and proximity of type localities, the two specimens show almost identical morphology of lips and radiolar appendages, which is rather peculiar among North American Myxicola species ( Figures 7D and 8D ), as well as the overall structure of mid-ventral lobe and peristomium, even though they are damaged in the second specimen ( Figure 8B ). Abdominal uncini also show similar morphology in those specimens, especially regarding the presence of a distinctive notch on the apical tooth, unveiling the presence of a secondary tooth, attached to the apical tooth ( Figures 9G–I and 10B–E ). Differences can be found in the number of thoracic eyespots (2 in M. monacis and 1 in M. affinis ) and in the presence of a more evident basal posterior prolongation of the abdominal uncini in M. affinis ( Figure 9G, H ). However, this could be due to compression by the cover slide as the original slide from Chamberlin is of a remarkable thickness, making it difficult to focus on the different elements. Despite an apparently good state of preservation, it was impossible to obtain information from new slides from M. monacis , forcing us to rely only on the original description, which does not include thoracic uncini or chaetae.

The lack of information about the morphology of thoracic uncini and radiolar structures prevents the possibility to obtain a full comparison between the specimens analysed. As result, the only clear difference between M. affinis and M. monacis concerns the number of eyespots, while a whole set of diagnostic characters is fully consistent between the two specimens. As consequence, at present we suggest that these two taxa should be regarded as synonymous (see Supplementary Material for the redescription of specimen MCZ: IZ: ANNb-2170).

Both M. affinis and M. monacis were synonymised with M. infundibulum by Hartman (1942), who did not examine the type material. However, this species can be differentiated from M. infundibulum based on pigmentation, morphology of lips and associated structures, radiolar skeleton, radiolar tip, mid-ventral lobe and morphology of both thoracic and abdominal uncini. The same characters distinguish M. affinis from M. polychroma , except for pigmentation and shape of radiolar tips.

Myxicola affinis is a small species of the ‘ infundibulum group’ (8 thoracic chaetigers), with a low number of abdominal chaetigers (<70) ( Putignano et al. 2023). This species has several similarities to M. mikacae Putignano, Gravili and Giangrande, 2023 , M. violacea Langerhans, 1884 , M. sulcata Ehlers, 1912 , M. fauveli Potts, 1928 and M. bushae sp. n. However, M. mikacae differs from M. affinis in having club-like radiolar appendages and radiolar skeleton composed of a single row of cells, and M. fauveli by the complex hyaline radiolar structures in subterminal position. Myxicola sulcata differs in the shape of radiolar appendages and thoracic uncini (showing a longer main fang and only one apical tooth in M. sulcata ) and abdominal uncini, showing a ‘minute handle’ ( Tovar-Hernández et al. 2017) and lacking the second, attached apical tooth, while M. violacea differs mainly by the presence of peristomial eyes and in the number of radiolar skeleton cells (one, in lateral view). Lastly, M. bushae sp. n. differs by the radiolar skeleton, abdominal uncini (which lack the second, attached apical tooth) and the morphology of radiolar appendages.

Distribution and ecology

Pacific Grove and Laguna Beach. The only information concerning its ecology is reported in the environmental notes of Chamberlin’s sample: ‘Taken on holdfasts of seaweeds’.

Figures

Figure 7. Myxicola affinis (YPM IZ 002772.AN). A. entire worm; B, C. peristomial rings, ventral and lateral views, respectively; D. complex of ventral and dorsal lips; E. radiolar tip; F. pygidium; G. scheme of radiolar cross section.

Figure 8. Myxicola affinis (as M. monacis in Chamberlin 1919; MCZ: IZ: ANNb-2170). A. entire worm; B, C. peristomial ring, ventral and lateral views, respectively; D. complex of ventral and dorsal lips; E, F. radiolar tips; G. pygidium.

Figure 9. Myxicola affinis (YPM IZ 002772.AN). A. peristomium notochaeta; B. 4th chaetiger thoracic notochaeta; C. 24th chaetiger abdominal neurochaeta; D–F. 4th chaetiger thoracic uncinus, lateral (2) and front views, respectively; G–I. 24th chaetiger abdominal uncini. Scale bars: 0.02 mm.

Figure 10. Myxicola affinis (as M. monacis in Chamberlin 1919; MCZ: IZ: ANNb-2170). A. abdominal neurochaeta; B–E. abdominal uncini. Scale bars: 0.02 mm.

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.