Laonome xeprovala, Bick & Bastrop & Kotta & Meyer & Syomin, 2018

Bick, Andreas, Bastrop, Ralf, Kotta, Jonne, Meyer, Maria & Syomin, Vitaly, 2018, Description of a new species of Sabellidae (Polychaeta, Annelida) from fresh and brackish waters in Europe, with some remarks on the branchial crown of Laonome, Zootaxa 4483 (2), pp. 349-364 : 352-358

Laonome xeprovala sp. nov. Bick & Bastrop, 2018

Holotype. Baltic Sea , Estonia, Pärnu Bay, September 2012, depth 6.2 m, 3.0 psu ( ZSRO-P2283 ).

Paratypes. Baltic Sea , Estonia, Pärnu Bay, September 2012, depth 6.2 m, 3.0 psu: 1 specimen ( ZSRO-P2245 ) ; 6 specimens, including 1 slide (ZSRO-P2246); 3 specimens, EtOH fixed material (ZSRO-P2247); 4 specimens (ZSRO-P2248).

Additional material. Laonome xeprovala sp. nov.: The Netherlands: Hollandse Ijssel , 51°56ʹ41.28ʺ N, 4°38ʹ12.84ʺE, 25.09.2012, 1 specimen ( ZSRO-P2281 ) ; Van Harinxmakanaal 53°11ʹ24.50ʺ N, 5°38ʹ31.94ʺE, 25.08.2014, depth 0.5 m,> 10 specimens ( ZSRO-P2414 ) ;>20 specimens (ZSRO-P2415). Sea of Azov , Taganrog Bay: 47°07ʹ9.5ʺ N, 39°09ʹ23.4ʺE, depth 2.7 m, 1.9 psu, 9 specimens ( ZSRO-P2475 ) ; 47°11ʹ30.84ʺN, 38°54ʹ34.2ʺE, depth 3.1 m, 3.3 psu, 11 specimens (ZSRO-P2476). Baltic Sea : Kiel ship canal, 54°20ʹ09ʺN, 9°44ʹ22ʺE, 2016, 13 specimens ( ZSRO-P2490 ) ; Sweden, Hallsfjärden 59°15ʹN 17°67ʹE, 3 specimens ( ZSRO-P2492 ) .

Comparative material. Laonome calida Capa, 2007 : Australia: Queensland, Gladstone, Calliope River , 23°51ʹS, 151°10ʹE, 1974: 1 specimen (AM W.31327, holotype) ; 23 specimens (AM W.31328, paratypes).

Laonome triangularis Hutchings & Murray, 1984 : Queensland, Gladstone, Calliope River, 23°51ʹ S, 151°10ʹE, 1974, 1 specimen (out of AM W199301, determined as L. calida ).

Laonome kroyeri Malmgren, 1866 : Baltic Sea, 54°01ʹ24.6ʺN, 11°08ʹ19.2ʹE, 0 2.09.1997, depth 13.0 m, 9 specimens (ZSRO-P400).

Diagnosis. Eight thoracic chaetigers (exceptionally six chaetigers). Up to eight pairs of radioles; radiolar skeleton composed of two interdigitated cell rows. Dorsal radiolar appendages absent. Dorsal pinnular appendages, ventral radiolar appendages, and parallel lamella present. Dorsal lips fused, forming a transverse lamellar process dorsal to the mouth. Ventral lips fused with one another and with the parallel lamella. Low palmate membrane present. First notopodium with narrowly hooded chaetae, sometimes with acicular spines and transitional chaetae; medium and posterior thoracic notopodia with superior narrowly hooded chaetae and one row of inferior paleate chaetae. Thoracic neuropodia with uncini and companion chaetae. Abdomen with broadly hooded neurochaetae and avicular uncini in notopodia.

Description. Holotype a complete specimen with 8 thoracic and 42 abdominal chaetigers, about 15 mm long, including a 4 mm branchial crown, and 0.5 mm wide. Other complete specimens between 4 and 17 mm long, and 0.5 to 0.7 mm wide; 8 thoracic and 35 to 45 abdominal chaetigers (only 2 out of 60 specimens with 6 thoracic chaetigers). Branchial crown is at maximum one-fourth to one-fifth the total length of the body, but usually less. Six to eight pairs of radioles, juveniles with only four pairs ( Figs 2A , 3A–D ); usually about one-tenth of the radiolar length without pinnules ( Figs 2A , 4B ); radioles quadrangular in cross-section, flanges absent ( Fig. 3A–D ). Radiolar skeleton composed of two interdigitated cell rows; transverse bar of branchial skeleton composed of about three cell rows ( Figs 3H , 5A,B ). Pinnules arranged in two alternating rows, diminishing in size distally ( Figs 2A , 4B ). Palmate membrane present, but only about 100 to 120 µm high ( Fig. 3A ). Dorsal lips low and rounded; ventral edges fused basally with one another, forming a transverse lamellar process ( Fig. 3A–G ). Dorsal radiolar appendages absent. Dorsal pinnular appendages present ( Fig. 3A–D ). Ventral lips shorter than dorsal lips, fused with one another and with the parallel lamella ( Fig. 3A–E ). Ventral radiolar appendages present ( Fig. 3A,B ).

Anterior peristomial ring low and even in height. Posterior peristomial ring collar present, with dorsal margins fused to faecal groove ( Fig. 6A ); midventrally slightly incised, bounded by low triangular lobes with rounded distal ends; ventral collar margin higher than lateral and dorsal margins; ciliary patch composed of the outer side of the parallel lamella on the posterior peristomial ring extending entire mid-ventral collar length ( Figs 4A , 6B ). Transverse ridge at the junction of the posterior peristomial ring and the first chaetiger present (best visible as a narrow white band after methyl green staining, see below) ( Figs 6B , 7A,B ).

Thorax distinctly longer than wide but each thoracic chaetiger wider than long ( Fig. 7A–C ). First notopodium with about 5–7 narrowly hooded chaetae; sometimes with 1 or 2 brass-colored, broadly hooded and 2 or 3 transitional chaetae (the thicknesses of these chaetae fall between those of the broadly and narrowly hooded chaetae) ( Figs 4C , 6D ), but these chaetae frequently broken off; without inferior paleate chaetae. Notopodium on chaetiger 2 with 3–5 narrowly hooded chaetae superiorly (sometimes with 1 additional brass-colored, broadly hooded and 1 or 2 transitional chaetae), and 5–6 inferior paleate chaetae arranged in 1 row ( Figs 4D , 6E ). Notopodium on chaetiger 3 with 3–5 narrowly hooded chaetae superiorly (sometimes with 1 transitional chaeta), and 1 row of 5–8 paleate chaetae inferiorly ( Fig. 4E ). Notopodia of chaetiger 4 to 8 with 3 to 7 superior narrowly hooded chaetae and 5–7 inferior paleate chaetae (a maximum of 8 on the largest and 3–4 on the smallest specimens) arranged in 1 row ( Fig. 4F ). About 14–23 thoracic neuropodial uncini (number size-dependent, the smallest specimens with only 3 uncini), with several rows of teeth above main fang, about 3–5 in profile and 4–6 per row; breast well developed, without a handle but with a small knob usually visible ( Figs 4G,H , 6G ). Companion chaetae present, with membranous, tear drop-shaped distal ends situated perpendicular to slender shafts; membrane with small denticles on less than one-third of its surface ( Figs 4G , 6F ).

Abdomen with about 5–8 broadly hooded neurochaetae arranged in a single row (smallest specimens with 2–3 neurochaetae) ( Fig. 4I ). About 10–22 abdominal notopodial uncini, with several rows of teeth above the main fang, about 3–5 in profile and 4–6 per row; breast well developed, without a handle, sometimes with a small knob visible ( Figs 4L , 6H ). Posterior end decreasing in width; ventral longitudinal groove on about the last 10–15 chaetigers ( Figs 4K , 6C ).

Pygidium triangular with conical papilla, without pygidial cirrus; pygidial eyes absent ( Figs 4K , 6C ).

Wall of tube thin and smooth; consisting of mucus and fine sediment particles.

Pigmentation. Living specimens yellowish-green with darker (green or brown) radiolar skeleton or blood vessel; radioles and pinnules crossed by a series of about 6–8 brown bands ( Fig. 2A,B ). Body pale in fixed specimens but brown bands on branchial crown remain unchanged.

Methyl green staining pattern. Ventral and lateral parts of thorax and abdomen intensively stained, except in regions around the noto- and neuropodia, around the ciliary patch on the posterior peristomial ring and the junction of the posterior peristomial ring and the first chaetiger ( Fig. 7A,B ). Dorsal part of thorax and abdomen weakly stained but with scattered dark blue dots ( Fig. 7C ).

Etymology. The species name refers to the invasion of this species from out of nowhere into different fresh and brackish waters in Europe ( xeprovala is Greek, like the genus name, and means “sudden appearance”).

Geographic distribution. Known from the Baltic and North Seas and their catchment areas: Pärnu Bay (type locality), Vistula Lagoon, Kiel Canal, Swedish, Finnish and Latvian coastal waters, Odra River (fresh water), and fresh and brackish waters in The Netherlands and Elbe estuary. Sea of Asov, Taganrog and Temtyuk Bays, Don River (about 150 km upstream). Invasive species (with yet unknown origin).

Ecology. Laonome xeprovala sp. nov. is obviously a euryoecious fresh and brackish water species. It was collected in fresh water (Odra and Don Rivers), low salinity areas (from 0.5 to about 7 psu), on hard bottoms (Van Harinxmakanaal, Pärnu Bay), mixed sediments (Taganrog Bay), sandy silt with detritus (Hallsfjärden), clean sand (Don River) and soft clay rich in organic material and decomposing microalgae (Pärnu Bay).

Laonome xeprovala sp. nov. has been recorded in shallow depths from 0.5 to 12 m. This species is able to survive very cold winters when ice cover lasts more than 4 months and seawater temperatures remain below 2 °C ( Kotta et al. 2015). Eggs of about 135 µm in diameter were observed in the anterior abdominal chaetigers (from chaetiger 9) in 3 specimens collected in August in Hallsfjärden, Sweden. Planktonic larvae of Laonome xeprovala sp. nov. were observed in the Baltic Sea in April and May ( Kotta et al. 2015). Specimens with eggs were found in the Taganrog Bay and in the Don River from August to September. In the same sampling event 5 to 12 juveniles were found in the tubes of adults which suggest that breeding obviously starts earlier in fresh water environments, and planktonic development could be replaced by incubating eggs.

Remarks. Laonome xeprovala sp. nov. is similar to L. albicingillum Hsieh, 1995 , L. calida Capa, 2007 , and two other undescribed species from California and China ( Capa 2007; Fitzhugh 2002). These species possess inferior thoracic paleate chaetae on chaetigers 2–8, thoracic companion chaetae as well as uncini with several rows of apical teeth above the main fang, and dorsal radiolar appendages are lacking. However, none of these species possess brass-colored, broadly hooded chaetae on the first thoracic chaetiger. Unfortunately, these chaetae are not always clearly visible, and they break off easily. Laonome xeprovala sp. nov. also differs from L. albicingillum in having up to 23 thoracic uncini per torus (but usually less), and up to 7 or 8 paleate notochaetae instead of 38–79 thoracic uncini and 14–17 paleate chaetae, respectively.

However, it is more difficult to distinguish between L. xeprovala sp. nov. and L. calida from Australia. Both species are about the same length, 15 mm and 16 mm, respectively, but L. calida has many more chaetigers. The holotype of L. calida has 6 thoracic and 87 abdominal chaetigers (not 5 and 92 as stated in the original description), and the paleate chaetae are arranged in 2 rows. None of the specimens of L. xeprovala sp. nov. has more than 45 abdominal chaetigers. There are usually 8 thoracic segments (58 of 60 specimens), and the paleate chaetae are always arranged in a single row. Specimens collected off the Swedish coast and having 35 abdominal chaetigers were ovigerous, indicating that they were already mature adults. Capa et al. (2014) compared specimens identified as L. calida from The Netherlands with L. calida from Australia. They found differences in the number of rows of thoracic paleate chaetae, and concluded to adapt the description by noting that L. calida may have one or two rows of paleate chaetae. They also mentioned other differences between specimens from The Netherlands and Australia, e.g., the number of pigmented bands of the branchial crown, and did not rule out the possibility that there could be two different species with an almost identical morphology.

Examination of the type material of L. calida from Australia revealed an interesting result. The type material contains two morphologically distinct groups of specimens without any intermediate individuals (see Fig. 8 ). The axes in this diagram correspond to the directions of the greatest variability within the dataset and explain 74.5% of the variability in our dataset. The holotype and 7 of the 23 paratypes (3 of these are shown here) have paleate chaetae in two rows and, if complete, between 83 and 105 abdominal chaetigers. The body length without the branchial crown of these specimens is between 12 and 14 mm; only one specimen was 22 mm long. The remaining paratypes (14 are shown here) belong to the second morphologically distinct group. They have only one row of paleate chaetae, 32–42 abdominal chaetigers, and a body length without the branchial crown between 5 and 10 mm. These specimens are similar to L. xeprovala sp. nov. ( Fig. 8 ). Since specimens of L. xeprovala sp. nov. with 35 abdominal chaetigers were ovigerous, we assumed that specimens with this many abdominal chaetigers were adults. This observation and the comparable sizes of the individuals despite a smaller number of chaetigers exclude them from being juveniles of L. calida . Whether the smaller L. calida specimens within the paratype series are juveniles of L. calida or belong to another species requires further examination.

Intraspecific variations in L. xeprovala sp. nov. involve the number of radioles, abdominal chaetigers, paleate notochaetae and uncini. These characters seem to be size-dependent. Smaller specimens (fewer than 40 thoracic chaetigers) have 6 pairs of radioles (8 pairs in larger specimens), 4–5 paleate chaetae (rather than 7 or 8) and 10–14 uncini (rather than 20–22). Similar relationships have already been described for L. albicingillum and other sabellids (e.g., Bick & Randel 2005, Hsieh 1995).

Figures

FIGURE 2. Laonome xeprovala sp. nov. Photographs from specimens collected in the Taganrog Bay, Sea of Azov. (A) Anterior end; (B) Complete specimen

FIGURE 3. Laonome xeprovala sp. nov. Semithin section (1 µm) of the branchial crown. (A) Seven pairs of separated radioles but connected by palmate membrane, with dorsal and ventral lips, dorsal pinnular appendages and ventral radiolar appendages visible; (B) Dorsal pinnular appendages fused with dorsalmost radioles; parallel lamella visible (14 µm basal of section A); (C) Left and right part of the ventral lip as well as the parallel lamella fused with one another (5 µm basal of section B); (D) Most radioles fused with one another; parallel lamella broadened (23 µm basal of section C); (E) Right part of dorsal lip fused with basal part of dorsalmost radiolus (22 µm basal of section D); (F) Right and left parts of dorsal lip fused with one another; parallel lamella disconnected in the middle (21 µm basal of section E); (G) Branchial crown fused with the collar ventrally; skeletal cell bars of the left and the right radiolar lobes distinctly separated (19 µm basal of section F); (H) Collar encloses branchial crown completely; mouth opening and dorsal pit visible; common stem of skeletal cells of the left and the right radiolar lobes (45 µm basal of section G); c collar, d dorsal, dl dorsal lip, dp dorsal porus, dpa dorsal pinnular appendage, m mouth opening, p pinnula, pl parallel lamella, pm palmate membrane, ra radiolus, tvp transverse lamellar process, v ventral, vl ventral lip, vra ventral radiolar appendage. Scale bars: A–H = 100 µm.

FIGURE 4. Laonome xeprovala sp. nov. SEM micrographs. (A) Anterior end, lateral view; (B) Distal end of radiolus; (C) First notopodium with one acicular spine and two transitional chaetae; (D) Second notopodium: superior with one acicular spine, one transitional chaeta, one capillary chaeta, and inferior with five paleate chaetae; (E) Third notopodium: superior with one transitional chaeta, three capillary chaetae, and inferior with five paleate chaetae; (F) Fourth notopodium: with four superior capillary chaetae and six inferior paleate chaetae; (G) Thoracic uncini and companion chaetae of chaetiger five; (H) Thoracic uncini of chaetiger six; (I) Transition between thorax and abdomen; (K) Posterior end, lateral view; (L) Abdominal uncini. Scale bars: A, K = 200 µm, B, I = 100 µm, C–F, L = 20 µm, G = 10 µm, H = 5 µm.

FIGURE 5. Laonome xeprovala sp. nov. Semithin section (1 µm) of the radioles. (A) Cross-section of two separated radioles with skeletal cells and blood vessel; (B) Longitudinal section of a radiolus with skeletal cells and blood vessel; bv blood vessel, scp skeletal cells of pinnules, scr skeletal cells of radioles, ss supporting sheath tissue. Scale bars: A, B = 50 µm.

FIGURE 6. Laonome xeprovala sp. nov. (A) Anterior end, dorsolateral view; (B) Anterior end, ventral view; (C) Posterior end, lateral view; (D) Notopodial spines of the first chaetiger; (E) Inferior paleate chaetae of thoracic notopodia; (F) Companion chaeta of thoracic neuropodial; (G) Thoracic uncini; (H) Abdominal uncini. Scale bars: A, B = 200 µm, C = 100 µm, D = 20 µm, E, G, H = 10 µm, F = 5 µm.

FIGURE 7. Laonome spp. Methyl green staining pattern of peristomium and thoracic region. A–C. L. xeprovala sp. nov. (A) Ventral view; (B) Lateral view; (C) Dorsal view; D–F. L. calida. (D) Ventral view; (E) Lateral view; (F) Dorsal view; G–I. L. kroyeri. (G) Ventral view; (H) Lateral view; (I) Dorsal view. Scale bars: A–F = 500 µm, G–I = 1000 µm.

FIGURE 8. Correspondence analysis of the character distribution similarities between specimens of the type series of L. calida and specimens of L. xeprovala sp. nov. from different European localities; holotypes indicated by red frame (for characters, see “Materials and methods”).