Bylgides sarsi (Kinberg ex Malmgren 1866)

Bylgides sarsi (Malmgren, 1865), variety

Antinoe sarsi.—Lovén, 1863:468.

Antinoe sarsii [sic] forma balthica Malm, 1874:75 [part].

MATERIAL EXAMINED.—BALTIC SEA: Östersjön, Gotland, Sweden, one specimen of original material of Antinoe sarsii forma balthica Malm, Malmgren collection, through S. Lov´n (USNM 74313, from BMNH 1865.9.23.18).

DISTRIBUTION.—Known only from the type locality, Östersjön, Gotland, Sweden.

COMPARISONS.—The variety differs from B. sarsi mainly in having 15 pairs of elytra instead of 14 and elytra with microtubercles instead of lacking them.

Bylgides sarsi (Malmgren, 1865)

Antinoe sarsi (Kinberg, MS) Malmgren, 1865:75, pl. 9: fig. 6 [part, Baltic form]; 1867:136 [part, forma minor].

Antinoe sarsii [sic].—Malm, 1874:75 [part, forma balthica].

Harmothoe sarsi.—Eliason, 1920:20.—Sarvala, 1971:231–309 [ecology].

Harmothoe sarsi sarsi.—Meunier, 1930:16 [Baltic form].—Mulicki, 1959:163–174 [ecology, in southern Baltic].

Harmothoe (Antinoella) sarsi.—Eliason, 1962b:14.

Harmothoe (Antinoella) sarsi sarsi.—Hartmann-Schröder, 1971:62, fig. 18a-e [part].

Antinoella sarsi.—Wolff, 1973:90.

Bylgides sarsi.—Uschakov, 1982:153 (M. Pettibone, in litt.).

MATERIAL EXAMINED.—BALTIC SEA: Gotland,östersjon, near St. Anne, 20 Aug 1862, Malmgren collection, 4 syntypes of A. sarsi (NMG 34). Off South and Southeast of Sweden and Gotland, 55°N, 15°E-59°N, 19°E, 4–110 m, sand and clay bottoms, F.A. Smitt and Hj. Widegren, collectors, 1864, Malmgren collection: Ystad; Karlskrona; Visby and Kapellshamn in Gotland; Oskarshamn;Örö Island near Västervik; Västervik; Gamleby; Bråviken; islands of Arkö and Marskär, 110+ syntypes of A. sarsi Malmgren and A. sarsii forma balthica Malm (NRS 5765–5776; BMNH 1865.9.23.18; USNM 48464). Baltic Sea, Lindstroem, collector, 13 small specimens (USNM 262). The Baltic, SE of Gotska Sandön, 58°07′N, 19°59′E, 165–180 m, O. Nybelin, collector, 15 Jun 1927, 2 large specimens (USNM 43218, from A. Eliason). Western Baltic near Bomholm, in stomachs of cod, April 1987, 8 specimens (USNM 119862, from ZMUC, M.E. Petersen).

DENMARK: Kattegat: Off Frederikshavn, 8–20 m, 3 specimens (ZMUC). Off D0kkedal, off entrance to Limfjorden, 2–10 m, 12 specimens (ZMUC). Limfjorden, 14 specimens (ZMUC; USNM 119861, from M.E. Petersen). Wadden Sea: SE of Rømø, 2 specimens (ZMUC).

NETHERLANDS: Province of Zuid-Holland, Isle of Goerce-Overflakkee, in burrow of Arenicola marina, W.T. Wolff, collector, 9 Sep 1963, 2 small specimens (USNM 48465).

MEASUREMENTS.—Type Material: Larger syntypes 21–25 mm long, 11–14 mm wide with setae, with 32–33 segments and 14 pairs of elytra. Young syntypes 3–6 mm long, 2.5–3 mm wide, with 13–25 segments.

Nontype Material: Two large specimens (USNM 43218) 39–42 mm long, 20–23 mm wide with setae, with 33–34 segments and 14 pairs of elytra.

DESCRIPTION.—Body flattened, tapering slightly anteriorly and posteriorly, with parapodia and setae of each side about equal to width of body. Color somewhat variable, dorsum uniformly greenish to blackish green with lighter transverse ciliated bands, 2 per segment, extending onto dorsal tubercles and elytrophores. Smaller specimens with pigment broken up. Elytra up to 14 pairs, on segments 2, 4, 5, 7, alternate segments to 23, 26, and 29, with 4–5 posterior segments with dorsal cirri. Elytra large, rounded to oval, covering dorsum; surface slick, shiny, dotted with cylindrical papillae with slightly clavate tips and round chitinous bases; papillae also along posterior and lateral borders; without microtubercles (Figure 4J; Malmgren, 1865, pl. 9: fig. 6C,E). Dorsal cirri with cylindrical cirrophores on posterior sides of notopodia, with long styles extending beyond neurosetae, with scattered short clavate papillae (Figure 4D; Malmgren, 1865, pl. 9: fig. 6B); dorsal tubercles nodular.

Prostomium bilobed, with small rounded cephalic peaks; anterior pair of eyes anterior to greatest prostomial width, slightly larger than posterior pair; ceratophore of median antenna large, bulbous, in anterior notch; style rather short, with short clavate papillae; ceratophores of lateral antennae short, rounded, fused medially ventral to median antenna, with styles short, subulate, papillate; palps stout, tapered, very finely papillate (Figure 4A; Malmgren, 1865, pl. 9: fig. 6A). Tentaculophores each with small projecting acicular lobe and 2–4 short notosetae on inner side; tentacular cirri similar to but slightly longer than median antenna, dorsal ones slightly longer than ventral ones; without facial tubercle (Figure 4A).

Second segment without nuchal fold, with first pair of prominent elytrophores and long ventral buccal cirri lateral to ventral mouth, similar to tentacular cirri, longer than following ventral cirri; notopodia and neuropodia subequal in length; notosetae similar to notosetae of following segments; neurosetae all with fine tips (Figures 4B,C). Pharynx with 9 pairs of border papillae and 2 pairs of amber-color jaws.

Parapodia prominent, biramous, with golden setae. Notopodium shorter than neuropodium, rounded, with projecting acicular lobe on lower side; neuropodium with longer presetal conical acicular lobe with supraacicular digitiform process, and with shorter, rounded postsetal lobe (Figure 4D,E). Notosetae numerous, forming radiating bundle, much thicker than neurosetae, with numerous spinous rows, some short, slightly curved, with short bare tips and some longer, straight, with longer bare tips (Figure 4F; Malmgren, 1865, pl. 9: fig. 6Dr). Neurosetae numerous, forming fan-shape bundles. Upper neurosetae slender, with long spinous regions, tapering to capillary tips (Figure 4G; Malmgren, 1865, pl. 9: fig. 6DS); middle neurosetae stouter, with shorter spinous regions, some tapering to capillary tips and some with slightly hooked bare tips (Figure 4H: Malmgren, 1865, pl. 9: fig. 6DS′); lower neurosetae shorter, tapering to capillary tips (Figure 4I; Malmgren, 1865, pl. 9: fig. 6DS″). Ventral cirri short, subulate, with minute bulbous papillae (Figure 4D,E).

Nephridial papillae short, cylindrical, beginning on segment 5. Pygidium with anus medial to parapodia of last segment, with pair of anal cirri (on smaller specimens, anal cirri very long, up to more than length of body).

BIOLOGY.—The very exhaustive study of B. sarsi (as Harmothoe) by Sarvala (1971) furnished a great deal of information on the population structures and dynamics of this species, which he studied chiefly in the mesohaline waters from bottom and plankton samples near Tvärminne, southern Finland, in the deeper parts of the Gulf of Finland, the Aland and Bothnian Seas, and the northern Baltic. The species was found in depths of 0.3–80 m, with maximum density at 21–40 m, mostly on soft bottoms, but not dependent on any specific type of bottom. The temperature range at which B. sarsi occurred was between 1.6°C and 14°C, (maximum 17°C), with greatest biomass between 3°C and 12°C (maximum between 3°C and 6°C). High temperature restricted the occurrence of the species in shallow water due to the cold-stenothermy of the larvae. The species is euryhaline. Its abundance decreased when salinity exceeded 17°‰, with highest densities occurring in 6–7 ‰. Bylgides sarsi was able to withstand low oxygen levels and thrive on hydrogen sulfide bottoms, similar in this regard to Arenicola marina.

Bylgides sarsi has a semipelagic mode of life, showing diurnal vertical movements. The worms creep on the surface of the mud, especially on moonless nights, and rise above the bottom to midwater during the night, swimming rapidly. This semipelagic mode of life allows the species to escape poor oxygen conditions near the bottom. Bylgides sarsi is carnivorous, with more than of its food consisting of small pelagic species, mainly crustaceans. Feeding occurs mainly at night, immediately above or on the bottom, also to some extent in upper-water layers and in bottom mud. The worms also appear to be scavengers to some extent, feeding on dead planktonic organisms that sink to the bottom. Interspecific competition appears to be low. In the northern Baltic, B. sarsi was the only polychaete inhabiting the oozy bottom, where it reached its greatest abundance. There is a strong predator pressure on the species, which serves as food for many fishes.

Bylgides sarsi reaches its maximum number of segments during the third summer, in worms 20 mm long. The largest specimens caught in the study area were 25–30 mm long by 10–13 mm wide, with 32–34 setigers (=33–35 segments). The maximum length attained in the Gotland Deep was 54 mm (mean length of 34 mm).

Some B. sarsi developed gonads during the first winter, 4 months after metamorphosis, but most did not mature until the second winter. Breeding took place only once a year within a relatively restricted period. The age of worms in the study area seldom exceeded two years. Early develomental stages of trochophores, metatrochophores, and nectochaetes were found in the plankton. Larvae were found in the plankton between February and August, but were numerous only from April to June-July. Larvae were very sparse at temperatures above 6°C, and their highest densities were found near the bottom. There was no selection of substratum. The larvae metamorphosed when they reached 7 setigerous segments, and recently metamorphosed young were found in the same water layers as the larvae, often high above the bottom. Larval development averaged 1.5–2 months, metamorphosis taking place in June. The main period of settling was in late June and early July. It was estimated that 16%–19% of settled B. sarsi reached the age of one year and less than 1% survived as long as two years.

Based on a study of Mulicki (1959) in the southern Baltic in the Gulf of Gdask, Poland, B. sarsi was found in depths of 22–230 m, with greatest biomass at 40–100 m, with temperature of 1.5°C–14°C, and salinity of 7–20 ‰.

DISTRIBUTION.—Baltic Sea, found almost everywhere except in inner parts of Gulf of Finland, Gulf of Bothnia, and in shallow gulfs and bays with low salinity and high temperatures. In deeper parts of Gulf of Finland, Åland and Bothnia seas in 0.3–80 m; Swedish east coast from Åland to Karlskrona in 4–110 m; Gotland Deep in 150–180 m; southern Baltic in Gulf of Gdańsk in 22–230 m; Øresund in 12–34 m; Eastern Kattegat in 2–29 m; Kiel Bay, Germany; Netherlands on Isle of Goerce-Overflakkee, with Arenicola marina (Sarvala, 1971).