Leitoscoloplos fragilis

Leitoscoloplos fragilis (Verrill, 1873)

Figures 4–5

Anthostoma fragile Verrill, 1873: 598–599.

Scoloplos fragilis: Verrill 1881: 301; Hartman 1942: 60–61, figs. 113-115; Brown 1982: 213–227.

Scoloplos (Scoloplos) fragilis: Pettibone 1963: 290–292, fig. 76a–f.

Haploscoloplos fragilis: Hartman 1944: 340, pl. 14, fig. 5, pl. 18, fig. 6; 1945: 30, pl. 6, fig. 5; 1951: 76–78, pl. 21, figs. 1–3; 1957: 271–272, p. 25, figs. 1–3; Trott 2004: 280.

Leitoscoloplos fragilis: Taylor 1984: 1.19–1.21, fig. 1.18a–g; Mackie 1987: 15–16, fig. 16; Blake et al. 2001: Appendix, 9-1; Fauchald, Granados-Barba & Solís-Weiss 2009: 763; Blake 2017: 18.

Material examined. (1,239 specimens) Northeastern USA, Maine, Damariscotta River Estuary, Upper Dodge Cove, coll. V. Walker, 05 Aug 1966, 43°59.658′N, 69°33.255′W, intertidal (18, MCZ 161580).— Massachusetts, Woods Hole, coll. G. Sene Silva, 08 Aug 2005, intertidal (1, MCZ 161581).— Massachusetts, New Bedford Harbor, Long-term Monitoring Program, 1999 September-October Survey: (Upper Harbor): Sta. 105-1, 14 Sep 1999, 41°36.407′N, 70°53.983′W, 6.4 m (1, MCZ 161540); Sta. 108-3, 06 Oct 1999, 41°40.496′N, 70°54.955′W, 1 m (3, MCZ 161541); Sta. 111-1, 05 Oct 1999, 41°40.42′N, 70°54.9′W, 1 m (1, MCZ 161542); Sta. 120-1, 18 Nov 1999, 41°13′N, 70°55.08′W, 1 m (5, MCZ 161543); Sta. 121-3, 18 Nov 1999, 41°40.154′N, 70°55.003′W, 1.4 m (5, MCZ 161544); Sta. 125-3, 29 Sep 1999, 41°40.01′N, 70°55.098′W, 3 m (2, MCZ 161545); Sta. 130-3, 01 Oct 1999, 41°39.848′N, 70°55.086′W, 1.8 m (2, MCZ 161546); Sta. 134-1, 01 Oct 1999, 41°39.756′N, 70°55.032′W, 2.3 m (46, MCZ 161547); Sta. 135-3, 01 Oct 1999, 41°39.753′N, 70°54.936′W, 1.7 m (40, MCZ 161548); Sta. 139-1, 29 Sep 1999, 41°39.675′N, 70°55.104′W, 1.4 m (71, MCZ 161549); Sta. 140-3, 01 Oct 1999, 41°39.685′N, 70°54.977′W, 3.3 m (30, MCZ 161550); Sta. 147, 28 Sep 1999, 41°39.594′N, 70°54.91′W, 1.7 m (2, MCZ 161551); Sta. 150-3, 28 Sep 1999, 41°39.513′N, 70°55.087′W, 3.4 m (13, MCZ 161552); Sta. 154-2, 28 Sep 1999, 41°39.415′N, 70°55.050′W, 3.4 m (3, MCZ 161553). (Lower Harbor): Sta. 202-1, 06 Oct 1999, 41°39.323′N, 70°55.032′W, 4.7 m (30, MCZ 161554); Sta. 207-1, 22 Sep 1999, 41°38.984′N, 70°55.270′W, 1.7 m (147, MCZ 161555); Sta. 208-2, 23 Sep 1999, 41°38.981′N, 70°55.022′W, 1.3 m (35, MCZ 161556); Sta. 212-2, 24 Sep. 1999, 41°38,826′N, 70°4.906′W, 3.3 m (2, MCZ 161557); Sta. 216-2, 22 Sep 1999, 41°38.660′N, 70°55.023′W, 2.2 m (10, MCZ 161558); Sta. 218-1, 27 Oct 1999, 41°38.663′N, 70°54.527′W, 9.5 m (26, MCZ 161559); Sta. 220-2, 22 Sep 1999, 41°38.507′N, 70°55.141′W, 11.1 m (31, MCZ 161560); Sta. 222-1, 23 Sep 1999, 41°38.501′N, 70°54.641′W, 3.1 m (79, MCZ 161561); Sta. 224-3, 22 Sep 1999, 41°38.338′N, 70°55.269′W, 10 m (37, MCZ 161562); Sta. 226-1, 21 Sep 1999, 41°38.336′N, 70°54.809′W, 3.5 m (39, MCZ 161563); Sta. 227-3, 21 Sep 1999, 41°38.330′N, 70°54.537′W, 3 m (71, MCZ 161564); Sta. 231-1, 21 Sep 1999, 41°38.166′N, 70°54.874′W, 4.1 m (21, MCZ 161565); Sta. 235-3, 20 Sep 1999, 41°38.011′N, 70°55.035′W, 8.8 m (17, MCZ 161566); Sta. 237-1, 24 Sep 1999, 41°38.006′N, 70°54.541′W, 6 m (1, MCZ 161567); Sta. 240-3, 20 Sep 1999, 41°37.873′N, 70°54.91′W, 10 m (28, MCZ 161568); Sta. 242, 20 Sep 1999, 41°37.67′N, 70°54.416’′W, 5.8 m (9, MCZ 161569); Sta. 245-3, 19 Sep 1999, 41°37.67′N, 70°54.780′W, 3.4 m (43, MCZ 161570); Sta. 249-1, 19 Sep 1999, 41°37.52′N, 70°54.67′W, 2.4 m (17, MCZ 161571); Sta. 250- 3, 19 Sep 1999, 41°37.51′N, 70°54.42′W, 8.5 m (10, MCZ 161572). (Outer Harbor): Sta. 306-3, 14 Sep 1999, 41°37.15′N, 70°52.237′W, 2.8 m (12, MCZ 161573); Sta. 310-1, 14 Sep 1999, 41°36.407′N, 70°53.983′W, 6.4 m (1, MCZ 161574).— Connecticut, Noank, coll. J.A. Blake, 3 May 1966, 41°19.48′N, 71°59.05′W, intertidal (1, MCZ 161582); Mystic, coll. J.A. Blake, 21 Dec 1965, 41°20.8′N, 71°58.29′W, intertidal (1, MCZ 161579).— New York, Long Island Sound, off Eastchester, Sta. IG 25, coll. P.L. Neubert, 28 Oct 1999, 40°52.45′N, 73°45.15′W, 23 m (18, MCZ 161585); Sta. IG 26, 28 Oct 1999, 40°52.20′N, 73°45.66′W, 13 m (115, MCZ 161586); off Throgs Neck, Sta. TN 6A, coll. P.L. Neubert, 40°48.6848′N, 73°48.6204′W, 4 m (150, MCZ 161583); Sta. TN12A, 40°48.7949′N, 73°47.1572′W, 16 m (45, MCZ 161584).

Description. Specimen from New Bedford Harbor (NBH) (MCZ 161559) complete, 3.5 cm long, 1.0 mm wide across thorax with 145 setigers, similar to largest specimens reported by Mackie (1987) from Nantucket, Massachusetts, of 3.9 cm long, 1.9 mm wide across thorax with 150 setigers. Pettibone (1963) reports larger specimens up 15

cm long, 3 mm wide, and with up to 250 setigers. Body elongate, with thoracic segments short, dorsoventrally flattened, about 4.5 times as wide as long (Fig. 4A). Thoracic segments of largest specimens with low transverse ridge across dorsum and venter between parapodia. Anterior abdominal segments becoming longer, about 1.5 times as wide as long in anterior segments; middle and posterior abdominal segments crowded, very short. Larger specimens with shallow mid-ventral groove present from middle abdominal segments to near posterior end. Color in alcohol light tan.

Pre-setiger region triangular, elongate, as long as first two setigers (Fig. 4 A–B). Prostomium conical, tapering to pointed tip; nuchal organs curved slits on posterior lateral margin; eyespots absent (but see juvenile morphology below). Peristomium a single smooth achaetous ring dorsally, ventrally encompassing mouth with upper lip formed of two large lobes and lower lip with curved row of eight or more narrow elongate lobes (Fig. 4B). Proboscis not observed but recorded as a multilobed sac when everted (Pettibone 1963).

Thorax with 14–16 setigers (13–16 recorded by Mackie 1987) with abrupt transition to abdominal segments with elongation of neuropodia and decrease in number of neurosetae. Thoracic notopodial postsetal lobes short, triangular in shape along most of thorax (Fig. 4C), becoming longer in last 1–2 thoracic setigers (Fig. 4D); all notopodial postsetal lamellae arising directly from body wall. Thoracic neuropodial postsetal lobes short, papillate, arising from broadly rounded base (Fig. 4C); last 1–3 thoracic neuropodia with extra postsetal lobe and 1–2 short subpodial papillae resulting in last 1–3 thoracic setigers with three short lobes (Fig. 4D). Abdominal notopodia becoming longer, fingerlike in middle and posterior segments (Fig. 4 E–F). Abdominal neuropodia longer and thickened in middle and posterior segments each narrowing to long narrow apical lobe with a short subterminal lateral cirrus; extra subpodial papillae retained only on first 3–4 abdominal neuropodia (Fig. 4E), papillae then merging with subpodial flange and continuing over middle and posterior abdominal setigers as large, subpodial flange with 2–3 distinct lobes (Fig. 4F). Prominent fingerlike interramal cirrus present on last 1–2 thoracic setigers (Fig. 4D), continuing on anterior and middle abdominal setigers (Fig. 4E), reduced to low interramal mound or process in posterior neuropodia (Fig. 4F).

Branchiae from setiger 14 or posterior thoracic segment on Connecticut specimens and larger specimens from New Bedford Harbor, but recorded from abdominal segments by Pettibone (1963) and Mackie (1987); first branchiae on Connecticut and NBH specimens minute, papilliform, becoming full size by about setigers 25–27 or anterior abdominal segments. Anterior branchiae triangular, tapering to rounded apex (Fig. 4D); branchiae of middle and posterior abdominal segments becoming longer and narrow apex more pointed (Fig. 4 E–F); all branchiae with transverse folds and cilia on inner and outer margins.

Notosetae all camerated capillaries; furcate and flail setae not observed; capillaries of thoracic segments numbering about 40 in 3–4 irregular rows in thoracic notopodia, reduced to about 15–20 in abdominal segments. Thoracic neurosetae all camerated capillaries with about 50 in 3–4 rows; abdominal neurosetae including 3–7 long stiff capillaries with barbs along one edge and 1–2 imbedded aciculae, none observed protruding.

Pygidium with two thin lateral anal cirri (Fig. 5 B–C).

Variability. The adult specimens encountered here differ from the accounts by Pettibone (1963) and Mackie (1987) in having branchiae from the last one or two thoracic setigers instead of the first abdominal setiger. In addition, the interramal cirrus is typically first present on posterior thoracic segments together with extra subpodial lobes. The interramal cirri are best observed by staining with Shirlastain A; they are more-or-less limited to the last thoracic and middle abdominal setigers. The cirri are reduced to a short, rounded process on small specimens and difficult to see.

Juvenile morphology. Juveniles were present in samples from the Throgs Neck (TN) (MCZ 161583) and NBH (MCZ 161564) collections. Several of these specimens were examined in an effort to determine at what size the characters that distinguish L. fragilis from local congeners were present. However, unlike the 300-µm-mesh sieve used to collect the L. acutus juveniles from Massachusetts Bay, the sieve size used to separate the fauna from the TN and NBH sediment was 500-µm mesh. As a result, there were fewer juveniles in these samples. However, the smallest sizes observed of L. fragilis were similar to those of L. acutus.

All juveniles have an acutely pointed prostomium; body segments are crowded, especially in posterior setigers. A 28-setiger juvenile from TN is 2.1 mm long with nine thoracic setigers and branchiae from setiger 10 (Fig. 5A). Low interramal processes are present on anterior abdominal setigers; these are the anlage of interramal cirri. At this stage, the prostomium is short and triangular. A 32-setiger NBH juvenile is 2.9 mm long with 11 thoracic setigers with branchiae from setiger 12 (Fig. 5B, E). At this stage, the prostomium has elongated. The characteristic interramal cirri are observed on a few anterior abdominal setigers as low mounds; in middle and posterior neuropodia short subpodial flanges are irregular in shape due to weakly developed subpodial lobes; posterior setigers are crowded and the characteristic lobed subpodial flange is evident (Fig. 5E). A 40-setiger NBH juvenile is 3.27 mm long with 12 thoracic setigers and branchiae from setiger 13 (Fig. 5C). A few interramal cirri were observed in middle thoracic setigers (Fig. 5D) and the neuropodia of the same segments were observed to have an irregular subpodial flange due to the presence of a superior subpodial papilla and medial notch. The prostomium has become longer and more pointed. Interestingly, a pair of eyespots was observed on the prostomium of the latter specimen and others of the same size (Fig. 5F). Such eyespots were not observed on adults. The pygidium of all juveniles consisted two lobes each bearing a long anal cirrus (Fig. 5B, F). Only camerated capillaries were observed in the noto- and neuropodia of these juveniles.

Remarks. Leitoscoloplos fragilis is most similar to L. robustus with which it sometimes occurs. The bilobed subpodial flanges of abdominal segments differ from those of L. robustus, which has relatively smooth subpodial flanges (see comments below for L. robustus).

The juveniles and adults reported here indicate that the fewest number of thoracic setigers observed is nine on a 28-setiger specimen with branchiae from 10; the largest juvenile reported is a 40-setiger specimen with 12 thoracic setigers and branchiae from setiger 13. The larger adults reported here are up to 150 setigers, 12 cm long and with a thorax of 14–16 setigers and branchiae from the posteriormost thoracic setigers. The characteristic interramal cirri were present, if only rudimentary, on the smallest juveniles collected and the lobed appearance of the subpodial flange was obvious on the 40-setiger juvenile. These results indicate that the most important characters required to identify L. fragilis are evident early in development.

Biology. Brown (1982) described some aspects of the distribution and biology of L. fragilis on a tidal flat in Delaware, USA. The species lives in the upper 15 cm of sediments having sand-size particles (1.36–1.44 mean phi Ø). Salinity at the site ranged from 28–30 PSU and a wide range of water temperatures from 2.0°C in December to 28°C in August. At the study site, a wave-swept sand bar, the species occurred on slopes, ridges, and troughs with the highest densities on slopes. At least three age classes were found, with reproductive maturity occurring after two years; the smallest specimens occurred in the August or mid-summer samples. Gametogenesis and reproductive biology were not investigated; egg diameters were not provided and cocoons with eggs or larvae were not observed.

In New Bedford Harbor, Massachusetts, the highest densities of L. fragilis occurred in lower portions of the Upper Harbor and throughout the Lower Harbor at stations having high sand and gravel content of 50% or greater (Blake et al. 2001).

Distribution. Eastern Canada to Florida; Gulf of Mexico; intertidal to shallow subtidal.