Arabella aracaensis, Steiner & Amaral, 2009

Steiner, Tatiana M. & Amaral, Antonia Cecília Z., 2009, Arabella aracaensis, a new species with growth rings on its mandibles, and some remarks on the endoparasitic Labrorostratus prolificus (Polychaeta: Oenonidae) from southeast Brazil, Journal of Natural History 43 (41 - 42), pp. 2537-2551 : 2545-2549

Arabella aracaensis sp. n.

( Figures 4–6 )

Material examined

Holotype: ZUEC-POL-103, Araçá Beach , São Sebastião Channel, State of São Paulo, Brazil, 45°24′28″W; 23°48′55″S, 3 December 2002, intertidal, mud, coll. Tatiana M. Steiner.

Paratype series: ZUEC-POL-2176 (1 specimen), Araçá Beach , São Sebastião Channel, State of São Paulo, Brazil, 45°14–30′W; 23°41–53′S, 3 December 2002, intertidal, mud, coll. Tatiana M. Steiner ; ZUEC-POL-2175 (1 specimen), Araçá Beach , São Sebastião Channel, State of São Paulo, Brazil, 45°14–30′W; 23°41–53′S, 3 December 2002, intertidal, mud, coll. Tatiana M. Steiner ; ZUEC-POL-2176 (1 specimen), Araçá Beach , São Sebastião Channel, State of São Paulo, Brazil, 45°14–30′W; 23°41–53′S, 3 December 2002, intertidal, mud, coll. Tatiana M. Steiner ; ZUEC-POL- 2179 (1 specimen), Araçá Beach , São Sebastião Channel, State of São Paulo, Brazil, 45°14–30′W; 23°41–53′S, 21 January 1997, intertidal, coll. A. Cecília Z. Amaral ; ZUEC-POL-2178 (1 specimen), Araçá Beach , São Sebastião Channel, State of São Paulo, Brazil, 45°14–30′W; 23°41–53′S, 25 October 1996, intertidal, coll. A. Cecília Z. Amaral.

Material used for SEM: from ZUEC-POL-2178 (parapodia 40 to 47, 70 to 76). Additional material: 2 slides from ZUEC-POL-2178 (ZUEC-POL-2178A1 – parapodia 6, 25, 62, 80, 115, 146; ZUEC-POL-2178A2 – jaws); 2 slides from ZUEC-POL-2179 (ZUEC-POL-2179A1 – parapodia 7, 21, 30; ZUEC-POL-2179A2 – jaws). SEM: parapodia 2, 7 and 50.

Description

Holotype 23 mm long, 0.8 mm wide, 90 setigers, with one posterior fragment, with pygidium, 18 mm long, 80 setigers. Smallest specimen is incomplete, 5 mm long, 0.6 mm wide, 19 setigers (ZUEC-POL-ST2179). Largest specimen is incomplete, 17.5 mm long, 1 mm wide, 55 setigers (ZUEC-POL-2175). One almost complete specimen, 56 mm long, 0.8 mm wide, 166 setigers (ZUEC-POL-2178). Median and posterior segments with a small median brown band, more conspicuous dorsally. Width uniform along the body, narrowing near pygidial region. Prostomium conical, inflated, not compressed dorsoventrally ( Figure 4A–C ), slightly wider than long, without pigmentation. Two pairs of eyes, arranged in line at base of prostomium; inner pair smaller and located superficially in the tissue. Peristomial rings slightly wider than base of prostomium, both equal in length and shorter than the following setigers. Mandibles joined by short ligament; distal plates with concentric growth rings ( Figure 4F ). Paired maxillary support four times longer than MI ( Figure 4D ); unpaired support wide at the base, tapering distally, two-thirds length of paired support. MI slightly asymmetrical ( Figure 4E ), distally falcate, with six to eight basal teeth; left MII with seven to eight teeth, right MII longer than left one and with about ten teeth plus one additional tooth on anterior margin ( Figure 4D , arrow); MIII and MIV with an additional broad dorsal chitinous plate ( Figure 4D , cp); anterior tooth of MII to MIV longer; next tooth shorter than following ones, except left MII where teeth are same length. Anterior tooth of MII to MIV extends laterally, through a ridge ( Figure 4D , rd) that ends in another tooth of same length ( Figure 4D , at). First two to three parapodia smallest, next parapodia increasing in size gradually along anterior region, and maintaining size in median and posterior regions. First setigers with short, truncate pre-setal lobule ( Figure 5A ); following setigers with rounded pre-setal lobule ( Figure 5B–D ). Post-setal lobule always conical and twice as long as pre-setal. Last parapodia slightly shorter. Notopodium represented by small papilla with one to two short slender inner aciculae. Simple geniculate limbate setae with one serrated blade, divided into four groups, 1 and 2 supra-acicular, 3 and 4 subacicular: (1) 1 superior, longer than others, and blade finely serrated ( Figure 6A, E, N ); (2) 1–2 mediodorsal ( Figure 6B, F, G, M, N ); (3) 1–2 medioventral ( Figure 6C, H, I, M ), shorter than mediodorsal and distal end more sloped. The slope is remarkable in the mediodorsal, which has a coarsely serrated blade ( Figure 6Q ); (4) 1, rarely 2, modified ventral setae ( Figure 6D, J–L, M, O, P ), with blade finely serrated, as wide as other setae of bundle. Setae of anterior parapodia slightly longer than rest of body, with narrower blades ( Figure 6A–D ). Acicula amber-coloured ( Figure 6V ), about three per parapodium, emerging slightly beyond the lobule. Narrow conical pygidium, as long as 2–3 last setigers ( Figure 4G ); one pair of digitiform lateral pygidial cirri, as long as the pygidium.

Remarks

The shape of the prostomium ranged from triangular ( Figure 4A ) to conical ( Figure 4B ). The colour of eyes in fixed animals ranged from black to light brown.

The jaws analyzed belong to specimens ZUEC-POL-2178, ZUEC-POL-2175 and ZUEC-POL-2176, and the smallest specimen ZUEC-POL-2179. The general morphology of the jaws is the same in all specimens. The asymmetry of MI is related to the size, with the left MI slightly larger than the right MI, except in ZUEC-POL-2176, in which the inverse occurs. Similarly, all specimens have a large right MII, except in ZUEC-POL-2176 where left MII is larger than right MII. In ZUEC-POL-2178 the asymmetry of MI is not easily perceptible.

The anterior tooth of the large right MII, as well as the MIII, MIV and MV, is large, easily visible, and is a character observed in all specimens. The only maxilla that does not have this large tooth is the short left MII, where all the teeth have similar lengths.

In the small specimen ZUEC-POL-2179 some differences were observed. The demarcation between the two peristomial rings is less apparent dorsally. Both lobules are slightly larger and robust ( Figure 5E ); the notopodial papilla has only one slender aciculum, and the neuropodium has one or two aciculae. The number of setae is about four per parapodium, and the blades have a short limb ( Figure 6R–U ). The unpaired support of MI was not observed. The asymmetry of MI is evident ( Figure 4E ). The shape and the pattern of dentition of MI, as well as the other maxillae, is basically the same, and the number of teeth was: MI = 7 + 6, MII = 9 + 6, MIII = 5 + 5, MIV = 4 + 3, MV = 1 + 1. Studies have shown that there is not necessarily a direct relationship between the number of teeth and the size of the maxilla: small maxillae can have the same number as large ones ( Kielan-Jaworowska 1966). The number of teeth can also vary according to the age of the animal.

Discussion

The character asymmetry of maxillae was considered as invariable, and the number of teeth as variable and size-dependent. The morphology of the jaws and the number of maxillae show only slight variation. Because of this, the new species was not compared with species that show wide variation in these characters, taking into account the size of the body. Therefore, the new species was compared with species that have asymmetrical maxillae as a constant character.

In general, most descriptions of species of Arabella do not describe in detail the morphology of the maxillae. Descriptions that mention the serrated aspect of the blades are also few. It is possible that the serrated blade is similar in all species of the genus, but this aspect is insufficiently studied. Perkins (1979) described three species of Cenothrix and mentioned the same pattern of serrated blades; the same occurs with A. semimaculata Moore, 1911 , A. asymmetrica Crossland, 1924 and A. protomutans Orensanz, 1990 .

Arabella aracaensis sp. n. was compared with many descriptions of A. iricolor ( Fauvel 1923; Pettibone 1963; Orensanz 1974; Gardiner 1975; Lana 1984; George and Hartmann-Schroder 1985; Hilbig 1995; Núñez-Fraga 1990). Basically, this species is characterized by the presence of asymmetrical maxillae and the absence of a modified ventral seta. However, a revision of specimens referred to this species is necessary.

Other species lack the modified ventral seta: A. asymmetrica , A. semimaculata , A. pectinata Fauchald, 1970 , A. cincta Hartmann-Schröder, 1962 and A. protomutans . Of these, A. protomutans shows some similarities to A. aracaensis sp. n., including the general morphology of the body, shape of the MI and paired maxillary support. The serrated blades show the same pattern as A. aracaensis , except the ventral seta which is smooth. Furthermore, the left MI is bifid, a character that is absent in A. aracaensis . Arabella logani Crossland, 1924 shows similarities in the shape of the maxillae and the pattern of the serrated blades of setae. However, Colbath (1989) studied the type species, found a bifid left MI, and noted that this character distinguishes this species from A. iricolor , since both have the same type of ventral modified setae. If the MI of A. logani is bifid, it is possible that A. protomutans is the same species.

Of the other species that have modified ventral setae, none resembles A. aracaensis , either because they have a symmetrical apparatus ( A. atlantica Crossland, 1924 ; A. novecrinita Crossland, 1924 ) or have a ventral seta that is very robust and tapers abruptly ( A. maculosa Verrill, 1900 ; A. multidentata ; A. monroi Colbath, 1989 and A. panamensis Colbath, 1989 ).

According to Orensanz (1990), oenonids have no growth rings on the mandibles, and growth occurs by intussusception: in other words, by gradual transformation and incorporation of the formative elements of the mandibles; rather than by apposition, i.e., growth through adding layers, which would form rings. Because of the intense dark coloration and dull surface of the mandibles, the rings may pass unperceived. However, by the use of optical microscopy with incident light, it is possible verify growth rings clearly. Also, according to Colbath (1989), the maxillae are not mineralized, as in other Eunicida , and are strongly sclerotinized and contain slight traces of metals, which can interfere with observation of the rings.

Distribution

Atlantic Ocean: Brazil (State of São Paulo, São Sebastião Channel, Araçá Beach).

Etymology

The species was named Arabella aracaensis in defence of the preservation of Araçá Beach, the collection locality of the two species described here. This beach supports a highly diverse biota which is threatened with extinction because of the expansion of the Port of São Sebastião in the State of São Paulo.

Figures

Figure 4. Arabella aracaensis sp. n. (A) anterior region of body (dorsal view, ZUEC-POL-2178); (B) anterior region of body (dorsal view, ZUEC-POL-103, holotype); (C) anterior region of body (ventral view, ZUEC-POL-103, holotype); (D) maxillae (dorsal view, ZUEC-POL-2178); (E) maxillae I (young, dorsal view, ZUEC-POL-2179); (F) mandibles (ventral view, ZUEC- POL-2178); (G) pygidium (ventral view, ZUEC-POL-103, holotype). Notes: AT, anterior tooth; CP, chitinous plate; RD, ridge; arrow indicates additional tooth of MII.

Figure 5. Arabella aracaensis sp. n. (A) Parapodium 6; (B) parapodium 25; (C) parapodium 80; (D) parapodium 45; (E) parapodium 21 (young specimen). Notes: All parapodia in anterior view; A–D from ZUEC-POL-2178; E from ZUEC-POL-2179.

Figure 6. Arabella aracaensis sp. n. (setae and aciculum). (A–D) From parapodium 6 (A, superior; B, mediodorsal; C, medioventral; D, ventral modified); (E–F) from parapodium 25 (E, superior; F, mediodorsal); (G) mediodorsal (parapodium 146); (H) medioventral (parapodium 25); (I) medioventral (parapodium 62); (J) inferior (parapodium 25); (K) inferior (parapodium 80); (L) inferior (parapodium 146); (M) from parapodium 73; (N) supra-acicular bundle (parapodium 45); (O) subacicular bundle (parapodium 45); (P) ventral modified; (Q) mediodorsal (detail of serrated blade); (R–U) from parapodium 30 (young specimen, ZUEC-POL-2179 (R, superior; S, mediodorsal; T, medioventral; U, ventral modified); (V) aciculum. Notes: INF, ventral modified seta; MD, mediodorsal seta; MV, medioventral seta; SU, superior seta.