Semiodera Chamberlin, 1919

Salazar-Vallejo, Sergio I., 2012, 3562, Zootaxa 3562, pp. 1-62 : 6-8

Semiodera Chamberlin, 1919 restricted

Semiodera Chamberlin, 1919:397 , Hartman 1959:419, Salazar-Vallejo 2009:205 partim.

Type species: Siphonostomum cariboum Grube & Ørsted in Grube, 1859, by original designation.

Gender: Feminine.

Diagnosis (emended): Body anteriorly swollen, with a posterior cylindrical cauda. Cephalic cage well developed. Dorsal shield rarely missing, extending over chaetigers 1–4 and often with an articulated anterior plate, rarely continued ventrally as a thinner coat. Body papillae small, capitate or clavate, in distinct belts, at least in anterior chaetigers. Anterior neuropodia with pseudocompound or with long transition hooks. Posterior neurohooks anchylosed, falcate, simple, sometimes flanged. Branchiae in two rows; distal row with four larger filaments, proximal row separated in two lateral groups, each with two or more filaments, often spirally arranged. Nephridial lobes between branchial rows. Posterior end with few neurohooks. Boring in hard or compact substrates.

Remarks. Semiodera Chamberlin, 1919 has been regarded as a junior synonym of either Pherusa Oken, 1807 ( Fauchald 1972) or Piromis Kinberg, 1867 ( Hartman 1965, Day 1973a, Fauchald 1977). However, Semiodera differs from Pherusa in three essential features: 1) Semiodera has an oval dorsal shield, made of cemented sediment particles, on the first few anterior chaetigers, but there is no shield in Pherusa ; 2) their branchial filaments are multiple, not just eight as in Pherusa , arranged in double or multiple rows; and 3) the body papillae in Semiodera are small, not large as in Pherusa . Day (1973a:108) redefined Piromis Kinberg, 1867 and regarded Semiodera Chamberlin, 1919 as a junior synonym for it. In fact, he should have written Balanochaeta Chamberlin, 1919 since that genus had, as type species, what we now call Piromis eruca ( Claparède, 1869) . This synonymy was followed by Fauchald (1977:117) and recently by Wehe & Fiege (2002:51). Additional information about this issue can be seen elsewhere ( Salazar-Vallejo 2011a); it must be emphasized that Semiodera differs from Piromis in three essential features: 1) Semiodera has the sediment particles cemented and often restricted to a small, anterior, dorsal region including few chaetigers, whereas in Piromis the sediment particles, if restricted, are present along the dorsal surface; 2) Semiodera has branchial filaments sessile on a depressed branchial plate, whereas in Piromis the branchial plate is tongue-shaped and carries abundant filaments; and 3) Neurochaetae in Semiodera are small and anchylosed, whereas in Piromis neurochaetae are large and multiarticulate. Consequently with this redefinition, several species previously included in Pherusa are transferred to Semiodera : this modification follows one of Fauchald’s (1972) species groups, although the included species differ slightly. Semiodera glabra Treadwell, 1928 , and Semiodera roberti Hartman, 1951 , pointed to the Chamberlin’s concept for the genus. However, the first species belongs in Trophoniella Hartman, 1959 (cit. Trophoniella Caullery, 1944 ), whereas the second species falls within Piromis , as shown elsewhere ( Salazar-Vallejo 2011a, 2012).

Semiodera includes species which bore into consolidated or calcareous substrates and build some inner layers with aragonite. Their bodies taper into distinct caudas with the anterior end depressed, often carrying a dorsal shield made of cemented sediment grains, and the body surface is covered by small, usually few body papillae which are often arranged in transverse rows. However, there are two distinct morphological patterns involved. The first pattern corresponds with Semiodera . The dorsal shields are variously developed, anterior chaetigers have pseudocompound or transitional hooks, and the posterior region is cylindrical with one to several neurohooks per bundle. The second pattern includes species with dorsal shields, anterior chaetigers with multiarticulate, aristate capillaries, and the posterior region is often depressed with many neurohooks per bundle. For the latter pattern, Daylithos , n. gen., is being proposed below. Because of these features, these genera were separated in a phylogenetic analysis of the Flabelligeridae ( Salazar-Vallejo et al. 2008) , and both formed a clade which was the sister group for what is now called Stylarioides delle Chiaje, 1831 and Treadwellia Salazar-Vallejo, 2011 ( Salazar-Vallejo 2011b).

The species included in Semiodera are, besides the type species S. caribea (Grube & Ørsted in Grube, 1859) new spelling, S. blakei n. sp., S. curviseta ( Caullery, 1944) n. comb., S. dubia ( Treadwell, 1929) n. comb., S. glynni n. sp., S. inflata ( Treadwell, 1914) n. comb., S. laevis ( Stimpson, 1856) n. comb., S. mezianei n. sp., S. nishii n. sp., S. salazarae n. sp., S. tenera ( Grube, 1868) n. comb., S. tovarae n. sp., S. treadwelli n. sp. and S. villalobosi n. sp.

As stated above, there is a group of species provided with multiarticulate capillaries in chaetigers 3–5(6), with the posterior region depressed, often with many neurohooks, and the branchiae are usually abundant. For these species, Daylithos n. gen. is being proposed, with Stylarioides parmatus Grube, 1878 , as the type species; besides this species, the other species being transferred to this genus are D. amorae n. sp., D. cinctus ( Haswell, 1892) n. comb., D. dieteri n. sp., D. iris ( Michaelsen, 1892) n. comb., and D. nudus ( Caullery, 1944) n. comb.

Morphology.

Body shape Semiodera and Daylithos have bodies separable into two regions: an anterior trunk, markedly thicker than the rest of the body, extending for nearly half of the total length of the body, and a posterior cauda, which is always thinner than the trunk, although sometimes it might be subdistally swollen. The cauda is often found bending over the back in living or preserved specimens. There are two distinctive patterns in the cauda; in Semiodera it is cylindrical and the neuropodia are usually provided with 1–4 neurohooks per bundle, whereas in Daylithos the cauda is depressed and there are 4-9 neurohooks per bundle, which are aligned along transverse rows. Tunic The tunic is thin and delicate throughout the body and slightly thicker along the first few chaetigers; it is probably easily damaged by formalin, since it is hardly visible in preserved or roughly handled specimens, where even the external muscle layers may be lifted off the remaining body wall. Although the tunic is easily detached from the body, its presence is revealed because of the eroded portions along it, because of the remaining parts especially over neurochaetal bases or along the cauda, or because it may carry minute, dark particles.

Body papillae Semiodera and Daylithos have, in comparison to other members of the family living within sediments, fewer and smaller body papillae, giving their bodies a rather smooth surface. The body papillae are usually arranged in transverse rows per segment, which could be close to each other or not, with one running along the anterior segment margin and the other placed about the median segmental region. The species differ, however, depending on the relative number of rows, which are better defined dorsally along few anterior chaetigers, although papillae become larger in posterior chaetigers, and the relative number of papillae increases per row. Some species have two transverse rows and the papillae of the anterior row can be of the same size, larger, or smaller than the posterior row of papillae. This feature is conservative and is used below to separate similar species. At the same time, when the specimens are fixed inside their tubes, the papillae are more difficult to detect than in specimens fixed outside their tubes, since body contraction results in an apparent enlargement of body papillae. Further, although the body papillae tend to appear larger along the anterior chaetigers, because posterior chaetigers are usually smaller the papillae become relatively larger. The interramal papillae, in turn, might be longer in posterior chaetigers.

Dorsal shield The dorsal or nuchal shield is made of cemented sediment particles and is only found in boring flabelligerids; it might vary among different species regarding their relative thickness, especially towards the posterior margin, and about their roughness because in some species the particles are completely embedded by the integument giving a smooth surface, whereas in others the particles are less perfectly fit and result in a rough surface. Another useful feature, although it may depend on the sediment particles available, is the relative color of the shield. Although it is frequently well-defined along its margins, in some Semiodera species it is reduced to a thin layer which also has poorly defined margins, being represented by a few cemented particles. However, this dorsally poorly defined shield may extend laterally reaching the base of the neuropodia and even tending to approach the midventral line. Another variation is the sediment particles are missing from the dorsal shield; one species found in floating objects has almost no sediment particles at all, whereas others found in boring rocks, might likewise have no particles on the dorsal shield, but the depressed anterior end gives a clear indication of its function to fit the tube opening. Under these conditions, these species are retained within Semiodera because of the anterior end modifications and the chaetal pattern along the body. It must be emphasized that sometimes the dorsal shield is removed from the specimens by an excessive peeling of foreign particles. In the latter case, there are some indications of its former presence which include an oblique, often smooth body wall portion and an ovoid ridge that used to fit along the shield margins, or some scars in the integument. A short-term staining with an oversaturated methyl-green ethanol solution may help in deciding this, because it reveals the surface micro-relief due to the former presence of the shield. It would be interesting to evaluate if the apparent lack of dorsal shield is independent of the treatment given to the specimens, since the shield may be confused with foreign particles, and some people working with the samples may completely remove it. However to eliminate this potential problem, those species lacking a well-developed dorsal shield have been included together with those with a poorly defined shield.

Branchiae The branchial filaments are separated into two lateral groups, although in some species the caruncle may not completely divide the branchial filaments and there may be two filaments present beyond the caruncle tip, such that branchiae are apparently arranged in a single continuous row, which becomes laterally spiraled. The filaments have different length and thickness along the branchial plate; the distal or more dorsal and upper lateral filaments are usually larger than the more ventral or inner filaments and their relative thickness may be useful to separate similar species, but because they are rarely exposed, this feature has not been utilised.

Neurochaetae The neurochaetae provide the most useful diagnostic features to separate similar species. The anterior chaetigers have multiarticulate neurochaetae, although they may be anchylosed and are very long in the first three chaetigers. From chaetigers 3, 4 or 5, chaetae become shorter and modified. There may be 1–2 pseudocompound neurohooks, 1–4 shorter transition chaetae without any pseudoarticulation, or 1–2 multiarticulate neurohooks; either way, these neurochaetae usually point backwards and become progressively shorter in subsequent chaetigers. Most body neurochaetae point forward and are arranged in a transverse or slightly oblique row. Pseudocompound hooks have a long handle, with medium-sized to long anchylosed articles (articulations transverse); the handle is distally widened and the pseudoarticulation is an oblique, pale line. The blade is a homogeneous continuation of the handle, tapering to a blunt, often falcate tip. Transition chaetae are also falcate but rarely pseudocompound; they progressively decrease in size and the overall shape progresses towards the typical neurohooks, showing a gradual transformation and hence the name, transition chaetae. Some species have progressively smaller neurochaetae but they are straight; they may be cylindrical, multiarticulate spines, often with a hyaline terminal article, or they may be aristate or mucronate, when the cylindrical handle terminates in a narrow, tapering distal part. Either type of neurochaetae is completely or gradually replaced by the stouter, non-annulate, or transparent and often falcate neurohooks, which are usually pointing forward, at least along anterior and median chaetigers. The detection of the first chaetiger where these neurohooks appear can be done by using the stereoscope, although small specimens may need to be examined under high magnification. The first chaetiger provided with only a single type of falcate or curved neurohooks can separate similar species, although there might be some slight size-related differences, this feature is conservative. Further, the nearby posterior chaetigers often show a marked change in relative size, increasing in size a few chaetigers before the start of the cauda. The number of neurohooks per bundle is also useful, although they are easily broken. Some median chaetigers should be scanned in order to detect their relative number. The trend includes one or a few chaetigers with 2–3 hooks, increasing in number up to 5–6 by around chaetiger 10 and then decreasing progressively towards the body constriction or cauda. From then onwards, as indicated above, there may be a progressive reduction of neurohooks per bundle along a cylindrical cauda, or remain high or actually increase in number on a depressed cauda.