Macellomenia schanderi, Kocot & Todt, 2014

Kocot, Kevin M. & Todt, Christiane, 2014, Three new meiofaunal solenogaster species (Mollusca: Aplacophora) from the north-east Pacific, Journal of Natural History 48 (45 - 48), pp. 3007-3031 : 3011-3016

Macellomenia schanderi sp. nov.

( Figures 2A, B , 3 , 4 )

Type material (10 specimens)

Holotype. ZMBN ( University Museum of Bergen , Norway) 94118, histological section series (6 slides), San Juan Channel near Reid Rock ( Figure 1 ); 48° 19' 26'' N, 122° 35' 08'' E.

Paratypes. ZMBN 94119 , histological section series (6 slides) ; ZMBN 94120 , 1 whole animal mounted on a glass slide ; ZMBN 94121 and 94122, sclerite mounts (2 glass slides) ; ZMBN 94123 , 2 whole animals fixed in 4% glutaraldehyde in 0.1 M sodium cacodylate buffer and preserved in 70% ethanol ; USNM ( Smithsonian Institution National Museum of Natural History ) 1231346, 3 whole animals preserved in 100% ethanol ; USNM 1231345 , 1 whole animal mounted on an SEM stub ; USNM 1231347 , 1 whole animal mounted on an SEM stub. All paratypes from type locality .

Macellomena schanderi full length COI sequence: NCBI accession number KJ568516 .

Diagnosis

Body up to around 2 mm long. Mantle sclerites up to 50 µm in length with ovate bases. Acicular spine of mantle sclerites flattened and broadest medially. Peripedal scales of one type, leaf-shaped and pointed. Atrium (= vestibulum) with unbranched sensory papillae. Mouth opening separate from atrium. Rostral midgut caecum present, frontally paired. Radula with seven equally sized denticles per tooth. Oesophagus longer than radula. Midgut without regular constrictions. Abdominal spicules present, attached to a retractable genital cone. Posterior mantle glands present. Mantle cavity without respiratory papillae. Dorsoterminal sense organ present.

Description

Habitus. Relaxed specimens are up to about 2 mm in length by 200 µm in width ( Figure 3A, B ). The body is without any keels or bumps. Specimens are creamy whitecoloured and slightly translucent in life with similar colour when fixed in formalin and preserved in 70% ethanol (although less translucent). Specimens have a shiny appearance (from the sclerites) when illuminated from above. Orange material was visible in the midgut of some specimens. Living animals maintained in glass dishes were observed to actively crawl around the dish, frequently raising their anterior end.

Mantle. Sclerites are typical of the genus with an ovate, cup-shaped basal plate sharply tapering to a solid, slightly recurved and flattened acicular spine ( Figure 3C, E ). The acicular spines are widest medially rather than proximally and are covered in minute spinelets. The mantle sclerites are up to 50 µm in total length with the basal plate being up to 18 µm long and 10 µm wide, and the acicular spine being up to about 6 µm in width. Only one type of scale was observed to surround the foot and pedal pit ( Figure 3C, D ). These peripedal sclerites are blade-shaped and distally pointed, measuring up to 30 µm in length. The cuticle is uniformly thin over the body with a maximum thickness of around 27 µm ( Figure 4 ).

Pedal groove and mantle cavity. The pedal pit is large and eversible ( Figure 3B ). Large, darkly staining pedal glands are associated with the pedal pit ( Figure 4B, C ). Small sole glands similar in appearance to the pedal glands (but more darkly stained) are associated with the foot. The foot in fixed specimens consists of a single fold ( Figure 4B, C ). The mantle cavity lacks respiratory papillae ( Figure 4D ). No adhesive glands (such as that of Meiomenia ) were observed but there are glandular cells in the posterior mantle rim (not shown). The mantle cavity has a tubular ventral pouch, which at its muscular end holds a paired group of about 30– 40 abdominal spicules. This pouch is projectable and the whole organ could be called a genital cone ( Figure 4D ; see below).

Digestive system. The mouth opening of M. schanderi is separate from the atrium ( Figure 4A ) but immediately posterior to it. There are no sclerites separating the two openings although there may be cuticle between them. The pharynx is narrow and muscular. It is about the same length as the distinct oesophagus ( Figure 4C ), which has dorsally located oesophageal glands. There is a tiny monostichous radula (16.5 µm wide) that bears seven equally sized denticles per tooth ( Figure 4B ). Each ventrolateral foregut gland consists of a long duct surrounded by densely packed subepithelial (extraepithelial) gland cells (type A). The midgut has a short rostral caecum that is paired at the frontal end. No nematocysts were observed in the digestive system of either of the histologically sectioned specimens examined.

Nervous system and sensory organs. The nervous system includes a cerebral ganglion that is large relative to the size of the animal. The cerebral ganglion spans 19 of the histological sections, making it about 38 µm in length. The vestibular sense organ (atrium or vestibulum) holds about 20 simple sensory papillae. A large, distinct dorsoterminal sense organ is present ( Figure 4D ). This organ was readily observed in living animals.

Reproductive system. In the region of the mantle cavity of M. schanderi there is a ventral muscular organ ( Figure 4D ) similar to the genital cone of Genitoconia ( Salvini-Plawen 1968) . Within this structure are up to 40 posterior-pointing abdominal spicules. Examination of histological sections indicates that this structure is not covered by cuticle. When relaxed, most specimens seemed to partially evert the cone and abdominal spicules ( Figure 2B ). One specimen imaged by SEM had mucous covering up part of its posterior end including the base of the ‘genital cone’ but it appeared that this specimen had either completely ‘ejected’ the abdominal spicules or they had broken off near their point of attachment to the body and were stuck in this mucous ( Figure 3A, D ). The unpaired portion of the spawning duct is very large, filling the ventral half of the posterior portion of the body. Only close to where the spawning duct fuses with the pericardioducts, the lumen of the spawning duct becomes paired. There are darkly stained vesicles in the cells of the spawning duct. The frontal end of the spawning duct is bilobed. One pair of tube-shaped seminal receptacles is attached to the spawning duct, just laterofrontal to where the pericardioducts fuse with its frontal end. The pericardioducts originate at the posterior end of the pericardium. The gonad is distinctly paired, with each half up against the body wall on either side of the midgut.

Distribution

Only known from type locality.

Etymology

This species is named to honour the late Dr Christoffer Schander, an expert on aplacophorans and other molluscs who was a mentor and friend to the authors.

Figures

Figure 1. Schematic map of the San Juan Islands showing the type locality of the species described herein with a star.

Figure 2. (A) Three new meiofaunal solenogaster species from Reid Rock, Friday Harbor: Ms, Macellomenia schanderi; Mm, Macellomenia morseae; Hs, Hypomenia sanjuanensis; note the similar size, colour, and body shape. Scale bar 1 mm; (B) posterior body end of M. schanderi showing: asp, abdominal spicules. Scale bar 150 µm; (C, D) epidermal sclerites of H. sanjuanensis sp. nov., light micrographs, polarized filter; (C) note the criss-cross arrangement of sclerites (cuticle removed with household bleach); scale bar 50 µm; (D) sclerite types: a, dorsolateral sclerites are acicular with small or almost reduced cavity, often serrated; b, most anterior sclerites are recurved and serrated; c, ventral scales; d, ventral scales found close to the pallial cavity opening; scale bar: 50 µm.

Figure 3. Macellomenia schanderi sp. nov. (A) SEM micrograph of entire specimen, oblique ventral view. Position of anterior end (ant) and pedal groove (pg) indicated. Scale bar 200 µm; (B) ventral view of anterior body tip showing the atrium (a), mouth (mo) and the partially everted, ciliated pedal pit (pp). Scale bar 50 µm; (C) SEM micrograph of epidermal sclerites. The small insert shows a pedal scale (ps), the larger image illustrates the morphology of the main sclerite type with broadened base (bs) and flattened, recurved spine (s) covered in minute spinelets. Scale bar 10 µm; (D) SEM micrograph of the partially everted genital cone and pallial cavity opening (pc) with abdominal spines (asp) covered in mucus (muc). Scale bar 50 µm; (E) light micrograph of sclerites. Most of the body sclerites are intact although the acicular portion has been separated from the basal plate from several. There is one pedal scale (ps). Scale bar 50 µm.

Figure 4. Light micrographs of histological cross sections through various body regions of Macellomenia schanderi. Scale bars 100 µm. (A) Anterior body-end with atrium (a) showing atrial papillae; cu, cuticle; ep, epidermal papilla; (B) radular region with cerebral ganglion (cer), radula (r), and pedal pit (pp) with pedal glands (pgl); the insert shows the radula in higher magnification; (C) section slightly posterior to B with oesophagus (es), lateroventral foregut glands (fgl), and radular sheath (rs); cer, cerebral ganglion; ep, epidermal papilla; (D) posterior body-end with pallial cavity (pc), part of the dorsoterminal sense organ (dts), hind gut (hg) and genital cone (genc).