Macellomenia morseae, 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 : 3016-3021

Macellomenia morseae sp. nov.

( Figures 2A , 5 , 6 )

Type material (5 specimens)

Holotype. ZMBN 94124 , histological section series (4 slides), San Juan Channel near Reid Rock ( Figure 1 ); 48° 19' 26'' N, 122° 35' 08'' E.

Paratypes. ZMBN 94125 , posterior body of one juvenile animal fixed in 4% formalin and preserved in 70% ethanol; sclerite mounts (2 glass slides), made from anterior body .

USNM 1231344 , 1 whole animal (a juvenile) fixed in 4% glutaraldehyde in 0.1 M sodium cacodylate buffer and preserved in 70% ethanol ; USNM 1231342 , specimen mounted on SEM stub ; USNM 1231343 , specimen mounted on SEM stub. All paratypes from type locality .

Diagnosis

Body to at least 2 mm long. Mantle sclerites up to 90 µm in length with ovate bases. Acicular spines of sclerites thickest proximally. With two types of peripedal scales. Atrium (= vestibulum) with sensory papillae. Mouth opening within atrium. Radula with eight equally sized denticles per tooth. Paired rostral caecum of the midgut present. Oesophagus shorter than radula. Midgut without regular constrictions. Mantle cavity without respiratory papillae. Abdominal spicules absent (?). Dorsoterminal sense organ not observed.

Description

Habitus. Relaxed living specimens are up to at least 2 mm in length by 200 µm in width ( Figures 5 , 6 ). 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. The sclerites give specimens of M. morseae a shiny appearance. Orange gut contents were visible through the body of some specimens. Living animals maintained in glass dishes partially immersed in a sea table were very active and were often found crawling upside-down on the water surface.

Mantle. The sclerites of M. morseae are typical of the genus with an ovate basal plate sharply tapering to a long and slightly curved solid acicular spine covered in minute spinelets ( Figure 5C, D ). The base of some spines is distinctly cup-shaped ( Figure 5C ), while in others it is more elongate-flattened. The mantle sclerites are up to 90 µm in total length with the bottom of the basal plate being up to 14 µm long and the acicular spine being up to about 3.5 µm in width. The acicular spine is of greatest diameter proximally ( Figure 5E ). Two types of flattened sclerites surround the foot. One type is ovate with a distal notch and a proximal rim and measures about 50 µm in length and 30 µm in width. The other type of peripedal sclerite is asymmetrically blade-shaped with a distal point and a proximal thickened rim at the attachment site. These scales are up to 45 µm long and 20 µm wide ( Figure 5D, E ). The cuticle is up to around 25 µm thick.

Pedal groove and mantle cavity. The pedal pit is densely ciliated and has associated pedal glands ( Figure 6B ), which are large and extend dorsally. Sole glands similar in appearance to the pedal gland, except for a much smaller size, are associated with the foot ( Figure 6C ). The foot is narrow (a single fold) and continuous with the mantle cavity. The mantle cavity lacks respiratory folds or papillae. No adhesive structures (such as those of Meiomenia ) are associated with the mantle cavity.

Digestive system. The mouth opening is located in the posterior end of the atrium (= vestibulum). The pharynx is narrow and muscular. It is much longer than the oesophagus, which is distinct but short. The oesophagus fuses with the midgut above the posterior end of the radular sheath. There is a tiny monostichous radula (14 µm wide) that bears eight equally sized denticles per tooth ( Figure 6B ). The ventrolateral foregut glands each consist of a muscular tube with a wide lumen, surrounded by densely packed subepithelial gland cells ( Figure 6C ). The midgut has a long, paired rostral caecum and lacks distinct constrictions. No nematocysts were observed in the gut.

Nervous system and sensory organs. There is a pre-vestibular sensory organ containing bundles of cilia, which is connected to the atrium via a groove lined with a thin layer of cuticle. There are around five small sensory papillae in the atrium ( Figure 6A ). The cerebral ganglion spans approximately 20 sections making it around 40 µm long. In the region of the radula, the cerebral ganglion is about 22 µm by 50 µm ( Figure 6B ). A dorsoterminal sense organ was lacking in the sectioned specimen and was not observed in living animals.

Reproductive system. The gonad of the single animal examined by histology is small, with few undeveloped oocytes arranged on both sides of the central septum; no spermatocytes could be detected. The remaining genital tract is not fully developed (not shown). The spawning duct is unpaired where it joins the mantle cavity ( Figure 6D ).

Distribution

Only known from type locality.

Etymology

This species is named to honour Dr M. Patricia Morse, an expert on meiofaunal invertebrates (especially molluscs) who described the first meiofaunal solenogaster from the area and alerted us to the presence of the species described herein.

Comparisons

Macellomeniidae Salvini-Plawen, 1978 , is a monogeneric family including only the genus Macellomenia Simroth, 1893 ( Table 1). This family is placed in the order Pholidoskepia on the basis of the presence of solid epidermal sclerites arranged in one layer, the presence of a thin cuticle, and the lack of epidermal papillae. Members of this family have unique sclerites, easily distinguishing them from the other families of the order. The two species of Macellomenia described herein increase the number of recognized species of the family from three ( M. palifera ( Pruvot 1890) as Paramenia ; M. aciculata Scheltema, 1999 ; M. adenota Salvini-Plawen, 2003a ) to five. Additionally, our collection of representatives of the genus in the Pacific greatly expands the known range of this family previously exclusively described from the European Atlantic and Mediterranean Sea.

Macellomenia schanderi sp. nov. and M. morseae sp. nov. can easily be distinguished from each other and all of the previously described species of Macellomenia on the basis of epidermal sclerite shape. The longer, straighter sclerites give M. morseae a much ‘spikier’ appearance than M. schanderi and this is readily apparent when directly comparing specimens using a stereomicroscope ( Figure 2A ). The solid, acicular portion of the body sclerites of M. morseae is usually quite elongate, thickest proximally tapering to its thinnest distally, and slightly if at all recurved. M. schanderi , on the other hand, has body sclerites with a much shorter acicular portion that is flattened, broadest medially, and more recurved than that of M. morseae . Notably, the acicular portions of the body sclerites of both species are covered with small spinelets that are not readily apparent unless examined using SEM. Also, a large dorsoterminal sense organ was easily observed in living specimens and histological sections of M. schanderi whereas no trace of this organ was observed in living specimens or histological sections of M. morseae . Additionally, a prevestibular sense organ is present in M. morseae only.

Macellomenia morseae appears to have two different kinds of scales accompanying the pedal groove, a character not described for any of the other species in the genus. M. schanderi , on the other hand, is the only known species with flattened sclerites. The dorsal sclerites of M. morseae are more similar in shape to those of the known species, but the relative length of the spine as compared to the base is much larger than in M. palifera and thus the specimens appear spinier, while the overall size of sclerites is smaller than in M. adenota . Unfortunately, Salvini-Plawen does not present any photographic documentation for M. adenota and the single specimen was not investigated with SEM. Therefore, it is not known whether the surface of the spines in M. adenota bears small spinelets, such as in M. schanderi and M. morseae . Similar spinelets have been found in one type of spine of the single specimen from the Irish Sea associated to M. palifera by Caudwell et al. (1995). Macellomenia aciculata differs from the new species in having very long (up to 300 µm) spines with a small base. Also here it is not known if the spines are smooth or covered in spinelets.

Macellomenia schanderi and M. morseae can also apparently be distinguished by the number of denticles per radular tooth: M. schanderi has seven denticles per tooth whereas M. morseae has eight denticles per tooth (although this character is not easily observed unless histological sectioning is conducted). Macellomenia palifera and M. adenota likewise have seven denticles per radular tooth, while M. aciculata has five denticles. However, we caution that this character could change with age and note that we (and most authors describing the other species) were able to examine this character in only a limited number of specimens (two for M. schanderi and just one for M. morseae ).

Interestingly, M. schanderi appears to be more closely related to the Atlantic species, M. adenota and M. palifera , rather than the sympatrically occurring M. morseae . Macellomenia schanderi and M. adenota both possess abdominal spicules presumably involved in copulation. In M. adenota , abdominal glands are associated with these abdominal spicules ( Salvini-Plawen 2003a). Macellomenia schanderi does not have abdominal glands and the arrangement of abdominal spicules is somewhat similar to the genital cone in Genitoconia spp. (Salvini-Plawen 1967), where they are called copulatory stylets. In the latter case, however, the stylets are embedded into the ventral musculature of the spawning duct, while in M. schanderi they are derivatives of a ventral pallial pouch. Therefore, we here keep to Salvini-Plawen’ s (2003a) interpretation as abdominal spicules in Macellomenia . It should be noted here that the single specimen of M. morseae examined by histology was a juvenile and it is possible that this species develops abdominal spicules as it matures. Additionally, M. schanderi and M. adenota are the only described Macellomenia species with the mouth opening separate from the atrium, although this character needs to be checked in M. palifera (see Salvini-Plawen 2003a).

Lastly, M. morseae apparently has a predilection to crawling on the surface tension of water in the dishes it is kept in. This behaviour was observed to a noticeably more limited extent in M. schanderi . It is in part because of this behaviour that so few specimens of M. morseae were examined for the present study – two additional specimens of this already rarely collected species were unintentionally lost during a water change of the dish.

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 5. Scanning electron micrographs of Macellomenia morseae. (A) Juvenile specimen mounted with ventral surface facing upwards; anterior end (ant) and pedal groove (pg) indicated. Scale bar 200 µm; (B) close-up of pedal groove showing two types of pedal scales, pointed (a) and broad with a notch (b). Scale bar 20 µm; (C) lateral sclerites, some in the centre of the image showing cup-shaped broadened bases Scale bar 25 µm; (D) pedal pit (pp) surrounded by pointed scales (a). Scale bar 25 µm; (E) light micrographs of epidermal sclerites (black background: polarized light filter). Note nail-shaped sclerites (n) and more asymmetrically shaped sclerites (asterisk) in lateral (to the left and to the right) and frontal (centre of image) views; both types of pedal sclerites (a, b) are very thin and delicate. Scale bar 50 µm.

Figure 6. Light micrographs of histological cross sections through various body regions of Macellomenia morseae. Scale bars 100 µm. (A) Anterior body end with atrium (a) showing atrial papillae; cu: cuticle; (B) radular region with radula (r) in the pharynx, cerebral ganglion (cer), and pedal pit (pp); the insert shows the radular apparatus in higher magnification; (C) section posterior to B with dorsal caecum of midgut (mg), oesophagus (es), lateroventral foregut glands (fgl), radular sheath (rs), pedal glands (pgl), and foot (f); (D) posterior body with pericardium (pc), hind gut (hg), and unpaired spawning duct (spd).