Archaeomenia nova Scheltema, 1999
Figs. 1G, H, 9, 10
Type locality. -- Off west Scotland, 57°25.5'N, 11°03.4'W, 609 m (INCAL [CENTOB] DS-03, 17.vii.1976).
Holotype. -- MNHN (alcohol specimen, spicule slide). 3.1 mm long, anterior, midbody, posterior heights 1.1, 1.0, 0.8 mm, respectively, width 0.8 mm anteriorly and posteriorly. Paratype 1. -- MNHN (dissected alcohol specimen, 2 spicule slides). Paratype 2. -- MNHN (dissected alcohol specimen, 2 spicule slides). Paratype 3. -- MNHN (spicule slide only).. Paratype 4. -- MNHN (dissected alcohol specimen, radula slide, 2 spicule slides). Paratype 5. -- MNHN (dissected alcohol specimen, spicule slide). Paratype 6. -- MNHN (radula and spicule slides only). All from type locality.

Etymology. -- archaeo = primitive; nova = new, referring to a newly found species in the genus.

Description. -- A small, squat species <5 mm in length, greatest midbody height 1.4 mm; anterior end rounded in lateral view, indented laterally in dorsal view, higher in diameter than truncated posterior end (Figs. 1G, 9A-C); dorsoterminal sense organ often evident; contracted mouth opening a longitudinal slit, spade-shaped when partially open; contracted pedal pit a longitudinal or lateral slit, bilobed when partially open; contracted mantle cavity opening a long, longitudinal slit extending posteriorly from paired ventral lobes, with brush of spicules overlying opening, often with copulatory spicules protruded. Cuticle and epidermis each about 10 µm thick. Spicules thickset, nearly flatlying (but not adpressed) except for two dorsal longitudinal bands of erect harpoon-shaped spicules (Fig. 9D), area between bands narrow and flat, slightly depressed, or slightly carinate; posterior spicules long, arranged axially. Rimmed, trough epidermal spicules from midbody to 142 µm long and 10 µm wide, and from posterior to 166 µm long, 11 µm wide (Fig. 9E, spicules 2), some with drawn-out tips to 47 µm (spicule 3); needles often slightly bulbous basally, to 214 µm long, 7 µm wide from midbody and to 250 µm long, 8 µm wide posteriorly (spicule 1); harpoon-shaped spicules from 58 to 85 µm long, distal hood 27 to 34 µm long by 13 µm wide, stem 29 to 45 µm long and to 4 µm wide, pedestal 9 to 11 µm wide (spicules 4-6); spicules beside pedal groove sharply pointed distally, with broad stem proximally, relatively large, to 112 by 20 µm posteriorly (spicules 7). Six radulae examined (two light microscopy, four SEM, measurements from single radula), entire radula 280 µm long, teeth and radular membrane weak, dissolution at distal end evident (Fig. 1H); tooth rows about 20, teeth per row 18 with 9 teeth on each side of median furrow, lateral teeth 36 µm long, greater than medial teeth, 28 µm long (Fig. 9G); teeth with single sharp, terminal point except small medial cusp on lateral teeth (Fig. 9F); tooth bases in adjacent rows nested, indented distally, rounded proximally (Fig. 9H, J). Paired copulatory spicules but not accessory copulatory spicules deciduous, five pairs measured; curved, with inner solid flattened rod partially enclosed by a thin, delicate sheath, both pointed with slight distal bend, both from 200 to 880 µm long in individuals 1.6 to 3.2 mm long, greatest width near distal end, rod 32 µm, sheath 70 µm wide (Fig. 10 spicules 1-3); paired groups of accessory copulatory spicules of two types: (1) hooked, 6 to 9 closely juxtaposed spicules per group arranged by increasing size; groups either undeveloped with short, flat-based spicules up to 115 µm long and 25 µm wide (spicule 4), or in most individuals >2.5 mm long fully developed, up to 230 µm long, 30 µm wide, basally narrow or recurved (spicules 5, 6); (2) groups of >40 solid, broad spicules next to hooked spicules, to 230 µm by 14 µm (not illustrated).

Reproductive system. -- To the description of the reproductive system in A. prisca (Thiele, 1906) may be added the presence of paired seminal vesicles in A. nova at the junction of gonads and gonopericardial ducts.
In A. prisca, Thiele (1906) noted a loss of calcium at the proximal ends of copulatory spicules in histologic sections. In dissected A. nova, spicules were either developing, broken, or lacking. Seventeen A. nova individuals from 1.6 to 4.7 mm long were examined for presence of copulatory spicules. In 7 individuals 23.0 mm copulatory spicules were present; in five individuals from 3.2 to 3.4 mm, three possessed copulatory spicules and 2 lacked them; and in five individuals >3.6 mm, copulatory spicules were lacking. Egg diameters were measured in 7 of these individuals. Although copulatory spicules were present in the three smallest individuals <2.5 mm long, they had only rudimentary gonads or eggs <80 µm in diameter. There was no unambiguous correlation between egg size and presence of copulatory spicules in four individuals from 3.2 to 3.9 mm: the largest eggs (124 and 129 µm) were found in two individuals, one with and one without copulatory spicules, whereas smaller eggs (106 and 108 µm) were found in two individuals without copulatory spicules, perhaps indicating that they had spawned. Likewise no correlation existed between size or absence of copulatory spicules and degree of development of hooked accessory copulatory spicules in individuals between 2.4 and 3.2 mm long; however, in all individuals >3.2 mm and lacking copulatory spicules, the accessory spicules were fully developed. The function of copulatory spicules in relation to spawning is difficult to assess but may be related to seasons in this upper slope species. It is not known whether copulatory spicules, once lost, are re-formed.

Remarks. -- Archaeomenia nova is differentiated from A. prisca by a more rotund, shorter body; by the proximity of the two dorsal bands of harpoon-shaped spicules to each other; and by larger size of spicules.

From Ophelia 51(1): 1-28 (1999).