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
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).