Fig 1.
Settlement of larvae in Scalpellum scalpellum.
Cypris larvae have two options for attachment, and thereafter follow completely different developmental pathways leading to either large, feeding hermaphrodites situated on hydroids (B, C, D) or dwarf males (E, F, G) situated in the receptacles of sexually mature hermaphrodites; yellow arrows indicate settling of cypris larvae; white arrows indicate development.
Fig 2.
Hermaphrodite metamorphosis in Scalpellum scalpellum.
Metamorphosing cyprids originally settled on hydroids but removed and incubated in vitro. (A) 1 day after settlement; with only a dorsal carinal primordium, but no other plates. (B) 2–3 days; CLSM, hermaphrodite now with both carina and clearly polygonally shaped primordia of scuta and terga. (C) 2–3 days as in B; specimen photographed still in situ on the hydroid. (D) 3–4 days; still enclosed in the cypris carapace, but with incipient peduncle formation at the anterior end (basal). (E) 5–6 days; incipient constriction between peduncle and capitulum; body stance stating to become elevated. (F) Detail of E, showing polygonally shaped scutal primordium (G) 7–9 days; cypris carapace now shed, peduncle has developed so the specimen is raised to near vertical stance. (H) 10–12 days; full armament of shell plates including three latus plates. Plates not calcified and thoracopods not fully functional. Further development of anterior larval tentacles. (I) 14 days; Thoracopods now clearly forming a cirral basket extending from the mantle cavity for feeding. Peduncle slenderer as the hermaphrodite continues to grow. (J) 17 days; juvenile hermaphrodite with calcified shell plates and cirral basket further developed. (K) 21 days; Sub-adult with completion of the capitulum and fully mineralized shell plates. Further development of cirral basket and presence of a transparent penis. Ce; compound eye; cp carapace; ppe pre-peduncle area; pe peduncle; to thorax; ad adductor muscle.
Fig 3.
Dwarf male metamorphosis in Scalpellum scalpellum.
All specimens settled in receptacles but removed and incubated in vitro. (A) 2–3 days after settlement; circular primordial of scuta and terga, but no carina; body shape beginning to assume an ovoid shape. (B) 3–4 days; no carinal plate, scutal and tergal primordia remain small and circular. (C) 4–5 days after settlement. (D) 5–6 days; cypris carapace shed; scutal and tergal primordia still small and circular, no carina; body shape ovoid. ce compound eye; cp carapace.
Fig 4.
Metamorphosis in Scalpellum scalpellum.
Summary of key events in hermaphrodite and dwarf male development. Explanatory text on the plate. ppe pre-peduncle area; ped peduncle; blue circles are scuta; green circles are terga; red circles are carina; pink circles are latus.
Fig 5.
CLSM of metamorphosis in Scalpellum scalpellum.
(A-D) hermaphrodites; (E) dwarf male. (A) 2–3 days enclosed in cypris carapace but incipient separation into peduncle and capitulum; thoracopods not yet of cirral form. (B) 3–4 days; peduncle, soma and thoracopods better developed; compound eyes moved ventrally and carapace close to being shed. (C) 7–9 days; carapace now shed; very clear separation of peduncle and capitulum; thoracopods further developed. (D) 10–12; hermaphrodite now with the juvenile shape; adductor muscles and soma fully developed but thoracopods not yet cirrus like. (E) 2–3 days old dwarf male; body form almost ovoid, scutal and tergal primordia circular and no carina; remnants of cypris thoracopods visible but no new ones developing. ppe pre-peduncle area; ped peduncle.
Table 1.
Presence/absence (+/-) of key characters in dwarf males of the studied species, Scalpellum scalpellum (Scalpellidae) and Octolasmis warwickii (Poecilasmatidae) and Smilium scorpio (Calanticidae).
Fig 6.
Dwarf males and adult hermaphrodites of Octolasmis warwickii.
(A, D) Hermaphrodites with dwarf male (squared) attached on the external surface; dwarf male shape identical to their large partner. (B, E) LM of (B) a larger dwarf male and (E) a smaller one, both with peduncle, carinal plate, cirri present and penis. (C, F) Histological section of B and E, showing presence of testes with sperm cells.
Fig 7.
Dwarf males and adult hermaphrodites in Smilium scorpio.
(A) A hermaphrodite with one scuta dissected exposing left scuta with a dwarf male encircled. (B) Morphology of the dwarf male and its position on the hermaphrodite partner. (C) Close-up of (B), note the presence of almost identical shell plate number and morphology between the dwarf male and hermaphrodite. Ped; peduncle.