Bogomolova E.V.1, Malakhov V.V. 2004. [Fine morphology of larvae of sea spiders (Arthropoda: Pycnogonida) from the White Sea] // Invertebrate Zoology. Vol.1. No.1: 3–28 [in Russian, with English summary].
Department of Invertebrate Zoology, Biology Faculty, Moscow State University, Moscow 119899, Russia. E-mail: 1 email@example.com
ABSTRACT: External morphology of pycnogonid larvae (Arthropoda, Pycnogonida) of four species with different types of postembryonic development was studied with scanning electron microscopy. The first type is characterized by the following features: the larvae hatch at the protonymphon stage; larval body length varies between 100 and 200 mm; early larvae stay attached to male’s ovigerous legs; later they become ectoparasites of hydrozoans. Two species, Achelia borealis (Schimkewitsch, 1895) (Ammotheidae) and Nymphon longitarse (Kröyer, 1845) (Nymphonidae) belong to the first type. The second type has the following set of characters: the larvae hatch at protonymphon stage; larvae are extremely small (about 50 mm); after hatching, they leave male and start endopararasitiс life on hydrozoans; larvae lack spinning spines and web glands. Phoxichilidium femoratum (Rathke, 1799) (Phoxichilidiidae) belongs to the second type. The third type is characterized by hatching of the big-size (up to 300 mm) lecitotrophic larvae with the buds of the walking legs (i.e. the first postembryonic stage does not correspond to the protonymphon stage of other species) staying attached to the ovigers of male up to late development stages. Pseudopallene spinipes (Fabricius, 1780) (Callipallenidae) belongs to the third type. The newly hatched larvae were studied in all the four species. The following structures are described: elaboration by small denticles on chela fingers, on spinning spines and on terminal segments of second and third legs of protonymphon; pores of proboscidean glands and of chela glands (the latter are described in P. spinipes too; for larvae with the third type of development chela glands have not been found before). Larvae have sensilla of different shape (simple or ramified into 2 to 4 hairs, smooth or plumose). The structure of sensilla can vary in different specimens of the same species and even in the same specimen. Protonymphon larvae show regular arrangement of sensilla. Number of lsensilla correlates with size of the protonymphon larva. Pycnogonid larvae have bilaterally arranged slit-like pores of integumentary glands. More advanced larvae (investigated in A. borealis and P. spinipes) bear long setae on the cuticle of legs. The number of sensilla and slit-like pores increases from stage to stage while the regularity of their arrangement disappears. In A. borealis, coupled pores appear on ventral side close to bases of the first and (later) second walking legs. They are supposed to be external openings of enigmatic ventral organs associated with developing ventral ganglia.
KEY WORDS: Pycnogonida, sea spiders, postembryonic development, larvae, fine morphology.