Ultrastructure of Adult Gonads and Development and Structure of the Larva of Rhabdopleura normani (Hemichordata: Pterobranchia)

Abstract

Lester, S. M . 1988. Ultrastructure of adult gonads and development and structure of the larva of Rhabdopleuru normani (Hemichordata: Pterobranchia). (Department ol' Biological Sciences. Clemson University, Clemson, SC, U.S.A.) -Acra zool. (Stockh.) 69. 95-109.Sexually mature adults, embryos and larvae of the pterobranch Rhabdopleura normani from Bermuda were studied with light and electron microscopy. The sexes are separate among the zooids of a colony, but a given colony may contain females and males. In zooids of either sex the single gonad is associated with a large haemal sinus in the trunk sac and is displaced laterally (to the right or to the left). The wall of the gonad is composed of three layers: an outer metasomal peritoneum. an internal lining of germinal epithelium and an intervening genital haemal sinus. The mature gametes lie in the lumen within the gonad. The spermatozoon is characterized by an elongate nucleus. no obvious acrosome, a long mitochondria1 filament in a midpiece appendix and a single flagellum with a 9+2 axoneme. Females brood 200 pm eggs and embryos in their distinctive. basally coiled tubes. The yolky eggs undergo radial cleavage and develop into ciliated. lecithotrophic, oblong larvae (400 p m in length) that are characterized by: ( I ) yellow coloration peppered with black pigment spots; (2) a deep ventral depression; (3) a posterior adhesive organ; (4) an anterior apical sensory organ; (5) an evenly ciliated epidermis. The ventral depression is not invaginating endoderm, but is instead a glandular epithelium that evidently secretes the larval cocoon and the adult tube. Internally. thc peritoneum of the coelomic cavities begins to split from the periphery of a large. central mass of yolky mesenchyme cells. The larva swims using cilia. but also undergoes contractions, evidently powered by the peritoneal cells, which constitute a myoepithclium. The discussion considers pterobranch affinities with other deuterostomes and with lophophorates.

An Externally Brooding Acorn Worm (Hemichordata, Enteropneusta, Torquaratoridae) from the Russian Arctic

“…There is, however, an earlier report of internal brooding in an enteropneust by Gilchrist (1925), who published a drawing of a single advanced larva within the trunk coelom of an adult female of Xenopleura vivipara (family Harrimaniidae). Some enteropneust species, like Saccoglossus pusillus (Davis, 1908) as well as pterobranch hemichordates in the genera Cephalodiscus (Sciaparelli et al, 2004) and Rhabdopleura (Lester, 1988;Sato et al, 2008), retain early developmental stages in the maternal burrow or tube.…”

Evolutionary crossroads in developmental biology: hemichordates

“…Pterobranchs were first defined as a class in 1877 (Harmer, 1887), but it was not until they were first found in shallow waters that the details of their development were comprehensively described (Stebbing, 1970;Dilly, 1973;Lester, 1988b;Lester, 1988a;. Bateson originally considered enteropneusts to be primitive chordates and therefore informative for understanding the origin of the vertebrates (Bateson, 1886).…”

Developmental biology of pterobranch hemichordates: History and perspectives

“…The colonies were found to grow on the concave side of disarticulated lamellibranch shells, mostly Glycymeris glycymeris (dog cockle), collected from the seabed at 21-to 24-m depth, 1-2 miles south of Stoke Point in Devon (Stebbing, 1968). Following this discovery, a species of Cephalodiscus (Lester, 1985) and R. normani (Lester, 1988b) were also discovered in shallow water, this time only 0.5-2 m depth in coastal waters of Bermuda. These discoveries have facilitated study of developmental biology in Rhabdopleura.…”

“…Of particular interest, it was demonstrated that Rhabdopleura has lecithotrophic planula-like larvae covered by uniformly arranged cilia (Dilly, 1973;Lester, 1988b;Stebbing, 1970). Larval behavior has been observed, and attempts have been made to observe settlement and development of settled stages in both R. compacta (Dilly, 1973) and R. normani (Lester, 1988a).…”

“…Larval behavior has been observed, and attempts have been made to observe settlement and development of settled stages in both R. compacta (Dilly, 1973) and R. normani (Lester, 1988a). For example, Lester reported that the larva of R. normani made a ''cocoon'' by eversion of a ventral depression on the larva immediately after settlement (Lester, 1988b).…”