The Biological Bulletin, Vol 190, Issue 1 24-44, Copyright © 1996 by Marine Biological Laboratory

DEVELOPMENT AND REPRODUCTION

Completely Direct Development of Abatus cordatus, a Brooding Schizasterid (Echinodermata: Echinoidea) from Kerguelen, With Description of Perigastrulation, a Hypothetical New Mode of Gastrulation

P. Schatt and J. P. Feral
Observatoire Ocbeeanologique de Banyuls, Universite Pierre et Marie Curie (Paris-VI), C.N.R.S. U.R.A. 117, F-66650 Banyuls-sur-mer, France

Development of the brooding schizasterid Abatus cordatus, a subantarctic echinoid endemic to Kerguelen, is described. Females spawn nonbuoyant eggs 1300 {mu}m in diameter, which are fertilized by elongated sperm (head 1 {mu}m wide and 15 {mu}m long). The main characteristics of this development are (1) incomplete cleavage beginning at the animal pole that becomes holoblastic, giving a filled wrinkled blastula 26 days after fertilization; (2) apparent (fate-mapping studies have not been done) external migration of mesenchyme cells, in the perivitelline space, from the animal to the vegetal pole during gastrulation while the archenteron invaginates; (3) hatching occurring at the end of the gastrulation (65 days after fertilization); (4) differentiation of the vestibule from a thickening of the oral epidermis as soon as the end of gastrulation is attained; and (5) production of a juvenile directly from the gastrula without any larval stage. The juvenile that leaves the brood chamber is 2 mm in diameter and about 250 days old. A. cordatus is a true completely direct developer (no larva and no metamorphosis). We propose to use (1) the term perigastrulation, as a tentative one until more definitive studies are available, to describe the hypothetical peculiar movement of cells during gastrulation and (2) the terms of direct development only for completely direct developing species and abbreviated development for species that have more or less transformed plutei.