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

Development of the brooding schizasterid Abatus cordatus, a subantarctic echinoid endemic to Kerguelen, is described. Females spawn nonbuoyant eggs 1300 μm in diameter, which are fertilized by elongated sperm (head 1 μm wide and 15 μm 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 ...