Metamorphosis and definitive organogenesis in the holothurian Apostichopus japonicus

Abstract

The structure of the late doliolaria, pentactula and 1-month-old juvenile of the holothurian Apostichopus japonicus was studied using light microscopy and 3D reconstruction methods. It was shown that metamorphosis in this species consists in the reorganization of the shape of the body and the destruction of provisional organs. The late doliolaria has a spindle-like form, ciliary rings and hyaline spheres shifted relative to the anterior-posterior axis of the body. Some provisional organs (ciliary rings, hyaline spheres) are destroyed during settlement, and others (hydropore and hydroporic canal) remain after metamorphosis. Definitive organogenesis in A. japonicus begins long before metamorphosis. The late doliolaria already has well-developed water-vascular and digestive systems, and the ectoneural part of the nervous system. Muscle and hemal systems begin to form in the pentactula. Moreover, the calcareous ring and connective tissue part of the body wall develop at this stage. The pentactula has anlages of the hyponeural part of the nervous system, which form in the mid-ventral and dorsal nerve cords. The hemal ring of the pentactula is located on the inner wall of the water-vascular ring. It remains unclosed in the left ventral radius. One-month-old juveniles have all the major organ systems except respiratory and reproductive systems. The hemal vessels of the intestine are well developed and begin to form the rete mirabile. Differentiation of the intestine into regions due to differential specialization of the enterocytes begins in 1-month-old juveniles. Obviously, emergence of new types of enterocytes enables the animal to consume a wider range of food items and indicates its increased feeding activity.