HATCHING PROCESS IN THE SIGNAL , LENIUSCULUS (DANA, 1852) (, )

BY

DOROTA PAWLOS1), KRZYSZTOF FORMICKI2,3), AGATA KORZELECKA-ORKISZ2) and ALEKSANDER WINNICKI2) 1) Westpomeranian Research Centre in Szczecin IMUZ, Czesława 9 Str., PL-71-504 Szczecin, Poland 2) Department of Fish Anatomy and Embryology, West Pomeranian University of Technology in Szczecin, K. Krolewicza 4 Str., PL-71-550 Szczecin, Poland

ABSTRACT

The need to elucidate the hatching process in the signal crayfish, Pacifastacus leniusculus, emerged as a result of the paucity of information on hatching mechanisms, perivitelline space, and egg hydration during embryonic development. To elucidate morphomechanical changes in the eggs during embryonic development of the signal crayfish, egg diameter and egg case strength were measured. Embryogenesis itself and hatching mechanisms were observed on live eggs in a special apparatus (a microscope coupled with a digital camera, a computer, and a video-recorder). Changes in egg case structure during embryogenesis and during hatching were analysed using light and scanning electron microscopy. During the period of observation, the eggs were found to continuously increase in volume, from an initial value of 8.87 ± 0.376 to 12.25 ± 0.371 mm3 at hatching. The egg case strength was initially considerable and exceeded 107.7±11.43 g. It was, however, observed to decrease with time to 38.6 ± 15.34 g of hatching. In addition, the structure of the egg case was observed, just before hatching, to loose its integrity mainly by local absence of the inner layer. On hatching, the case would break down along the embryo’s cephalothorax, where it was most conspicuously stretched.

RÉSUMÉ

La nécessité d’étudier le processus d’incubation chez l’écrevisse « signal », Pacifastacus leniusculus, est apparue du fait du manque d’information sur le mécanisme de l’éclosion et le processus de formation de l’espace péri vitellin et des relations hydriques au cours de développement embryonnaire. Pour élucider les changements morpho-mécaniques des œufs de l’écrevisse signal pendant son développement embryonnaire, le diamètre des œufs et la résistance de l’enveloppe de l’œuf ont été mesurés. L’embryogénèse et le mécanisme de l’éclosion ont été observés sur des œufs vivants à l’aide

3) Corresponding author; e-mail: [email protected] © Koninklijke Brill NV, Leiden, 2010 Crustaceana 83 (10): 1167-1180 Also available online: www.brill.nl/cr DOI:10.1163/001121610X526997 1168 DOROTA PAWLOS ET AL. d’un dispositif composé d’un microscope couplé avec une caméra numérique, un ordinateur et un magnétoscope. Les changements de structure de l’enveloppe de l’ceuf au cours de l’embryogénèse et de l’éclosion ont été analysés à l’aide d’un microscope optique et d’un microscope électronique à balayage. Pendant l’observation une croissance continue du volume des œufs a été observée depuis une valeur initiale de 8,87 ± 0,376 jusqu’à 12,25 ± 0,371 mm3 au moment de l’éclosion. La résistance de l’enveloppe de l’œuf au début du développement a été élevée et a dépassé 107,7 ± 11,43 g. Cependant, au cours du temps, cette valeur a diminué jusqu’à 38,6 ± 15,34gaumomentde l’éclosion. De plus on a constaté que, juste avant l’éclosion, la structure de l’enveloppe de l’oeuf perd son intégrité par la disparition locale de la couche interne. A l’éclosion l’enveloppe se fissure le long du céphalothorax de l’embryon là où il y avait la plus grande tension.

INTRODUCTION Hatching is one of the critical times during the early ontogenesis of a crayfish. The otherwise ample literature on hatching mechanisms in species lacks, however, an unequivocal opinion on the way the newly hatched individuals release themselves from the egg. The process during which the hatchlings leave their egg cases may follow one of several pathways. The first involves mechanical rupture of the egg case by the hatching larva. Another way is for water to be “pumped up” into the egg throughout the entire embryonic development, which eventually results in break-down of the egg case. Still another way involves “digestion” of the inner surface of the egg case by eclosion enzymes secreted by the embryo shortly before hatching; finally, hatching may occur as a result of a combined action of some or all of those factors. In the first case, the egg cases are mechanically broken by, on the one hand, different processes on the embryo’s body, which may serve as specific tools to break the egg case’s integrity; this has been observed in the Euphausiacea (cf. Gómez-Gutiérrez, 2002, 2003; Gómez-Gutiérrez & Robinson, 2005). On the other hand, the egg cases may be broken down as a result of rapid movements performed by the embryo, which disrupts the egg case’s integrity; this is the case in the crab, Pugettia quadridens (De Haan, 1850) (cf. Iwata, 1970). Another process that leads to hatching and has been reported from is related to the presence of a hatching (eclosion) enzyme in the perivitelline fluid. Crayfish were found to contain metalloproteinases representing an astacin family of the enzyme (Stöcker et al., 1988; Dumermuth et al., 1991; Stöcker & Zwilling, 1995; Geier & Zwilling, 1998). The Embryonic Astacin (AEA) metalloproteinase was isolated from eggs of the noble crayfish, (Linnaeus, 1758). The enzyme was contained in the developing embryo and its concentration was observed to increase from day 8 to day 4 (maximum concentration) before hatching (Geier & Zwilling, 1998).