Patterns of comb row development in young and adult stages of the ctenophores Mnemiopsis leidyi and Pleurobrachia pileus

Abstract

The development of comb rows in larval and adult Mnemiopsis leidyi and adult Pleurobrachia pileus is compared to regeneration of comb plates in these ctenophores. Late gastrula embryos and recently hatched cydippid larvae of Mnemiopsis have five comb plates in subsagittal rows and six comb plates in subtentacular rows. Subsagittal rows develop a new (sixth) comb plate and both types of rows add plates at similar rates until larvae reach the transition to the lobate form at ∼5 mm size. New plate formation then accelerates in subsagittal rows that later extend on the growing oral lobes to become twice the length of subtentacular rows. Interplate ciliated grooves (ICGs) develop in an aboral-oral direction along comb rows, but ICG formation itself proceeds from oral to aboral between plates. New comb plates in Mnemiopsis larvae are added at both aboral and oral ends of rows. At aboral ends, new plates arise as during regeneration: local widening of a ciliated groove followed by formation of a short split plate that grows longer and wider and joins into a common plate. At oral ends, new plates arise as a single tuft of cilia before an ICG appears. Adult Mnemiopsis continue to make new plates at both ends of rows. The frequency of new aboral plate formation varies in the eight rows of an animal and seems unrelated to body size. In Pleurobrachia that lack ICGs, new comb plates at aboral ends arise between the first and second plates as a single small nonsplit plate, located either on the row midline or off-axis toward the subtentacular plane. As the new (now second) plate grows larger, its distance from the first and third plates increases. Size of the new second plate varies within the eight rows of the same animal, indicating asynchronous formation of plates as in Mnemiopsis. New oral plates arise as in Mnemiopsis. The different modes of comb plate formation in Mnemiopsis versus Pleurobrachia are accounted for by differences in mesogleal firmness and mechanisms of ciliary coordination. In both cases, the body of a growing ctenophore is supplied with additional comb plates centripetally from opposite ends of the comb rows.

Cambrian Sessile, Suspension Feeding Stem-Group Ctenophores and Evolution of the Comb Jelly Body Plan

“…The ctenes emerge near the aboral side between the oral region and the apical organ and grow toward the mouth. Furthermore, each ctene is paired in development [56]. The nervous system is centralized into the apical organ, congruent with our proposal that this is homologous to the ''dinomischid'' calyx, which originally carried the tentacles and the digestive system.…”

Cilia and the life of ctenophores

“…In support of this view, free‐swimming cydippid larvae of lobates do not initially possess ICGs, and their comb plates are about five times longer than their distance apart, the greatest degree of overlap of adjacent plates yet observed (Tamm ; see ). By the time the body grows larger with oral lobes and becomes more fragile, ICGs have developed between the plates, which are now farther apart.…”

“…Lobates like Mnemiopsis are somewhat compressed in the tentacular plane and expanded in the sagittal plane, and have two oral lobes on either side of the slit‐like mouth. As a result, the four subsagittal comb rows bordering the sagittal plane are about twice as long as the four subtentacular rows bordering the tentacular plane (Mayer ; Tamm ).…”

Defecation by the ctenophore Mnemiopsis leidyi occurs with an ultradian rhythm through a single transient anal pore

“…When cydippid larvae of M. leidyi reach a size of ~5 mm long, two developing lobes appear in the sagittal plane on either side of the mouth, marking the transition from cydippid to lobate body plans. Progression to the lobate form continues with larval growth and typically is attained in 1.2–1.5 cm larvae (Tamm, ). Later stages of growth are referred to here as young animals and adults.…”

“…Progression to the lobate form continues with larval growth and typically is attained in 1.2-1.5 cm larvae (Tamm, 2012b). Later stages of growth are referred to here as young animals and adults.…”