Origin of planktotrophy—evidence from early molluscs

Abstract

The size of early ontogenetic shells (protoconchs) of ancient benthic molluscs suggests that feeding larvae occurred at about 490 myr (approximately, transition from Cambrian to Ordovician). Most studied Ordovician protoconchs were smaller than Cambrian ones, indicating smaller Ordovician eggs and hatchlings. This suggests substitution of nutritious reserve matter such as yolk by plankton as an energy source for larvae. The observed size change represents the first direct empiric evidence for a late Cambrian to Ordovician switch to planktotrophy in invertebrate larvae. It corroborates previous hypotheses about a possible polyphyly of planktotrophy. These hypotheses were primarily based on molecular clock data of extant clades with different types of larva, change in the overall body size, as well as increasing predation pressure on Early Paleozoic sea floors. The Early Ordovician is characterized by an explosive radiation of benthic suspension feeders and it was suggested that planktotrophy would prolongate escape from benthic predation on hatchlings. This biological escalation hypothesis does not fully explain why planktotrophy and suspension feeding became important at the same time, during a major biodiversification. An additional factor that probably included availability of nutrients must have played a role. We speculate that an increasing nutrient supply and availability of photoautotrophic plankton in world oceans have facilitated both planktotrophy and suspension feeding, which does not exclude a contemporaneous predation-driven escalation. It is very likely that the evolution of planktotrophy as well as increasing predation contributed to the Ordovician radiation.

Pulse of atmospheric oxygen during the late Cambrian

“…Indeed, the surviving shelf biota, dominated by trilobites and inarticulate brachiopods, appears to have increased in diversity at the peak of the SPICE (14). On longer timescales, some important transitions in the plankton (12,31,32) and benthos (33) may have begun following the SPICE oxygenation event in the late Cambrian Furongian Series (Fig. 6).…”

Understanding the Great Ordovician Biodiversification Event (GOBE): Influences of paleogeography, paleoclimate, or paleoecology

“…Planktonic feeding larvae developed after the Late Cambrian, possibly as an escape strategy from increasing predation pressure due to the appearance of benthic suspension feeders (Signor and Vermeij, 1994). Nützel et al (2006) found the first direct evidence for planktotrophy in gastropods at the Cambrian-Ordovician transition. Molecular-clock data and analysis of the fossil record also support this interpretation, with Peterson (2005) noting that planktotrophy evolved independently between the latest Cambrian and Middle Ordovician at least four different times in multiple lineages.…”

The morphology and evolutionary significance of the ciliary fields and musculature among marine bryozoan larvae

“…One main question regarding larval form is whether similarities in larval morphology are phylogenetically linked or if the body plans of larvae are free to evolve, arriving at similar morphologies based on convergent selective pressures (McEdward and Janies, 1993;Strathmann and Eernisse, 1994;Wray, 2002;Santagata, 2004;Raff and Byrne, 2006). Several investigations have concluded that the transition from a feeding to a nonfeeding larva (or vice versa) is a common evolutionary switch among marine invertebrates having no significant phylogenetic signal (Hart et al, 1997;Duda and Palumbi, 1999;Nü tzel et al, 2006). However, in some cases, the structure of larval and presumptive juvenile tissues is conserved among closely related species (Lyke et al, 1983;Santagata and Zimmer, 2002).…”