Who wins in the weaning process? Juvenile feeding morphology of two freshwater mussel species

Abstract

The global decline of freshwater mussels can be partially attributed to their complex life cycle.Their survival from glochidium to adulthood is like a long obstacle race, with juvenile mortality as a key critical point. Mass mortality shortly after entering into a juvenile state has been reported in both wild and captive populations, thus weakening the effective bivalve population. A similar phenomenon occurs during metamorphosis in natural and hatchery populations of juvenile marine bivalves. Based on a morphological analysis using scanning electron microscopy of newly formed juveniles of the freshwater species Margaritifera margaritifera (L.) (Margaritiferidae) and Unio mancus Lamarck (Unionidae), we show that a second metamorphosis, consisting of drastic morphological changes, occurs that leads to suspension feeding in place of deposit feeding by the ciliated foot. We hypothesize that suspension feeding in these two species improves due to a gradual development of several morphological features including the contact between cilia of the inner gill posterior filaments, the inner gill reflection, the appearance of the ctenidial ventral groove and the formation of the pedal palps. Regardless of the presence of available food, a suspension feeding mode replaces deposit feeding, and juveniles unable to successfully transition morphologically or adapt to the feeding changes likely perish. K E Y W O R D Sbiodiversity conservation, development, juvenile mortality, metamorphosis

An unexpected host for the endangered giant freshwater pearl mussel Margaritifera auricularia (Spengler, 1793) as a conservation tool

“…Large numbers of juveniles can be produced in captivity before being reintroduced to the wild. The release of juveniles raised to a large size is desirable because of the high natural mortality of newly metamorphosed juveniles (Araujo et al, ; Jones, Hallerman, & Neves, ; Kyle, Reid, O'Connor, & Roberts, ). The action plans of many species already include artificial propagation as necessary for their recovery (e.g., Araujo & Ramos, ; USFWS, , ).…”

Captive breeding of Margaritifera auricularia (Spengler, 1793) and its conservation importance

“…According to Hastie and Young (), the pedal‐to‐filter feeding transition represents a critical period for the survival of juveniles in captivity, yet the age for this transition is still unknown for M. auricularia . Araujo et al () found that for M. margaritifera and Unio mancus (Lamarck, 1819) the second metamorphosis occurs at an age of 150–200 and 70 days, respectively. Lavictoire et al () reported that at age 12 months, M. margaritifera continue to feed with the foot and it is only at an age of around 25 months that it becomes a filter feeder, although gills are not completely formed until the mussels are 3 years old.…”

“…High mortalities in juvenile freshwater mussels may have many causes, including hypoxia, metabolite accumulation (Eybe et al, ), and the transition process during the second metamorphosis in the laboratory, which many juveniles cannot overcome (Araujo et al, ). In the future, more studies are needed to assess the importance of this transition phase to decrease such high mortality rates in captivity.…”

“…auricularia. Araujo et al (2017) found that for M. margaritifera and Unio mancus (Lamarck, 1819) the second metamorphosis occurs at an age of 150-200 and 70 days, respectively. Lavictoire et al (2016) reported that at age 12 months, M. margaritifera continue to feed with the foot and it is only at an age of around 25 months that it becomes a filter feeder, although gills are not completely formed until the mussels are 3 years old.…”

The growth of juvenile native mussels (Elliptio complanata) in lakes varies with sediment characteristics and site exposure

“…In the laboratory, this metamorphosis is completed within a year (Araujo et al, 2018), but this would take longer in situ, depending on ambient conditions (e.g. In the laboratory, this metamorphosis is completed within a year (Araujo et al, 2018), but this would take longer in situ, depending on ambient conditions (e.g.…”