Abstract
The naiads, large freshwater mussels (Unionida), have very long life spans, are large-bodied, and produce thousands to millions of larvae (glochidia) which typically must attach to host fish tissues to metamorphose into a juvenile mussel. Glochidia develop within a female's marsupial gill demibranch, thus their number is restricted by female size. However, larger mussels acquire more energy, which could be invested in either larger-sized glochidia, in a more glochidia, or a combination of both. The high level of host specialization seen in many naiads may constrain glochidial size and shape around a narrow optimum, while naiads that use a wide range of host fishes may be predicted to possess greater plasticity in glochidial morphology. In this paper, we investigated the relationship between maternal body size and progeny body size and shape, aided by modern digital microscopy. We analyzed the between- and within- species variation of glochidia size and shape relative to female size in four widespread species of European naiads: Anodonta anatina, Anodonta cygnea, Unio crassus and Unio tumidus. Whereas the total reproductive output is collinear with female body size, substantial differences between species in glochidia size were found within genus Anodonta, but not genus Unio where glochidial size is remarkably consistent. The glochidial shape, however, differed within both Unio and Anodonta. We interpret this constant within-species glochidial size in Unio as reflecting a constraint imposed by the likelihood of successful transmission onto and off from a narrow range of hosts, whereas their shape seems to be less constrained. The Anodonta species, inhabiting a wide spectrum of habitats and using more than twice the number of fish hosts than Unio spp., have larger glochidia with greater variation in size and shape. Our results suggest that measures of glochidial variability may also serve as an indicator of host specificity in other naiads.
Highlights
The naiads, large freshwater mussels (Unionida), have very long life spans, are large-bodied, and produce thousands to millions of larvae which typically must attach to host fish tissues to metamorphose into a juvenile mussel
Some research suggests that developing glochidia are supplied by the female with calcium carbonate for shell production[14,15,16,17], which may indicate an additional investment of females into their offspring
Sizes of female mussels were [all measurements are expressed in mm]: A. anatina (N = 37, mean length = 120, SD = 13, range 73–140); A. cygnea (N = 33, mean = 113, SD = 18, range 84–146); U. tumidus (N = 24, mean = 91, SD = 16, range 51–116); U. crassus from the river Biała (N = 11, mean = 64, SD = 5, range 54–69); U. crassus from the river Cedron (N = 5, mean = 58, SD = 3, range 56–63); U. crassus from the river San (N = 15, mean = 47, SD = 9, range 36–62)
Summary
The naiads, large freshwater mussels (Unionida), have very long life spans, are large-bodied, and produce thousands to millions of larvae (glochidia) which typically must attach to host fish tissues to metamorphose into a juvenile mussel. Some research suggests that developing glochidia are supplied by the female with calcium carbonate for shell production[14,15,16,17], which may indicate an additional investment of females into their offspring This group provides a interesting opportunity for the analysis of life-history. Fewer than one in 1 million find a suitable host and survive[22,23,24], suggesting that selection will act strongly towards optimised infestation efficiency and driving a very narrow variation around optimal glochidial phenotypes.The degree of host specificity varies among species of naiads from specialists able to successfully parasitize only one or a few closely related fish species to generalists which can complete development on a taxonomically wide range of fish species[25]. The low investment in individual glochidia is reflected by their low flesh content[27], simple m orphology[9], and microscopic size (from 0.05 to 0.45 mm[12])
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