Abstract
Bitterlings are small freshwater fish that use long ovipositors to lay eggs in host mussels, and they have morphological adaptations to increase larval survival. The most well‐known adaptation is the minute tubercles on the skin surface of larvae; they are developed in early‐stage larvae with weak swimming ability and disappear in free‐swimming larvae before they leave the host mussel.In the present study, I comprehensively analyzed the developmental stages of Rhodeus pseudosericeus larvae, their morphological and physiological characteristics, their migration inside mussels, and the development of minute tubercle in order to elucidate the morphological function of the minute tubercles. These tubercles began to develop 1 day after hatching (formation stage), grew for 2–5 days (growth stage), reached the peak height after 6–7 days (peak stage), abruptly reduced in height after 8–10 days (abrupt reduction stage), and gradually reduced in height (reduction stage) until completely disappearing 27 days after hatching (disappearance stage).The larvae remained in the mussels’ interlamellar space of the gill demibranchs until 10 days after hatching and began to migrate to the suprabranchial cavity 11 days after hatching. At this time, the larvae had a heart rate and the caudal fin began to develop. At 24 days after hatching, the minute tubercles had almost disappeared, and some individuals were observed swimming out of the mussels.The results presented herein elucidate that the minute tubercles are the developmental dynamic structures that the bitterling larvae have morphologically adapted to prevent premature ejection from the mussel.
Highlights
Coevolution is a process that involves reciprocal evolutionary changes resulting from the interrelationship between a group of organisms and associated populations and plays an important role in the adaptation and speciation of almost all living organisms (Thompson, 1994)
I comprehensively analyzed the developmental stages of Rhodeus pseudosericeus larvae, their morphological and physiological characteristics, their migration inside mussels, and the development of minute tubercle in order to elucidate the morphological function of the minute tubercles
The results presented elucidate that the minute tubercles are the developmental dynamic structures that the bitterling larvae have morphologically adapted to prevent premature ejection from the mussel
Summary
Coevolution is a process that involves reciprocal evolutionary changes resulting from the interrelationship between a group of organisms and associated populations and plays an important role in the adaptation and speciation of almost all living organisms (Thompson, 1994). Besides the reproductive success of an individual, the choice of type and site of spawning in species with parental care are important factors that influence larval survival (Kitamura, 2005; Mills & Reynolds, 2002; Refsnider & Janzen, 2010; Smith, Reynold, Sutherland, & Jurajda, 2000). To prevent ejection by host mussels, bitterlings have several unique physiological, behavioral, and morphological adaptations (Aldridge, 1997; Kitamura, 2006a; 2006c; Methling et al, 2018; Smith et al, 2004; Spence & Smith, 2013). No comprehensive studies correlating the developmental stages of larvae with their morphological and physiological characteristics, their migration inside the mussels, and the development of the minute tubercles have been conducted. I discuss the evolutionary advantages of the development of the minute tubercles and migration of larvae inside mussels to increase survival
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