Morphological differences in tardigrade spermatozoa induce variation in gamete motility

Area of Science:

• Zoology
• Reproductive Biology
• Evolutionary Biology

Background:

• Fertilization initiates ontogeny, relying on evolved gamete-uniting strategies.
• Tardigrade reproduction involves external sperm release, female storage, and species-specific sperm morphology (coiled nucleus, acrosome).
• Sperm motility in tardigrades remained uncharacterized despite known mating behaviors and morphology.

Purpose of the Study:

• To investigate and analyze individual spermatozoon motility in two tardigrade species.
• To compare the swimming patterns and speed of spermatozoa with differing lengths.
• To correlate sperm morphology with motility and reproductive success in tardigrades.

Main Methods:

• Utilized high-speed camera recordings to capture spermatozoon movement.
• Analyzed motility patterns of spermatozoa from Paramacrobiotus sp. (longer) and Macrobiotus shonaicus (shorter).
• Observed sperm behavior post-mating around the female cloaca.

Main Results:

• Paramacrobiotus sp. spermatozoa exhibited faster movement than M. shonaicus, despite equal tail beat frequencies, indicating longer tails enhance acceleration.
• Sperm heads remained largely immobile, while tails propelled movement; head-swinging was more pronounced during turns in Paramacrobiotus sp.
• Shorter M. shonaicus spermatozoa displayed wider tail movements relative to their heads, suggesting different locomotive strategies.
• Post-mating, spermatozoa aggregated around the female cloaca in both species.

Conclusions:

• This study presents the first analysis of individual tardigrade spermatozoon motility.
• Morphological differences significantly impact sperm motility: longer sperm swim faster, shorter sperm exhibit greater stability.
• Tail movement primarily drives swimming, but the elongated head of Paramacrobiotus sp. spermatozoa aids in turning.
• Sperm head morphology appears crucial for aggregation at the female cloaca, facilitating entry.