The life cycle
The lab tardigrade Hypsibius exemplaris goes from a single-celled egg to an adult laying its own eggs in about two weeks at room temperature. Every animal is female, and each moult doubles as a nursery: she lays her eggs into the skin she has just shed.
- Egg. A smooth, round egg about 55 µm across. It completes meiosis after it is laid and develops without fertilisation (Gabriel et al. 2007). Our embryo grows through all 19 stages.
- Hatching. After 4–4.5 days the embryo punctures the shell with its stylets and crawls out head first, with about 1,140 cells (Gabriel et al. 2007; Quiroga-Artigas & Moriel-Carretero 2024). It walks and feeds straight away.
- Growing. The young animal grows from about 125 µm to about 240 µm by moulting its cuticle (in tardigrades a moult takes 5–10 days); while it moults it sheds its mouthparts too and cannot feed. Most new cells are storage cells, which rise from about 72 to about 281 (Quiroga-Artigas & Moriel-Carretero 2024; Altiero et al. 2018).
- Laying. From about day 13–14 an adult lays eggs as she moults, into the cuticle she has just left, usually 3–12 at a time, and keeps doing so every moult for the rest of her life (Gabriel et al. 2007; Gąsiorek et al. 2018).
How to keep them through the whole cycle: Find and keep tardigrades.
How this is built
- Data
- Sizes, timings and counts from Gabriel et al. 2007, Quiroga-Artigas & Moriel-Carretero 2024, Altiero et al. 2018 and Gąsiorek et al. 2018, each cited where it is used.
- Built with
- We drew the cycle by hand as an SVG in this page, with the numbers written on it, and a CSS animation that carries a dot round the ring.
- Checked
- Not tested against data. Each number carries its citation, so it can be checked at the source.
- Limits
- The ring is a diagram, not a clock: its four quarters are equal, not scaled to time. Timings are at "room temperature", for which the sources give no number.
- More
- How the whole project is built
Inside a moult
A moult is more than a change of skin: the animal throws out its whole feeding apparatus and builds a new one. In cultures of three species, Marcus (1929) timed each phase. The mouthparts are expelled first, about six days before the cuticle splits; for 1–3 days the animal has no mouth parts at all and cannot feed; then it rebuilds them over 2–4 days. The cuticle splits 0–2 days later, and the old one comes off over 0–3 more days (Ramazzotti & Maucci 1983, pp. 74–76).
- Who rebuilds what. The two salivary (stylet) glands make the new buccal tube and stylets; the pharynx's own cells make the new thickenings that grind food; the stylet muscles are kept and reused. New claws grow beforehand, inside the tip of each leg, from small claw glands (pp. 30, 43–44, 54, 75).
- Still walking. The old cuticle stays attached at the muscle insertions until the very end, so a moulting animal can still move (p. 76).
- How many moults? Estimates for Macrobiotus and Hypsibius run to 6–12 in a lifetime, worked out from body-size distributions with many pitfalls; nobody has followed one animal from hatching to death (pp. 77–78).
- Warmth speeds eggs up. Milnesium eggs take 21 days at 8–10 °C, 13 at 18 °C and 10 at 24 °C (p. 70). That works out to a rate that rises about 1.6 times for every 10 °C, which gives us a first estimate of how temperature changes the timing of H. exemplaris embryos, whose published schedules say only "room temperature".
How this is built
- Data
- Marcus 1929's moult phases and the Milnesium egg times, as summarised with page numbers by Ramazzotti & Maucci 1983.
- Built with
- We drew the timeline by hand as an SVG, each bar at the middle of its range. We worked out the temperature effect from the three egg times: 21, 13 and 10 days at 8–10, 18 and 24 °C.
- Checked
- Not tested against data. The page numbers are given so each phase can be checked at the source.
- Limits
- The phases come from three other species, not H. exemplaris, and the warming factor from Milnesium, so it is a first estimate only.
- More
- How the whole project is built
Eggs and young, from the papers
