# Reproductive Biology and Early Life History of the Apodid Sea Cucumber Chiridota laevis

## TL;DR

This study explores the reproduction and early life stages of the sea cucumber Chiridota laevis, revealing unique traits like seasonal sex change and a long development period.

## Contribution

The study provides the first detailed account of the reproductive biology and early life history of Chiridota laevis, a poorly understood sea cucumber species.

## Key findings

- Chiridota laevis undergoes seasonal sex change and spawns in late winter at cold temperatures.
- Development is lecithotrophic with no pelagic larval stage, and juveniles take up to 7 years to reach adult size.
- Eggs form a sticky mat on sediment, and embryos hatch after 7 weeks with feeding beginning at 10 weeks.

## Abstract

Sea cucumbers are keystone species of marine ecosystems around the world that demonstrate diverse life history strategies. Chiridota laevis is a burrowing species found in temperate to cold waters of the Arctic, North Atlantic, and North Pacific Oceans. The widespread distribution of this species underscores the value of studying the currently unknown fundamentals of its reproduction and development. In the present study, adults displayed distinct male or female sexual cells (sperm or eggs) for most of the year before undergoing a seasonal sex change in the fall months. Spawning and fertilization occurred in late winter, coinciding with the coldest water temperatures. The sticky eggs of C. laevis sank immediately after being released and formed a mat on the muddy sediment. The embryos were ~350 μm in diameter and did not undergo a feeding larval stage, indicating that yolk reserves sustained the developing offspring. The development of C. laevis was relatively long, with individuals hatching at 7 weeks and beginning to feed at 10 weeks. The measured growth rate suggests that the species may take up to 7 years to reach the adult size. Assessing the life history strategy of this understudied species helps us understand its unique evolutionary position and ecological role.

The apodid sea cucumber Chiridota laevis has been a documented member of endobenthic marine communities in northern waters for over a century and the rare studies available on its biology identify it as distinctive species and promising model for research. The present study sought to elucidate fundamental aspects of its life history that remained unresolved. Adults were determined to be protandric, with individuals primarily demonstrating solely male or female gametes from winter (close to spawning) to the spring and summer months before undergoing a sex change in the fall months. Additionally, gametes of both sexes reached maturity synchronously in late winter (February to March). In mesocosms, free spawning occurred in February, as the temperature reached ~2.0 °C. The negatively buoyant eggs were encased in a sticky casing and fell to the sediment where they adhered to each other to form a mat on the muddy substratum. The realized fecundity was ~15,000 offspring. Development was lecithotrophic, demersal, and abbreviated, characterized by the absence of a pelagic larval stage. Embryos reached the gastrula stage after about 7 days post fertilization; the calcareous ring appeared at 6 weeks, and juveniles hatched from the sticky casing at 7 weeks, immediately becoming endobenthic. The size of late embryos and juveniles remained similar (~350 μm) until they began actively feeding at about 10 weeks of age. Feeding juveniles more than doubled in size in the first week (740 μm), reached 3.5 mm by year one, and measured up to 11 mm by year two. This growth rate suggests that it may take this species up to 7 years to reach adult size at ~24 mm contracted length.