Living a Double Life

There is a spectacular amount of biological diversity in the sea. Most of it usually goes by unnoticed.

The vast majority of marine species live a double life split across two (or sometimes more) distinct life stages. Marine species usually begin life as a small larva, typically less than 1mm long, before undergoing the process of metamorphosis, where abrupt morphological and functional changes occur, to develop into a drastically different adult.

The life cycle of the sea squirt Pyura dalbyi (image order clockwise: fertilised egg, hatching larva, free-swimming larva, newly settled larva, two-month old adult, and fully reproductive adult). Images: Evatt Chirgwin

The life cycle of the sea squirt Pyura dalbyi (image order clockwise: fertilised egg, hatching larva, free-swimming larva, newly settled larva, two-month old adult, and fully reproductive adult). Images: Evatt Chirgwin

Most of the life we encounter during our ventures into the ocean and the seafood that ends up on our plates represent the adult stages of species’ life cycles. The tiny size of larvae means that they generally go unseen without the aid of a microscope. While larvae tend to go unnoticed by the everyday observer, they account for arguably the most fascinating diversity and ecology in the sea.

It’s a hard life for larvae - they’re small critters in an enormous environment. Yet despite their size, larvae are far from being simple, helpless drifters. They have evolved a range of adaptations that enhance their chances of survival. For instance, many avoid predators by hiding at great depths during daylight hours, only rising to the more productive and food-abundant shallow waters under the cover of night. Additionally, several species also exploit stratified horizontal currents as a sort of conveyor belt, which allows them to migrate between near-shore and offshore habitats to suit their needs at different times.

Marine larvae are remarkably diverse members of ocean ecosystems. Images: Evatt Chirgwin & Wikimedia Commons

Marine larvae are remarkably diverse members of ocean ecosystems. Images: Evatt Chirgwin & Wikimedia Commons

Seemingly the most important role of larvae is that it's typically the stage in an individual critter’s life where the largest or only (for species with a sessile adult stage) dispersal occurs. Some larvae travel immense distances, covering hundreds of kilometres over the course of months, while others travel only a few metres in a few hours. Crucially, the larvae need to use this dispersive period to find a habitat with sufficient resources and mating opportunity or escape the trappings of a poor habitat. This allows for success in their subsequent adult stage. Consequently, many larvae have adapted intricate ways to assess habitat quality using chemical cues in the water column or the biofilm (a bacterial layer covering solid surfaces) to obtain information on vital factors, such as food, oxygen, predators, and possible mates.

Larvae of species such as the bryozoan Bugula neritina have been shown to use chemical cues in biofilm to assess habitat quality. Images: Evatt Chirgwin

Larvae of species such as the bryozoan Bugula neritina have been shown to use chemical cues in biofilm to assess habitat quality. Images: Evatt Chirgwin

Following metamorphosis, an adult often shows remarkably little resemblance to its larval stage. However, there is a good reason for this disparity, as it reflects the fundamental role of each life stage. Whilst the larval stage is typically tasked with undergoing dispersal, the adult stage must achieve reproduction. The variations between adults and larvae have repeatedly been extended due to the two stages typically occupying vastly different environments and ecological niches. For instance, many species live their larval stage in the pelagic before settling and living their adult stage in the intertidal zone. In doing so, many face huge changes in factors such as temperature, oxygen, and salinity.

The tubeworm Galeolaria caespitosa spends its first few weeks of life as a free-swimming pelagic larva before settling and living in the intertidal as an adult. Images: Evatt Chirgwin

The tubeworm Galeolaria caespitosa spends its first few weeks of life as a free-swimming pelagic larva before settling and living in the intertidal as an adult. Images: Evatt Chirgwin

An unfortunate consequence of being unnoticed is that you can all too easily slip into trouble. Larvae are nearly always more sensitive to environmental stress than their more visible adult counterparts. Consequently, the actual effect of anthropogenic stressors such as climate change and pollution on marine populations can often be not truly appreciated - at least not immediately. With environments globally changing at increasingly faster rates, it’s important that we work to understand and manage all parts of species’ lives, and not just the parts most visible to us.


Evatt Chirgwin

Evatt is an evolutionary ecologist whose research focuses on how natural populations can adapt to environmental change. He is currently undertaking his PhD at Monash University.

You can find him on Twitter at @EvattChirgwin


Banner image courtesy of Evatt Chirgwin.