Brave new propagules: terrestrial embryos in anamniotic eggs

Area of Science:

• * Evolutionary biology
• * Herpetology
• * Ichthyology

Background:

• * Many fish and amphibian species exhibit terrestrial reproduction, a strategy not requiring the amniotic egg characteristic of reptiles and birds.
• * Their eggs are typically smaller, possess simple membranes, and hatchlings usually return to aquatic environments.
• * Terrestrial incubation offers benefits like warmer temperatures and avoidance of aquatic hypoxia, but poses risks such as desiccation and predation.

Purpose of the Study:

• * To investigate the evolutionary strategies of fish and amphibians that reproduce terrestrially without amniotic eggs.
• * To explore the physical challenges, hatching constraints, egg size effects, and parental care involved in terrestrial incubation.
• * To identify diverse early life history patterns among anamniotic embryos in terrestrial environments.

Main Methods:

• * Review and synthesis of existing literature on terrestrial reproduction in fish and amphibians.
• * Analysis of case studies, including the California Grunion, to exemplify trade-offs in embryonic development.
• * Identification and classification of different terrestrial incubation strategies.

Main Results:

• * Eight distinct types of early life histories were identified for anamniotic embryos undergoing terrestrial incubation.
• * Terrestrial incubation involves trade-offs between embryonic growth and incubation duration, influenced by yolk energy reserves.
• * Environmental cues can trigger hatching in some species.
• * Physical challenges like desiccation and predation are significant factors.

Conclusions:

• * The evolution of terrestrial reproduction in anamniotes demonstrates diverse adaptations for life on land.
• * These strategies represent alternative routes for vertebrates to colonize terrestrial environments.
• * Understanding these adaptations provides insights into the broader evolutionary transition from water to land.