Thyroid hormones determine developmental mode in sand dollars (Echinodermata: Echinoidea)

Area of Science:

• Marine Biology
• Developmental Biology
• Evolutionary Biology

Background:

• Evolutionary transitions in larval nutritional modes are common in marine invertebrates.
• Mechanisms driving the shift from feeding to nonfeeding larval development are poorly understood.
• Facultative feeding larvae represent a potential intermediate stage in this evolutionary transition.

Purpose of the Study:

• To investigate the role of the thyroid hormone thyroxine in larval metamorphosis.
• To determine if thyroxine can induce a transition from obligate feeding to facultative nonfeeding development in sand dollars.
• To explore the influence of egg size and endogenous hormone production on larval development and metamorphosis.

Main Methods:

• Larvae of the sand dollar Leodia sexiesperforata were treated with thyroxine and compared to starved controls.
• Metamorphosis success, juvenile viability, size, and energy content were assessed.
• Experiments were replicated with different egg sizes and included a second species, Mellita tenuis.

Main Results:

• Thyroxine treatment enabled obligately feeding Leodia sexiesperforata larvae to complete metamorphosis without food.
• Thyroxine-treated juveniles were viable but smaller and had less energy than fed controls.
• Larger egg size in L. sexiesperforata correlated with a higher rate of metamorphosis in starved larvae, but thyroxine accelerated this process.

Conclusions:

• Thyroxine plays a significant role in echinoderm metamorphosis and the evolution of nonfeeding larval development.
• Egg size and endogenous hormone production are critical factors in the evolution of marine invertebrate life history transitions.
• Facultative feeding larvae are a key intermediate stage, facilitated by hormonal signaling.