As animals grow and develop from birth to adulthood, many physical changes occur. In fishes, these changes can be especially dramatic as eggs hatch into tiny larvae that must quickly develop while avoiding predators. Larval fishes face some unique challenges because of their small size. They are unable to feed themselves, so they carry a yolk sac which provides nutrients. They also may have difficulties while swimming, which means that they could get swept up by the water or made into a quick meal by predators. Some larval fishes use a unique organ to stick to plants or rocks to stay safe and avoid being swept off into the current. This organ, called the larval attachment organ, is a temporary structure found on, or near, the head of the fish. While examples of larval attachment organs have been found in the largest group of ray-finned fishes, the teleosts, Amanda Pinion from Texas A&M University, USA, and colleagues from Germany, Mexico and the USA found a larval attachment organ in the tropical gar, Atractosteus tropicus, which is a type of primitive ray-finned fish. They wanted to better understand how the organ is shaped and how it works, so they studied images of the tissues in and around the snout throughout the fish's development.
Each day, the authors collected 20 tropical gar larvae over the course of their development from hatching to 6 days of age, measuring the larvae's length and the size of their larval attachment organs using both scanning electron microscopy and light microscopy.
They discovered that the larval attachment organ is a well-developed disc located on the snout when the gar larvae hatch. The gar larvae grow quickly, so while the larval attachment organ does not change much in size or shape, it appears to be proportionally smaller on the snout tip of the larvae. At 5 days old, the larvae resorb the larval attachment organ. This is also around the time at which they resorb their yolk sac and begin to swim freely in search of external food. After examining microscopic images of the cells in and around the larval attachment organ, the authors also found that the larval attachment organ is a complex super-organ composed of a circular clustering of about 35 individual attachment organs surrounded by a border of supporting cells. These individual attachment organs are made up of two different types of cells: support cells, with dense ridges for holding fluid, and attachment cells, which are filled with tiny sacs that may release a fluid from the attachment disk. These attachment cells probably secrete a sticky adhesive which helps to secure the fish to a plant or rock surface.
While the structure of cells in the larval attachment organs of gar is similar to that of previously studied fishes, the organ itself is unique in that it is composed of dozens of clustered individual attachment organs fused together. Future studies of larval attachment organs in fishes could shed light on the relatedness of these groups. Additionally, the underwater adhesive that helps to hold larval gar secure at the most vulnerable stage of their lives could inspire scientists to design a similar underwater adhesive, which has been challenging to develop so far. This work uncovers some of the mystery behind a unique adhesive organ in larval gar and, hopefully, will allow us to better understand some of the ways larval fish survive their first few vulnerable days of life.