THE INTRIGUING ROLE OF LUNULES IN MELLITID SAND DOLLARS

 

Lunules, the unique structures found in mellitid sand dollars, are fascinating not only for their appearance but also for their essential functions in the organism's life. Here’s a closer look at what lunules do and how they are formed:

• Hydrodynamic Functions: One of the primary roles of lunules is to assist in hydrodynamics. They help manage water flow around the sand dollar, which is crucial for both movement and feeding. The evolution of ambulacral lunules is linked to a series of developmental changes, including the bifurcation of food grooves and the creation of pressure drainage channels, leading to their complete formation.

 Encope grandis Agassiz 1841 from the pliocene of Sonora (California-USA).

• Developmental Modifications: The formation of lunules is a result of specific modifications in the growth patterns of the test plates. This means that lunules are not simply added features; they arise from the organism's growth processes. Notably, the paper states that the formation of lunules does not require the resorption of the test, highlighting their developmental nature.

 

Encope micropora californica Verrill, 1870 from the pliocene of Baja California (Mexico).

 

• Distinct Origins of Anal Lunule: The anal lunule, while also serving a hydrodynamic purpose, has a different origin compared to the ambulacral lunules. It is hypothesized to have formed as an all-or-nothing event due to the forward migration of the periproct (the waste opening) and a failure to resume normal growth at the junction of specific plates. This distinct pathway emphasizes the complexity of lunule development.

Monophoraster darwini (Desor, 1847) from the Miocene (Burdigalian ~ 20 million years old) of the Peninsula Valdes, Argentina.

• Variability in Position: The position of the periproct varies among different species of sand dollars, which can influence the formation and function of lunules. For instance, in Scutella, the periproct is located between the first and second post-basicoronal plates, affecting how lunules are structured and operate.

                                 Echinodiscus bisperforatus Leske, 1778 from the miocene of Sukabumi (Java-Indonesia).

In summary, lunules are remarkable structures that play vital roles in the hydrodynamics and overall functionality of mellitid sand dollars. Their formation is a complex process influenced by growth patterns and evolutionary adaptations, making them a fascinating subject of study in marine biology.

More info: Malcolm, Telford. (1988). Ontogenetic regulatory mechanisms and evolution of mellitid lunules (Echinoidea, Clypeasteroida). Paleobiology, 14(1):52-63. doi: 10.1017/S0094837300011799