Tubercle loss in Spatangoids (Echinodermata, Echinoides): Original skeletal structures and underlying processes

Abstract

Circular marks, flush with the test or slightly depressed, exist on the test surface of various echinoid species. Fifty-six species belonging to regular and irregular echinoids were examined in order to describe the diversity and structure of these marks and to discuss their origins, with particular emphasis being put on the spatangoids Heterobrissus niasicus and Maretia planulata. Investigations combine statistical, light and electron microscopical methods. The marks correspond to surrounding ordinary tubercles in their size, their distribution on the test, their structure and their microstructure (stereom meshwork). Marks recolonized by miliaries and marks overlapping each other attest that they were made during the life of the sea urchins. This hypothesis is strengthened by comparisons between marks and artificially extracted tubercles. The microstructure of numerous marks displays original patterns with blunt broken surfaces or concentric structures suggesting that these marks result from skeletal fracture and resorption processes. From the structure and distribution of these marks it is argued that they are formed by the natural removal of tubercles. Two possible origins are retained: they are scars resulting from a traumatic extraction of spines and tubercles, or they are due to an autotomy of tubercles and associated spines, a process ontogenetically controlled and always combined with sparse or heterogeneous tuberculation. The marks resulting from these two processes are not randomly distributed either on the test or between different groups of echinoids. Their distribution and abundance are strongly involved in the ornamentation of the test.