Applying Records of Extant and Extinct Horseshoe Crab Abnormalities to Xiphosurid Conservation

Xiphosurids are marine chelicerates that have been subject to extensive biological and palaeontological scrutiny over the past two centuries. This research effort is fuelled by the unique anatomical and physiological characteristics of the group, a long fossil record with conserved morphology, and use as modern analogues for understanding extinct arthropod groups. Despite this extensive literature, abnormal xiphosurid specimens are somewhat understudied. Recent studies have documented malformed specimens, the majority of which are attributed to injuries and developmental complications. To augment this recent research, we present records of Limulus polyphemus and Tachypleus tridentatus with malformed with malformed appendages, cephalothoraces, thoracetrons, and telsons. Causes of abnormalities are discussed and attributed to moulting issues and injuries. Three new examples of abnormal fossil xiphosurids are also presented: Euproops danae and Mesolimulus walchi specimens with cephalothoracic injuries and one specimen of M. walchi displaying a curved telson. We conclude that documenting abnormalities within populations may aid identification of spawning areas that require conservation attention. These oddities represent a potential avenue to minimize the population threats currently facing these unique chelicerates.