symbionts of sea urchins. They cling to the spines on the outer surface of the sea urchin test using their highly specialized walking legs. Because their ability to walk on substrates like sand is impaired, zebra crabs spend their entire benthic life stage attached to sea urchins, switching between hosts only during the mating season. Usually only one zebra crab is attached to an individual sea urchin outside of the mating season, but larger sea urchins can be hosts to two (very rarely more). The area of the test they inhabit is characteristically smooth; completely devoid of spines, pedicellariae, tube feet, and even epidermis. It is unknown if they physically destroy and/or consume these appendages or if they use other stimuli to induce the host sea urchins to autotomize. Although previously considered harmless commensals, authors have since reclassified them as parasites. In addition to the visible external damage, a 1974 study also observed abnormal behavior and coloration among infected sea urchins. They also appear to be immune to the flower urchin's venom. Reproduction Flower urchins are dioecious (having separate male and female individuals), but it is almost impossible to determine the sex of an individual by external characteristics alone. A possible method is by examining the external characteristics of the genital pores (gonopores). In males, they are generally short, cone-shaped, and extrude above the body surface; while in females they are usually sunken. However this is not reliable, as 15% of the cases can exhibit the opposite characteristics. All other external features, like shape and size of the tests or color of the spines are indistinguishable between the two sexes. Flower urchins have a chromosome number of 2n = 42. Relatively little is known of the spawning behavior of flower urchins. Like other sea urchins, fertilization happens externally. Males and females release free-swimming gametes (eggs and sperm) directly into the water currents in mass spawning events. In Okinawa, Japan, a 1994 study identified the spawning season of flower urchins as occurring in winter, at the same time as the closely related and sympatric Toxopneustes elegans. It also recorded possible natural hybrids resulting from instances where the eggs of Toxopneustes pileolus are fertilized by the sperm of Toxopneustes elegans. In Taiwan, a 2010 study observed flower urchins spawning in May on the years 2007 and 2009. They occurred on highly similar conditions: in the afternoon low tide of the spring tide