From my trips to the beach and aquarium this weekend, where I overheard people misidentify fish, it’s clear than many people don’t know the difference between skates and rays. While both are flattened, cartilaginous fishes with enlarged pectoral fins within the superorder Batoidea, there are some subtle differences. Unlike rays (order: Myliobatiformes), skates (order: Rajiformes) do not have barbs. Also, according to new research led by Dr. Cristopher Martinez at the University of California, Davis, skates exhibit strong sexual dimorphism compared to rays.
Sexual dimorphism is when male and female individuals of the same species look different, like how male deer have antlers and are larger than females, or how male peacocks are much more colorful and ornate than females. By measuring dozens of skates from over 20 genera, Dr. Martinez’s team found sexual dimorphism to be widespread in skates.
Aside from males having male reproductive organs (paired, cartilage-supported, tube-like structures called claspers) and females having female reproductive organs, they also have differently-shaped pectoral fins, pelvic fins, and jaws. While mature females have relatively flat edges to their pectoral fins, mature males have concave pectoral fins. Interestingly, though, immature males and females look roughly the same, suggesting dimorphism increases as the skates mature. To test this, Dr. Martinez and colleagues measured a time series of the pygmy skate Fenestraja plutonia. They found that male skate pectoral fin shape diverges from females during sexual maturation, developing curves as their claspers dramatically elongated. Therefore, the same processes that cause clasper elongation may also cause curves to develop in male skate pectoral fins.
A mature female clearnose skate (R. eglanteria) that I caught at the beach. Note how that compared to the male above, the pectoral fins have relatively flat edges.
The degree of sexual dimorphism between males and females varies greatly depending on the species of skate, as well as the size of claspers. However, clasper size doesn’t matter when it comes to the curviness of males; Dr. Martinez’s team found no correlation between the two variables. Alternatively, they propose that the rate of clasper development may be causal to the degree of sexual dimorphism. In species in which claspers elongate rapidly, the high metabolic cost of growing the claspers would take place over a relatively short period of time. The males of these species may have to allocate more resources toward clasper development and away from other bodily processes, like pectoral fin growth, causing a difference in pectoral fin morphology between males and females.
Sexual dimorphism in skates may contribute to their remarkable number of species, which has been puzzling due to their seemingly low ecological and morphological variation. Different degrees of dimorphism between species may indicate there are subtle variations in the life history of skates that have yet to be described. Future research into these diamond-shaped sea pancakes may reveal the causes to their sexual dimorphism and more secrets to their surprising diversity.
Noah Bressman is a PhD Candidate at Wake Forest University studying fish functional morphology, biomechanics, and behavior, with a special focus on amphibious fishes. You can find more at NoahBressman.wixsite.com/Noah or @NoahwithFish, or contact him at NoahBressman@gmail.com