IMPROVING CAPTIVE REARING METHODS FOR THE ENDANGERED ORANGEBLACK HAWAIIAN DAMSELFLY Megalagrion xanthomelas

The  endangered orangeblack Hawaiian damselfly (Megalagrion xanthomelas)  was once common in lowland aquatic habitats throughout the high Hawaiian Islands, but t he introduction of nonnative mosquito fish reduced this species to  a  few disparate populations. On Oahu, M. xanthomelas rests on the brink of extirpation, with one remaining population known. In 2018, t he  arthropod  captive rearing laboratory at the Hawaii Department of Land and Natural Resources  began  a program to rear and release M. xanthomelas naiads (aquatic immatures) with the intent to establish additional populations on Oahu. Initial rearing efforts yielded a high survival rate, but naiad coloration was much lighter than that observed in the wild, and the reasons for this were unclear.  We were concerned  that the pale coloration  may  increase vulnerability to predation, and that it may be a symptom of a nutrient or mineral deficiency.

We tested the effects of diet supplementation and substrate variation on naiad developmental rates and pigmentation in captivity. To measure the effect of diet diversity, we compared naiads fed an exclusive diet of brine shrimp (Artemia spp.) to those fed brine shrimp and a diversity of freshwater zooplankton. To explore the roles of substrate color and composition, we compared naiads reared on six different substrates spanning a wide range of colors and textures.

We found that naiads  fed a more diverse diet developed significantly faster and were slightly darker than naiads fed only brine shrimp. However, the color difference between the two groups, though statistically significant, was subtle , and  supplemented naiads were still much paler than those observed in the wild . Substrate had a much greater effect on naiad coloration (Fig. 1) , with  naiads  raised on dark backgrounds being significantly darker than those raised on light backgrounds, and more closely matching the color of wild naiads. These results have been used to modify  our rearing protocols for this endangered insect . Naiads are now fed a diversity of zooplankton  to promote faster growth,  and their rearing cups are lined with dark gravel to produce a more natural pigmentation.