Feeding and related biology in the masking crab Notomithrax ursus (Herbst 1788) (Oxyrhyncha: Majidae)

This study is an investigation of feeding and related biology of Notomithrax ursus (Oxyrhyncha:Majidae). N.ursus is a spider crab which masks itself with materials from its surrounding environment, particularly algae. Of particular interest was the investigation of the hypothesis that the algal mask of N.ursus functions as a food store. N.ursus was observed to remove a variety of red, green and brown algae from its mask to eat. Mask ingestion involved the removal of algae from the carapace, with removal from the legs occurring less commonly. When there was no other available source of food, N.ursus removed an average of 4.1% of the mask per day to eat. When there was an alternative source of food (mussel flesh), mask ingestion still occurred, but at a significantly lower rate, 2.8% of the mask being consumed per day. Although N.ursus removed algae from its mask to eat, it was not observed to remove and eat other types of masking material such as bryozoans or sponges. N.ursus was shown to prefer masking with some species of algae more than others. The types of algae most preferred as masking material were branched algae such as Halopteris spicigera and Corallina officinalis. Thallose-like algae, such as Ulva sp., and strap-like algae, such as Cladhymenia oblongifolia, were less preferred. It was also shown that algae N.ursus prefers masking with are also the algae that N.ursus prefers to eat. Examination of the morphology of the feeding apparatus and diet of N.ursus revealed that it has an omnivorous diet, with no specialisation. The setose mouthparts and gastric mill were well-developed and generalised in structure. They were also similar in structure, as well as in the way they were used, to those of other macrophagous brachyuran crabs. No variations in the mouthparts and gastric mill were evident for crab size or sex. The left and right chelae of N.ursus were morphologically identical in shape and dental pattern. The chelae of juvenile male and female crabs were similar in shape and size but the chelae of mature male crabs were enlarged. Relative growth of the chelae was described from measurements made on the propodus height and length, using carapace width as the reference measure. Relative growth of the chelae was positively allometric for both male and female crabs. Growth of the female chelae remained constant, but the chelae of male crabs exhibited discontinuous growth with an increase in allometry after the pubertal moult. The chelae of female crabs had a mean mechanical advantage of 0.200, while those of male crabs had a higher mean mechanical advantage of 0.284. The natural diet of N.ursus from the intertidal zone at Oaro, was examined over a 13 month period from November 1990 to November 1991. Examination of stomach contents revealed that N.ursus is an omnivore, feeding on a variety of food items. Algae was the single most important dietary component. Small gastropods, isopods, amphipods and decapods were also significant dietary components. Other types of food eaten by N.ursus included bryozoans, polychaetes, sponges, ostracods, nemerteans, copepods and bivalves, however, these were relatively unimportant as dietary components. Seasonal variations in the consumption of various food types by N.ursus, particularly algae, were observed. Although they fed on similar food materials, differences in dietary composition were evident for crab size and sex. Observations on the range of intertidal organisms that N.ursus could include in its diet revealed that N.ursus can feed on a wide range of both animal and plant organisms. N.ursus was not observed to prey on bivalves or large gastropods. The location of food items by N.ursus is mediated largely through mechano­ and chemoreception; eyesight does not seem to play a large role in food location