How Efficient are Emerald Crabs Mithraculus sculpus as Bubble Algae Valonia spp. Controllers?

Event Name/Location

Aquaculture America 2006 and Marine Ornamentals 2006, Las Vegas, NV, February 13-16, 2006

Document Type

Conference Proceeding

Publication Date



Emerald crabs (Mithraculus spp.) are used to control nuisance algae particularly, bubble algae (Valonia spp.) in marine reef aquaria. Although Mithraculus crabs feed on algae, they are omnivorous and can be fed with frozen as well as formulated food. The objective of this study was to determine the feeding efficiency of M. sculpus on Valonia spp. in the presence of alternative foods, particularly those that are commonly used to feed fishes in reef aquaria. Crabs were maintained in a 210L tank at 26°C, 35 ppt salinity, 14L:10D photoperiod and fed on Valonia spp., frozen mysis and pellet food (MarineGro, Red Sea Fish Pharm Ltd.). Crabs of three size classes (small <1.5g, 1.5 ≤ medium < 3g and large ≥ 3g) were used to conduct the following experiments: (1) Control: crab was offered 1g Valonia spp. only. (2) Alternative food 1: crab was offered simultaneously 1g Valonia spp. and frozen mysis in excess; (3) Alternative food 2: crab was offered simultaneously 1g Valonia spp. and pellet food (MarineGro, Red Sea Fish Pharm Ltd.) in excess. Ten replicate tanks (40L) of each treatment (1 crab per tank) were carried out at 26°C, 35 ppt salinity and a 14L:10D photoperiod. . Food preference as well as food handling and processing time was recorded. At the conclusion of each trial (6 hours), uneaten Valonia spp. was removed and weighed. Experimental results indicated that emerald crabs do feed on Valonia spp., however, they frequently preferred the alternative foods when provided. The weight of Valonia spp. ingested during the 6-hour experiment also decreased significantly when other food was provided. There was a positive correlation between crab weight and Valonia spp. consumed; however, there was no significant difference in algal consumed between the weight classes of the crabs. To determine the relationship between food preference and time/energetic cost of processing each of the food items, experiments were conducted to determine handling time of different bubble algae sizes (small ≤0.03g, 0.03 < medium ≤0.1g and large >0.1g), frozen mysis and pellet food (MarineGro, Red Sea Fish Pharm Ltd.) by crabs of different weights and sexes. In an attempt to further understand the feeding performance of M. sculpus while feeding on Valonia spp, we recorded the feeding behaviour of M. sculpus using digital high-speed videography. M. sculpus grasp and tear the bubble algae with their claws, and then ingest pieces of the algae. Review of the video suggests that the liquid inside the bubble algae that contain young Valonia spp. spores is released into the water, which may contribute to algal dispersal. Results of this study contribute to our understanding of the efficacy of emerald crabs as aquarium-algal cleaners.