Are relatively low densities of nocturnally-active freshwater shrimps an important component of energy flow in tropical streams draining mainland Costa Rica

Background/Question/Methods

While freshwater shrimps can comprise an obvious and significant proportion of the overall invertebrate biomass and secondary production on tropical island streams, little is known about their role in mainland tropical streams where they often occur at much lower densities and are largely-nocturnally active.  In this study we estimate density, growth rates, and secondary production for three species within the shrimp genus, Macrobrachium, in seven stream reaches within  La Selva Biological Station (LSBS), a mainland site (1600 ha) in Costa Rica, Central America. We determined shrimp growth rates of M. olfersi, M. carcinus and M. heterochirus (>14 mm carapace length)  in situ using mark-recapture of individually marked shrimp at LSBS. Generalized linear models were used to investigate environmental factors (discharge, substrate, and solute level) affecting growth rate and secondary production. Huggins Closed-Capture Models were implemented in Program “Mark” to derive the probability of detection (p) and probability of recapture (c), which were then used to estimate abundance. Finally, for one of our study streams, shrimp biomass and secondary production were combined with aquatic insect invertebrate biomass and secondary production (generated by our previous studies) to estimate total invertebrate biomass and secondary production.

Results/Conclusions

Surprisingly, we found that shrimps account for the majority of invertebrate biomass in streams at our mainland tropical site in Costa Rica, despite their relatively low abundance:  Model-averaged densities of M. olfersi  and M. carcinus ranged from 0-2.83 individuals m^-2 .and  0-0.21 individuals m^-2, , respectively.  Nonetheless, shrimps represents 94% and 87% of the standing invertebrate biomass in the wet and dry seasons, respectively.  Mean instantaneous growth rates for M. olfersi  and M. carcinus were 0.002 g DM g^-1 day^-1 (SE+ 0.0003) and 0.004 g DM g^-1 day^-1, SE+ .007), respectively. Also, we found that M. olfersi growth rate was influenced by sex, original biomass, and density of M. carcinus.   Lowland rainforest streams draining LSBS yielded a surprisingly high rate of shrimp secondary production; the highest estimate of secondary production for any Macrobrachium spp. that has been published. Total shrimp production at a site ranged from 0.005-10.83 g DM m^-2 yr^-1. Shrimps clearly contribute a significant percentage (40%) of stream invertebrate secondary production at our mainland study site in Costa Rica. Our findings indicate that shrimp can be an important component in the energy flow of mainland tropical streams.