Abstract

Freshwater baths are a common treatment method to remove external parasites on a variety of saltwater species, particularly invertebrates.¹ Very little information exists in the literature on how this abrupt change in salinity impacts these animals' ability to osmoregulate and maintain normal body homeostasis. One study in American lobsters documents a drop in serum electrolytes with low salinity baths up to 1 hour in duration.² The objective of this study was to evaluate the effects of a 5-minute freshwater bath on hemolymph concentrations of electrolytes in two species, the Jonah crab (Cancer borealis) and the horseshoe crab (Limulus polyphemus). The horseshoe crab tolerates a wide range of salinities,¹ whereas the Jonah crab is more stenohaline than other members of the Cancer genus.³ Observations by aquarists suggest the Jonah crabs appear more agitated with freshwater bath treatment when compared to horseshoe crabs. We hypothesized that this agitation was secondary to disturbance in osmoregulation, and that this disturbance would manifest as changes in hemolymph concentration of electrolytes.

All individuals in the study were acclimated to a full salinity system at 33.7 ppt for 24 hours. The Jonah crabs were divided into a control group (n = 8) and a freshwater treatment group (n = 7). All horseshoe crabs were subjected to a freshwater bath (n = 10), and hemolymph was collected immediately before and after. A Welch's t-test was used to compare the difference in hemolymph concentrations of sodium (Na), potassium (K), chloride (Cl), and calcium (Ca) between the Jonah crab control and treatment groups. Results show a significant difference in Na and Cl concentrations (p < 0.001) between groups. Mean Na in the treatment group dropped from 469 mmol/L (± 5.0) pre-bath to 457 mmol/L (± 8.4) post, in comparison to the control group, which rose from a mean of 472 mmol/L (± 9.6) to 476 mmol/L (± 8.8). Cl concentrations dropped from a mean of 519 mmol/L (± 9.0) pre-bath to 503 mmol/L (± 11.3) post with the opposite trend for the control group with Cl rising from a mean of 520 mmol/L (± 14.6) to 522 mmol/L (± 17.8). Horseshoe crab data was analyzed using a paired t-test, and results show less significant drops in Na and Cl, with mean Na at 448 mmol/L (± 7.9) before treatment and 445.4 mmol/L (± 10.2) after (p = 0.05), and mean Cl at 549.3 mmol/L (± 9.6) before and 544.6 mmol/L (± 12) after (p < 0.05). Potassium concentrations significantly increased (p < 0.01) over treatment from a mean of 11.89 mmol/L (± 0.34) to 12.15 mmol/L (± 0.15). Concentrations of Ca did not differ significantly across groups or species. These findings suggest that a 5-minute freshwater bath is sufficient time to disrupt body salt homeostasis in Jonah crabs and that horseshoe crabs appear more tolerant to abrupt changes in salinity. With low-salinity baths causing serum electrolyte disturbances in American lobsters for up to a week,² this simple treatment may cause long-term disruptions in osmoregulatory balance, and greater understanding of species sensitivities is important in future applications of this treatment method.

Acknowledgements

The authors wish to thank Mystic Aquarium's Samantha Dale, coldwater gallery aquarist and Rebecca Bray, medical aquarist, for technical assistance and planning associated with this project. The authors also wish to thank Dr. Paul Anderson and Justin Richard for statistical advice.

* Presenting author
⁺ Student presenter

Literature Cited

1.  Lewbart GA. Invertebrate Medicine. Ames, IA: Blackwell Publishing; 2006:303.

2.  Speare DJ, Cawthorn RJ, Horney BS, MacMillan R, MacKenzie AL. Effects of formalin, chloramine-T, and low salinity dip on the behavior and hemolymph biochemistry of the American lobster. Can Vet J. 1996;37:729–734.

3.  Charmantier G, Charmantier-Daures M. Ontogeny of osmoregulation and salinity tolerance in Cancer irroratus; elements of comparison with C. borealis (Crustacea, Decapoda). Biol Bull. 1991;180:125–134.