M. Fields, et al. Infection of the planktonic copepod Calanus finmarchicus by the parasitic dinoflagellate, Blastodinium spp: effects on grazing, respiration, fecundity and fecal pellet production. J. Plankton Res (2014) 0(0): 1– 10. doi:10.1093/plankt/fbu084
Copepods are the most abundant multicellular organisms found in the ocean (Fig. 1). They are dominant grazers of phytoplankton and provide an essential link in oceanic food webs. Accumulated carbon is deposited by copepods in packaged fecal pellets that sink down through the water column. This carbon becomes an available food source to creatures on the seafloor. Therefore, small changes in copepod abundance, reproduction and grazing rates can have enormous impacts on entire marine environments. As can be expected, it is not all “fun and games” for these zooplankton. Copepods, mainly females, lie victim to frequent parasitic infection from smaller organisms called dinoflagellates. Dinoflagellates are free-swimming microorganisms that can act as primary producers, carnivorous predators, or even parasites.
The dinoflagellate Blastodinium spp. is the primary culprit and lives in the gut of infected female copepods. During early life stages, copepods ingest unicellular forms of the parasite, which are called dinospores. Once inside the gut, the parasitic dinoflagellate divides into a multicellular structure called a trophont. This parasitic invader continues to grow with the copepod host! At maturity, Blastodinium spp. is released through the anus of the host copepod as free cells or contained in fecal pellets. Though non-lethal, not much is known about Blastodinium spp. occurrence and infection. In a collaborative effort, researchers set out to elucidate the impacts of Blastodinium spp. on copepod reproduction, grazing, fecal pellet production and respiration.
The copepod, Calanus finmarchicus, was sampled at two locations off the coast of southern Norway during April 2013 and 2014. The collected copepods were sorted and a portion of females examined under microscopes. The frequency of Blastodinium spp. infection was determined by observing the presence of parasites in the gut of female copepods. Four females were dissected to remove the trophont from the gut (Fig. 2).
Subsets of experiments were conducted to compare the functional responses of infected versus uninfected female C. finmarchicus. The amount of grazing and respiration per day was determined for both infected and uninfected copepods. Additional parameters such as daily copepod egg and fecal pellet production were also assessed. All experiments were done in replicate and copepods were fed on diets of their favorite algae, Rhodomonas Baltica.
Overall, proof of infection was present in 58% of collected copepods. When fed, uninfected female copepods ate significantly more food than infected females (Fig. 3). A healthy copepod is generally a happy one and in this case consumed on average 2.93 x 104 cells/day. Infected female copepods did not register a measurable grazing rate over the 24-hour period (They were not happy!). Respiration rates followed suit; Blastodinium-infected females respired at nearly half the rate of uninfected females.
Uninfected and infected copepods were fed Rhodomonas Baltica for four days and daily fecal pellet production rates (FPRs) were measured (Fig. 4). By the fourth day, the mean FPR was 2.5 times higher among uninfected females than for the infected (Starting to sense a trend?). In addition to producing fewer pellets over time, infected female copepods had much smaller sized fecal pellets (~ 417% smaller on average than healthy copepods). Finally, egg production rates (EPRs) were measured and to no surprise were found to be higher for uninfected females. Healthy copepods produced mean rates of 20.8 eggs/day, whereas infected copepods had mean rates of 0.6 eggs/day.
On average, infected female copepods had lower rates of respiration, consumption, fecal, and egg production compared to uninfected copepods. In this study, infected copepods did not ingest any prey and were essentially starved. Extended periods of starvation has been shown to lead to underdeveloped or disintegrated ovaries as well as decreased copepod respiration. Infected copepods that cannot properly feed or respire will be less fit to survive and reproduce in their environment. Equally important, a reduction in copepod egg production means less food will be available to larger organisms in the food web.
Infected female copepods produced fewer, less healthy fecal pellets. Fecal matter excreted by copepods represents an important source of energy for many marine organisms, particularly those on the seafloor. Thus, infection from Blastodinium spp. could strongly impact the amount of organic matter that reaches deeper waters. The effects of Blastodinium spp. infection may alter the survival success of individual copepod species. Furthermore, organisms that directly prey on copepods or indirectly benefit from copepod fecal matter could be in serious jeopardy. The exact distribution of Blastodinium spp. in the ocean is still relatively unknown. Further investigation is needed to understand this parasitic invader and the long-term ecological consequences associated with it.