First evidence of tumor-like anomaly infestation in Copepods from the central Indian Ridge

Abstract

To investigate the distribution and abundance of mesozooplankton in the Indian Ocean, zooplankton sampling was conducted along the Central Indian Ridge (CIR) from the equatorial waters to 8 degrees S latitude. Of the seven sampling locations, cysts or Tumor Like Anomalies (TLA) of ectoparasites like Ellobiopsis sp (Protista, Ellobiopsidae), being known as TLA were observed at the 5 locations. The highest frequency of TLA was observed at Stn. MPN-03 at all the 3 depths sampled (0-500 m, 500-1000 m and 1000-1500 m). Frequency of infected specimens among all the sampled copepods varied from 3.0- 6.9%. Among the infected genera, Oncaea sp (Copepoda: Poecillostomatoida) was the most dominant accounting 21.2% followed by Clausocalanus sp (13.6%), Centropages sp (12.9%), Pleuromamma sp (9.4%) and Acrocalanus sp (7.8%). Two types of tumors were observed. Type 'A' was elongated and full of small granular structures in the tumor and type 'B' was spherical with few or no granules in the structure. Type 'B' was the most common that dominated in terms of frequency (96.60%), where as type 'A' which was observed rarely, was in 3.4% of the total parasitized population. Consequent to the physico-chemical anomalous signatures of active hydrothermal mineralisation recorded at station MPN-03, it is hypothesized that high levels of potentially toxic chemicals erupted from the hydrothermal plume caused the weakening of the exoskeleton of Oncaea and other copepod species, making them more susceptible to the parasitic attack, especially to ectoparasites like Ellobiopsis sp. These ectoparasites attack the host and feed on their body fluids which may lead to the death of the host. This study emphasis the consequences of parasitic infection in dominant planktonic copepods like Oncaea to the deep-sea food chain