What Lies Beneath (part 4) – Harpacticoida

The Harpacticoida are a mainly free-living benthic aquatic organism, distinguished by other copepods by the presence of a short pair of first antennae. The greater majority of those found in freshwaters are benthic or interstitial, that is living between the grains of sand and gravel within inland waterways, although many species have successfully exploited other habitats, including semi-terrestrial habitats. They have even been living found in pitcher plants in the temperate rainforests of Tasmania.

Photo courtesy Robert Berdan

 A harpacticoid copepod begins life as a fertilized egg, which then hatches into a nauplius larva. The nauplius has six stages. With each moult, the body of the nauplius elongates and additional appendages appear.  

In the copepodite stages which resemble the adult, distinct body segmentation appears, appendages are added, and older appendages develop further.  

The duration of development from egg to adult is governed primarily by temperature. A complete cycle may take three to four weeks at warmer temperatures and up to several months at low temperatures. Some species can be found year-round, others only during the warmer months, and yet others only during late fall, winter, and early spring. The development rate can also vary depending on the productivity of the waterbody. 

In addition, growth rates vary among species. As a result, a species may have one to several generations per year in a given body of water. 

Under unfavourable conditions, harpacticoids may produce resting eggs or encyst as adults to bridge the gap between seasons or periods of drought. The cyst encasing an adult is a hollow sphere of silt and detrital material glued together with secretions. The main function that encysting serves is to protect individuals from prolonged dehydration. 

In addition to improving a copepods chance of survival under adverse conditions in its home body of water, these resting stages provide a mechanism for the transfer of species from one locality to another, by wind, water or passive transport on waterfowl, animals or even humans.

Harpacticoid copepods occur in a variety of aquatic and semi-aquatic habitats. In standing or flowing water, they may be found in association with aquatic plants, detritus, or bottom sediments. They also may be found in interstitial sands or shoreline debris of streams, rivers, lakes, springs, and seeps; sometimes up to several meters above the shoreline. In general, they are found to a depth of 40 m, with the highest abundances found in littoral areas. 

Harpacticoids are primarily surface-feeders that use their mouthparts for scraping or seizing food from a variety of substrates. They may consume algae, fungi, protozoa, bacteria, and detritus. Carnivory has been reported in harpacticoids, but predation is not thought to be an important mechanism for obtaining food in most species.   

Harpacticoids are very important as primary and secondary consumers and are a major food source for both larger invertebrates and many small or juvenile fish species. Researchers have found that stream harpacticoids consumed up to 22% of the daily carbon production.  

Harpacticoid populations may double in size in less than two weeks, under optimal food and temperature conditions. It is not uncommon to find densities of harpacticoids ranging from 30,000-40,000/m2 in lake systems of the northern hemisphere. 

The Harpacticoids are a very diverse group of copepods both in terms of morphological diversity and in the species-richness of some of the families. However, because of their small size and habitat, they are frequently overlooked in ecological studies.