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Six-legged mermaids

A major portion of our planet is swaddled in water bodies and host a wide variety of organisms. Fishes would be the first to pop up in our heads, but we also have mammals and reptiles and oodles of invertebrates. One of them is beetles, or water beetles to be precise. What do we know about these? Well, for starters, the adults of the aquatic beetles live submerged fully or partly in water and belong to the insect order called Coleoptera. They exhibit a wide range of characters, both physically and behaviour-wise to suit the underwater environment. The body of these ‘true water beetles’ is designed for swimming as opposed to their land-mates.

A group of researchers from Pune, Maharashtra set out on a journey to identify and study aquatic beetles present in manmade ponds of Western Ghats. The ponds which were part of the study are 40-year-old abandoned stone quarries. From these ancient water reservoirs, they collected and identified a total of 22 water beetle species!

One of the study sites for collecting aquatic beetles

Photo: Sheth et al.

Each of them was different from the other in so many aspects. To pick one out, they all like different snacks. Some munch on algae (called algivores), while some are happy with junk (yep, the detritivores that we have heard of) and some are plain old herbivores. There are also a bunch of them that are vicious predators. The researchers decided to classify them into 6 different groups according to their body shape and based on how they liked to swim. The group one members are easily the Michael Phelps among the beetles. They are the fastest swimmers and their body is designed for the same. They are super flat and narrow with short, compact heads. However, the second group are totally opposite! They are round and chubby and just flap around in globular patterns. We cannot really call group three members as the experts in the swimming department, probably due to their undefined bodies. Members of group four have broad front ends and move in a convex manner, while groups 5 and 6 are a mixed bunch. The researchers did not note them to be having particularly extreme characters. They probably find food by complex techniques like scavenging and thus needn’t be great swimmers.

Image showing different beetles with different swimming types

All these different beetles with all these swimming types! Why do they all do it differently though? It indeed serves a purpose. Swimming competitions may be just competitions for humans but it aids these Coleopterans to perform essential activities. It is through their delicate swimming techniques that they cunningly entangle their prey and swiftly escape from their enemies and even find their partners to make babies with. Pretty interesting, huh?

The researchers discovered that the speed of their swimming and their movement type is determined by the shape and size of their bodies. If we look at flat streamlined species, they are fast swimmers, but not that good at turning (Something to do with physics and reduced drag forces). This implies that the difference in body size of these beetles may cause them to perceive the same habitat differently. As a result, they all have exhibit variation in their ‘lifestyle.’ They hunt in diverse patterns, like dissimilar shelters and even have distinct methods and preferences for laying eggs. What’s more? As in the case of other animals, body size affects their metabolic rates and growth. Their interactions are shaped by how big or small the beetles are. This in turn controls how the food webs are formed.

Let’s see a few cool observations that helped the researchers propose this theory. We have two predatory beetles in the picture; Laccophilus parvulus and Hydaticus satoi (from the beetle Olympics gold medal team). The general trend was that whenever two predatory beetles were found together, their body size differed by a ratio of 1.3 or more. These beetles tend to attack preys that are proportionally smaller in size. Our protagonists L. parvulus and H. satoi differ by a size ratio of 3.98. Hence they prefer preys of different sizes.

Therefore, with less competition, easy coexistence, both parties end up happy (and alive).

Predatory beetles: Hydaticus satoi, Laccophilus parvulus with a difference in the size ratio of 3.98. Photo: Sheth et. al.

But how do these predatory beetles hunt? Do they stealthily wait in the dark until the prey comes or do they daringly jump in and attack? Well, our predatory beetles are active prey capturers. They have streamlined bodies that can help them swim fast (might come in handy in chasing when food gets flighty). Amidst the shady dark areas with lots of submerged plants like the brave warriors they are. This could be because the greater the vegetation; the greater the prey base.

When the vegetation changes or gets complex, this affects the type and availability of the residing prey species. Due to this, aquatic vegetation is one of the important factors for deciding the diets and roles of the species involved in the scene.

But some species have similar body sizes too! Could they be co-occurring in harmony? Yes, but conditions applied. We have another set of beetles Hygrotus musicus and Berosus pulchellus that hang out in the same area. They are almost the same size. However, the first one is a predator and a maneuverer whereas the second one is a scavenger species that barely swim. Since their food habits are different, they need not fight.

Here we see that it is the body size or other physical traits of water beetles that are influencing their swimming abilities. What does this imply? We can predict the abilities of the organism by observing its morphology and can understand how it could affect their fitness by influencing their growth, survival, and reproduction. Imagine looking at a beetle and foreseeing how its life would turn out! Scientists say that characters of an organism have functional impacts affecting their ecological activity and are important for even the formation of groups. It is the traits of the individuals that shape the community.

However, it is not just the organismal traits that have all the power. There are certain abiotic factors also that play an important role in shaping aquatic communities. For example; calcium hardness and salinity can affect the distribution of these beetles. The beetle species in this pond have developed a tolerance for these factors during evolution. They have been specialized for this way of life and this relationship of water and beetles grew over 300 million years of evolutionary period. That is one long relationship indeed!

The study elucidates the relationships between environment and physical characteristics of co-occurring water beetles in anthropogenic ponds from the Western Ghats, India, for the very first time. Natural and man-made ponds are precious reservoirs of biodiversity that harbour many unique species like our six-legged mermaids. It is in these little spaces that they come alive, grow, thrive, feed, meet their mates and die. These researchers have put their magnifying lenses on all the drama happening down there, and we are entitled to conserve the same.


Source: Sayali D. Sheth, Anand D. Padhye and Hemant V. Ghate (2021). Effect of environment on functional traits of co-occurring water beetles. Ann. Limnol. Int. J. Lim., 57,2


Article written by: Apoorva Gopinath, edited by Anuja Vartak A huge vote of thanks to Dr Sayali Sheth for fact-checking the article and providing us with all the required media.

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