Funny how being a Masters student makes you ask such fundamental questions, when I should have asked this question to myself when I first started studying biology.
We tend to take so many things for granted. If we humans consume food by inserting it in our mouths, chewing it and swallowing it, does that mean all other organisms do the same? Well, birds have a mouth, so do snakes and turtles, and frogs and salamanders, and even fishes. Food enters it in a way similar to our own, and they just…eat! Well, not really. And anyway this list covers only the vertebrates which account for 2 per cent of all life on earth. What about the rest of the animals?
I was fascinated today to learn that a basic process such as feeding is carried out in a myriad different ways. Animals such as star fish use sieves to filter feed. Tentacles are used by worms. Mucus gets coated on these tentacles and food particles stick to them, which is then consumed by the organism. Many animals trap food particles in mucous strands, webs, or sheets secreted from the mouth or some other specialized feeding structure and ingest the food-laden mucus. These are all examples of ‘Suspension Feeding’, where an animal removes food particles suspended in the ambient medium, (mostly water but sometimes air, a spider ‘filter feeds’ via its web) and consumes it.
There are 3 other major types of feeding patterns- manipulative mechanisms for acquiring large food particles, sucking fluids or soft tissues and surface absorption of nutrients with no specialized feeding mechanism and often no digestive tract.
You might expect suspension feeding, to be present in phylogenetically older animals. But surprisingly, this feeding pattern exists in chordates as well. It is rare in vertebrates, which comes under the group of chordates, but is present nonetheless. For example, tadpoles filter feed. And so do baleen whales. One of the largest animals on earth, the blue whale depends on the tiniest organisms to sustain it! There are examples of birds filter feeding too. The highly modified beaks of flamingoes act as seives, helping them eat algae, diatoms ans small crustaceans.
Manipulative Mechanisms For Acquiring Large Food Particles- A number of animals simply swallow large inactive food particles. Crocodiles’ teeth are meant for grabbing prey but they can’t tear meat apart. You must have seen some documentary on the National Geographic where 2 crocs are holding a piece of wildebeest at 2 ends and rotating on opposite sides to break it apart, like the way you unwrap a candy from its wrapper. They just gulp large food pieces down their throat. The egg-eating snake dasypeltis is a remarkable example of this ability to swallow large objects. Not only is swallowing large eggs a seemingly impossible task, but so is breaking the egg shell and the shell membranes inside the stomach quite difficult. Since these animals don’t chew their food, I suppose they must have some pretty strong enzymes in their gastric cavities to do the job of breaking the food into its constituent particles.
Surface Nutrient Absorption- In contrast to this, some parasites that live inside other organisms, i.e. endoparasites can absorb the required nutrients directly from their rich environment. For example, consider a tapeworm residing in the gut of a human or pig. The surrounding environment is full of rich nutrients. It simply absorbs these nutrients to sustain itself. This capacity has totally eliminated the need for a digestive system and indeed these organisms do not possess any digestive apparatus as such.
Sucking Fluids or Soft Tissues- Many animals feed on fluids or soft plant or animal tissues by piercing and sucking. Many invertebrates use this type of feeding pattern such as mosquitoes and butterflies. But, as before, vertebrate examples do exist, such as the vampire bat.
These examples are just a tip of the iceberg. Any basic physiology textbook abounds with many many more such examples. What we can see from this, I guess, is that a given goal (ingestion of food) can be performed in so many different ways. The environment these animals lived in acted as a selection pressure for the evolution of these feeding patterns. Maybe prehistoric tapeworms did have a digestive tract, but it slowly degenerated as it was of no use. But at the same time, they may have had to evolve better membrane porosity to let the surrounding nutrients through. A tough job considering they should have strong membranes to live in an environment that also has protease enzymes- enzymes that can break a membrane. Afterall, they are living in a place where all organic matter gets broken down into its constituent molecules.