Who knew that creatures traditionally used by people as natural bath sponges could be the longest-living animals known?

One step further from a colony of unicellular animals, an individual sponge is a multicellular and largely immobile animal. These fascinating organisms adopted all sorts of interesting strategies in the fight for survival and they seem to have made it.

Most sponges feed by filtering food particles. Hence they live in quiet, undisturbed waters. Since water flow is so important to extracting food and oxygen, they evolved something we could only dream of: sponges are able to remold themselves. In other words, they can change their body shape according to the environment. This card allows them to mold according to their substrate, incrusting rocks and other hard surfaces such as shells and corals.

Sponges are able to change their body shape by employing two mechanisms:

• The cells making up their external layers such as pinacocytes and choanocytes are not bound tightly like epithelial cells in our skin and mucosae.
  

• Their endoskeleton called the mesophyl can be continuously remodeled by specialized cells, the lophocytes.
  

All this can be summed up as living on unstable ground. Yet flexibility pays off.

There are two features potentially immortal species have and sponges make no exception. They reproduce asexually and their adult somatic cells are pluripotent.

Let’s take them one by one.

Sponges reproduce sexually. Most of them are hermaphrodites, the same individual producing sperm and eggs, without having true gonads. Yet they preserved the ability to reproduce asexually and they do this in 3 manners:

• by fragmentation

• by budding

• by producing gemmules

Not all sponge fragments are able to recreate another individual from scratch. In order to do that, the fragment must contain at least these two types of cells: the collencytes which produce the mesohyl ( their endoskeleton) and archaeocytes from which all other types of cells are derived.

When times are stressful, usually when temperature drops, sponges – and many other species – degenerate into gemmules. These tiny ‘survival pods’ stay dormant until better times come and the little creatures regenerate. Often such gemmules are retained within the parent sponge.

Asexual reproduction is not enough. Many species reproduce asexually without displaying extraordinary longevity. The three Rs come next: reorganization, rejuvenation, regeneration. Sponges have a huge advantage here: they are simple creatures. This time minimalism pays off. Sponges have no true tissues. They have between 5-10 cell types depending on the species. They have no body symmetry. A symbol of flexibility, dare I say. Most sponge cells are able to move around the body. A few of them are able to dedifferentiate – transforming themselves from one type of differentiated cell into another, by-passing the usual stem cell route. There is another interesting ability in some sponge species: if you take one such animal, blend its contents and put those cells into water, they will reorganize themselves into a new sponge. How cool is that?

I previously mentioned that one fragment must contain archaeocytes in order to recreate a sponge individual. Archaeocytes are totipotent cells. They can differentiate into any type of cell the sponge needs. This ability is totally lost in species like humans which are only able to express totipotency as embryos.

Sponges lack a complex immune system like we do, yet they are able to reject grafts from other species. What is fascinating though is that sponges will accept grafts from members of their own species.

I mentioned sponges being potentially immortal. The word ‘potentially’ is important here, because accidents are a part of the sponge’s life just like they are in our lives. They are predated upon by echinoderms, turtles and some fish.That is when they aren’t turned into commodities such as natural bath sponges for human use.

But sponges understand the sharing economy better than anyone. They collaborate freely with anyone that will give them an advantage in survival.

For example, they’ll often team up with photosynthesizing organisms: green algae, cyanobacteria, dinoflagellates. What’s more, glass sponges – one of the three main classes of sponges, the other two being calcareous sponges and demosponges – have silica spicules which conduct light into the mesohyl, the endoskeleton where the green algae live in symbiosis with the sponge. The sponge provides safety with plenty of light and the green algae provide some oxygen and organic matter as food. That looks like a fair barter to me!

Sponges will often host shrimps too. Each shrimp species of the genus Synalpheus will inhabit a different sponge species, enjoying not only safety from its host, but the larger food particles the latter can’t digest.

Sponges are the animals with the maximum known lifespan. They lack any protective shell. Sponges are immobile, lacking any means of escape. But they are flexible and evolved to synthesize a variety of unusual substances which pave the way to better drugs for humans.

Their lifespan varies wildly. Some sponge species survive for only a couple of years. At the same time, some desmogens grow their spicules very slowly at a rate of 0.2mm/year. The spicules is where researchers look for the chronological age in such species. If we suppose the growth rate is constant, then a sponge with a diameter of 1 m could have at least 5,000 years old. Another Antarctic specimen is estimated to be around 15,000 years old according to its growth curve.

Sponges are fascinating organisms that can be easily grown at home, not to mention in a lab. Why aren’t they used as a biological model of aging or rather the lack of aging?

 

Note: This blog post is an excerpt from ‘The aging gap between species’ book.

Anca Ioviţă is the author of Eat Less Live Longer: Your Practical Guide to Calorie Restriction with Optimal Nutrition ,The Aging Gap Between Species and What Is Your Legacy? 101Ways on Getting Started to Create and Build One available on Amazon and several other places. If you enjoyed this article, don’t forget to sign up to receive updates on longevity news and novel book projects!

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