Dicyema erythrum (Phylum Dicyemida) (Image: Used with the kind permission of Professor Hidetaka Furuya)

Yes, of course

Odd sort of question, if I may say. Now then, animals (grown-ups call them Metazoa) certainly evolved from the protistans (which in the old days were named Protozoa), but so far as the former are concerned their nearest chums go by the name of choanoflagellates. Yes, these are protistans and belong to an enormous group (the unikonts if you need to know, more or less meaning “single flagellum”) that happens to include amoeba. But the relationship between any animal and an amoeba is pretty remote. So no traction here. Maybe you were thinking of size? Certainly animals as tiny as an amoeba are known. Please step forward those masterpieces of miniaturization in the form of tiny insects that fly using a comb and have brains the size of a human neuron. Yet despite their Lilliputian status not only are they fully functional, but the economy of construction can be mind-boggling.

In this context consider a marine group known as the cycliophorans. Here the males are dwarfed (not as rare as you might think) and at 40 μm in length are twice as small as any arthropod. But are the boys correspondingly simplified? Not a bit of it! With a hundred cells or less, they still construct a complex body, complete with nervous system, muscles and those all-important gonads. No traction here either. Alright, what about the parasites known as the myxozoans? Never heard of them? You certainly would if you were running a fish-farm where they are a major scourge and induce unpleasantness in fish such as whirling disease. Certainly myxozoans are very protistan-like. But parasitism plays strange tricks on animals and all the evidence points to myxozoans actually being derived from jellyfish and cleverly redeploying the iconic stinging cells (the nematocysts) as attachment devices to grasp the host tissue. So we don’t seem to be any closer to making an animal into any sort of protistan, do we?

No, don’t be ridiculous

Think again! Consider the mesozoans. How did they come by their name? Good question, because for a long time they were thought to represent a half-way house between the Metazoa and Protozoa. In fact they are not only animals, but belong to a group (in the trade known as the lophotrochozoans) that includes the squid and octopus (cephalopods). And that is where you need to go to find mesozoans, where they form dense thickets on the surfaces of the kidneys. Despite retaining genes like Pax-6 (that help to make the camera-eyes of their hosts), the mesozoans have made a very fair stab at turning themselves into protistans. The body is hugely simplified, with no trace of a gut or nervous system but consisting of about 40 cells equipped with beating cilia and a unit that allows them to attach to the kidney. But it gets better because whilst bottom-dwelling cephalopods play host to these fake protistans, the kidneys of their swimming counterparts are infested with remarkable look-alikes but these are the genuine article in the form of ciliates (belonging to the chromidinids).

It all depends on the question

Spending a life drenched in octopus urine may seem a pretty short straw, but mesozoans provide object lessons to keep any biologist happy. First, despite their apparent simplicity they have an engagingly complex life-cycle and a nifty way of constructing their sperm that breaks all the rules. Next object lesson is their convergence with the protistan ciliates. This extends to a similarity not only of body shape, but mode of attachment and life cycle. All this suggests that if you are going to spend a life thrashing around in octopus pee then there is only one way to do it. Then there is the question: are they just parasites or actually favoured symbiotic guests? The latter appears to be the case, but there is a further intriguing thought. In many ways squid and their relatives are honorary fish and are connected by all sorts of convergences that go far beyond the famous example of the camera-eyes. But a cephalopod bodyplan imposes limitations, and in principle a fish kidney outstrips the squid equivalent. Maybe, however, the mesozoans (and their ciliate counterparts) play a vital role in the excretory process, assisting with fluid flow and dealing with the waste products?

Text copyright © 2015 Simon Conway Morris. All rights reserved.

Further reading

Aruga, J. et al. (2007)  Dicyema Pax6 and Zic: tool-kit genes in a highly simplified bilaterian.  BMC Evolutionary Biology 7, e201.

Czaker, R. (2011)  Dicyemid’s dilemma: structure versus genes.  The unorthodox structure of dicyemid reproduction.  Cell and Tissue Research 343, 649-658.

Furuya, H. et al. (2004)  Renal organs of cephalopods: A habitat for dicyemids and chromidinids.  Journal of Morphology 262, 629-643.

Holland, J. W. et al. (2011)  A novel minicollagen gene links cnidarians and myxozoans.  Proceedings of the Royal Society of London, B 278, 546-553.

Huber, J.T. and Noyes, J.S. (2013)  A new genus and species of fairyfly, Tinkerbella nana (Hymenoptera, Mymaridae), with comments on its sister genus Kikiki, and discussion of small size limits in arthropods.  Journal of Hymenoptera Research 32, 17-44.

Kobayashi, M. et al. (2009)  Molecular markers comparing the extremely simple body plan of dicyemids to that of lophotrochozoans: insight from the expression patterns of Hox, Otx, and brachyury.  Evolution & Development 11, 582-589.

Nesnidal, M.P. et al. (2013)  Agent of whirling disease meets worm: Phylogenomic analyses firmly place Myxozoa in Cnidaria.  PLoS ONE 8, e54576.

Neves, R.C. (2009)  Cycliophoran dwarf males break the rule: High complexity with low cell numbers.  Biological Bulletin 217, 2-5.

Some 300m below the ocean surface it is always twilight, and cold…  The water is barely above zero.  Fast-moving squid hide here from predatory fish which stay near the surface; at this depth, their nerves would be so slowed by the cold that their eyes could no longer see for them to hunt effectively.

But there are exceptions; a stealthy predator dives into this semi darkness.  Whilst the swordfish’s body temperature matches that of the water, its eyes and brain, crucially, stay toasty warm at around 23⁰C.

Why do swordfish have warm eyes?

Close-up of a swordfish’s eye from a caught specimen. The eyes sit in a bony eye cup surrounded by a thick insulating layer of fatty tissue – part of which is visible here below the eyeball (Image: Wikimedia Commons... more)

A fish’s body temperature usually matches that of the water, meaning they are ‘cold blooded’ (poikilothermic).  Swordfish nerves, like ours and those of other vertebrates, operate only within narrow temperature limits.  The squid is also ‘cold-blooded’, but their elongated nerve cell axons however, are unusually wide, around 0.5mm diameter, and operate well in the cold, allowing them to maintain their fast movements and escape predators.

A few fish species, however, have evolved methods to generate heat in some of their tissues.  Under the chilly, low light conditions of the deep sea, the warm eyes of the swordfish keep its optical nerve signals rapid.  This allows it to register more visual signals per second than can other vertebrate predators.  This fast image resolution ‘slows down’ apparent time and amplifies details, allowing these stealthy hunters to discern the brief flashes of silver that reveal the fleeting movements of small fish and squid.

This prompts some key evolutionary questions;

– How is the swordfish’s eye heat generated?

– How does the swordfish keep the heat localised to its eyes and brain?

– How does keeping body parts at different temperatures adapt swordfish for survival?

How is the swordfish’s eye heat generated?

Heat generation is not limited to animals.  Some plants such as this Voodoo Lily (Amorphophallus titanium) have developed their own form of cellular heat generation, termed ‘non-shivering thermogenesis’. These unus... moreual plants heat parts of their floral organs to liberate scent messages into the air. This attracts insect pollinators, and may also protect its delicate reproductive tissues from the sometimes very cool night temperatures in its native tropical forest habitat (Image: Wikimedia Commons)

Swordfish eye muscles contain many brown-coloured cells that produce heat without shivering (non-shivering thermogenesis).  They have a high metabolism (respiration rate) and contain many of the organelles known as mitochondria.

Mitochondria are formerly free-living bacteria found inside nearly all animal, plant and fungal cells. They ‘breathe’ for their cell, converting sugars and oxygen into carbon dioxide and water.  This releases energy, which they use to pump hydrogen ions (H⁺, protons) from the internal matrix into their inter-membrane space.  They use the chemical energy gradient this creates to produce adenosine triphosphate (ATP), life’s energy storage compound.  These cellular energy factories are found in all animals and plants.

Humans and other mammals have brown adipose cells, also called ‘brown fat’.  The mitochondria in these cells make very little ATP.  Instead, ‘uncoupling proteins’ rearrange negatively charged fatty acids in the mitochondrial inner membranes to face into the inter-membrane space.  These associate with the positively charged protons, then ‘flip-flop’, carrying them back into the matrix and dissipating the energy gradient as heat.

How does the swordfish keep the heat localised to its eyes and brain?

A pod of sperm whales (Physeter macrocephalus) diving off the coast of Mauritius. These animals are insulated by a thick layer of blubbery fat (Image: Wikimedia Commons)

When our bodies generate heat in a cold environment, this sets up an energy gradient; the bigger the differences between our internal and external temperature, the faster we cool.  Warm bodies in a cold environment lose heat quickly, unless insulated.  Birds use feathers, most mammals use fur and whales have blubber.

Fatty insulation over the swordfish’s skull retains heat, and helps keep its eyes and brain at a near constant temperature.  These tissues are homeothermic (maintaining a stable temperature), whilst the rest of its body is poikilothermic (allowing temperatures to vary with the environment).  Blood vessels supplying oxygen to the swordfish’s eye muscles are also arranged to retain heat.  These vessels run in parallel, allowing outgoing veins to warm incoming arteries (this is known as a ‘counter-current’ heat exchange system).

Emperor penguins (Aptenodytes forsteri) at Atka Bay, Weddell Sea, Antarctica. The wide webbed feet of these birds have a large surface area. Reducing the skin temperature here reduces the steepness of the heat energy gr... moreadient at the place where their bodies contact the ice. This reduces the heat loss from these uninsulated body tissues (Image: Wikimedia Commons)

Insulation (fur, feathers or fat), combined with a blood supply arranged to allow counter-current heat exchange, are found in many cold-adapted animals.  Lowering surface temperatures reduces the energy difference between a body and its surroundings, so minimising heat loss.  Warm-bodied migrating species such as wolves and many birds from polar regions use a counter-current exchange to reduce the temperatures of their legs and feet.  This means that their body parts in contact with snow or ice remain at just above zero.

How does keeping body parts at different temperatures adapt swordfish for survival? 

Keeping your body at a different temperature from your environment requires a lot of energy.  The swordfish’s ‘dual temperature’ body isolates the heat and keeps it in one well-insulated region; this is the most energy efficient way for these ‘wait and sprint’ hunters to survive in this environment.  Tuna are another example of a fish with warm and cool tissues.  Their red muscles along their spine are warm, and sustain constant ‘slow’ strokes of the tail during their long distance migrations.

An elephant dust-bathing in the ‘W du Niger’ trans-border national park, Niger Elephants cool down by ear flapping, and water and dust bathing. Their ears have a large surface area for their volume, and strong blood... more supply. Dilating the capillaries in the ears to increase blood flow to the skin allows these surfaces to lose heat to the air. At higher temperatures elephants lower their metabolic rate, reducing their resting body temperature (Image: Wikimedia Commons)

When we sweat, water evaporates and cools our skin surfaces.  Dogs and many other mammals pant to evaporate water from their tongue and mouth cavity.  Elephants lack both sweat glands and a panting reflex; these are possible remnants of their aquatic ancestry.

In very high temperatures they enter a whole-body heterothermic state.  They slow their metabolism, lowering their morning body temperature.  They then absorb daytime heat, raising their temperature above 36.7⁰C, and radiate this ‘stored’ heat at night.

Varying the temperature at times like elephants, or in certain tissues like swordfish, is known as heterothermy.

Text copyright © 2015 Mags Leighton. All rights reserved.

References

Carey, F.G. (1982)  A brain heater in the swordfish.  Science 216, 1327-1329.

Fritsches, K.A. et al. (2005)  Warm eyes provide superior vision in swordfishes.  Current Biology 15, 55-58.

Guderley, H. et al. (2005)  Why are some mitochondria more powerful than others; insights from comparisons of muscle mitochondria from three terrestrial vertebrates.  Comparative Biochemistry and Physiology, B 142, 172-180.

Hulbert, A.J. et al. (2006)  How might you compare mitochondria from different tissues and different species?  Journal of Comparative Physiology, B 176, 93-105.

Kowaltowski, A.J. (2000)  Alternative mitochondrial functions in cell physiopathology; beyond ATP production.  Brazilian Journal of Medical and Biological Research 33, 241-250.

Nespolo, R.F. et al. (2011)  Using new tools to solve an old problem; the evolution of endothermy in vertebrates.  Trends in Ecology and Evolution 26, 414-423.

Warrand, E.J. and Locket, N.A. (2004)  Vision in the deep sea.  Biological Reviews 79, 671-712.

Weissenbrock, N.M. et al. (2012)  Taking the heat; thermoregulation in Asian elephants under different climatic conditions.  Journal of Comparative Physiology, B 182, 311-319.