It has been known for a long time that whales and dolphins are incredibly intelligent animals but it’s not often we see that intelligence so impressively displayed as when orcas (often called killer whales) hunt. Orcas can actually be divided into several different ‘types’ which are found in different areas of the world and often specialise in hunting different prey. Some, such as those around Norway and Greenland, are particularly adept at hunting herring and follow the fishes migration path. Others, such as those in the north-east Pacific are skilled salmon hunters, and some have even learnt to strip tuna fish from fisherman’s lines. There is one group however that outclasses them all, the orcas of the Antarctic peninsula have become specialised at taking seals from floating ice and the way they do it is simply breathtaking. Ingrid Visser and her colleagues were lucky enough to observe the attack in 2006 and described it like this
…one killer whale remained in position with its rostrum against the ice floe while four killer whales moved away from the ice floe with the seal on it. These four killer whales reappeared simultaneously, approximately 20 seconds later in line-abreast with all submerged just under the surface. All four were coordinated-swimming, with their left sides orientated towards the surface. A trail of bubbles emanated from each of the animals blowholes as they accelerated and passed directly under the ice floe, two on each side of the stationary killer whale. This generated a large wave, which tipped the ice floe initially towards the wave, then as the wave poured over and crested under the ice, it pivoted and tilted the ice in the other direction where the attacking whales were now waiting. The breaking wave washed the seal into the water…
This same hunting technique, sometimes termed “wave-washing”, was later filmed by the BBC for the series Frozen Planet (highly recommended if you haven’t yet seen it).
As someone who has worked with seals a lot over the last few years I have mixed feelings about this. On the one hand the seal is clearly distressed and is tormented for a very long time before it is finally killed, but on the other I can’t fail to be impressed by the skill and intelligence of the orcas that is required to pull off an attack like this. For this hunting strategy to be succesful there must be forward planning, and a high level of communication and coordination between individual orcas. These characteristics are not often associated with animals.
What is really interesting is that in the case described by Visser and her colleagues the seal was caught after around 15 minutes but then released and allowed back onto the ice. It then had to endure a second wave-washing attack before being finally killed almost 15 minutes later. Why did the orcas not kill and eat the seal immediately? The answer is not known, it could simply be play behaviour or, more interestingly, it may be that the adults are training their young to hunt. We clearly have a lot more to learn from these amazing animals and I expect there will be many more discoveries in the future.
For a detailed description of this behaviour see:
Visser I.N., Smith T.G., Bullock I.D., Green G.D., Carlsson O.G.L. & Imberti S. (2008). Antarctic peninsula killer whales (Orcinus orca) hunt seals and a penguin on floating ice, Marine Mammal Science, 24 (1) 225-234. DOI: 10.1111/j.1748-7692.2007.00163.x
It’s probably fair to say that slugs are not the most well loved of animals. To most people these gastropods are dull coloured, slimy and unattractive, to gardeners they’re a sworn enemy. While this might be true of typical garden, or land, slugs, there is another group of slugs that are in an altogether different league. The marine slugs, or Opisthobranchia, are a little known group comprising some 5000 to 6000 species of beautifully coloured animals, many of which possess fascinating biological adaptations that are either unique or exceptionally rare in the animal kingdom.
Take for example, Glaucus atlanticus. This species, which is found throughout the world’s tropical and temperate seas, is immune to the stinging cells of Cnidarians (jellyfish and related species) called Cnidocytes. This means that despite being only 3cm long it is able to feed on dangerous and highly venomous Cnidarian species including the Portugese man o’ war, Physalia physalis, and the the by-the-wind-sailor, Velella velella.
That would be impressive enough on its own, however, G. atlanticus goes further by storing Cnidocytes taken from its prey for later use against would-be attackers. Quite how it does this is not yet known. The sting of a Cnidocyte cell is fired when a hair-trigger, called the Cnidocil, is released. These cells can only be used once yet somehow G. atlanticus is able to consume its Cnidarian prey without causing its victims’ Cnidocyte cells to fire. One possibility, supported by studies of closely related species such as Aeolidia papillosa, is that mucus secreted from glands in the oral tube may be what is responsible for preventing the Cnidocytes from firing. Another possibility is that only immature Cnidocytes are stored while mature ones are digested. Alternatively, it may be that the Cnidocyte cells and the slug become acclimatised to one another in the same way that anemones become acclimatised to anemone fish. Whatever the mechanisms behind it, it is impossible not to admire the impressive suite of adaptations possessed by G. atlanticus and its relatives.
Another group of marine slugs, belonging the clade Sacoglossa, may not have the same striking appearance as G. atlanticus and its relatives but what they lack in appearance they make up for with an adaptation that is found in no other animal group. Sacoglossan slugs, such as Elysiella pusilla pictured above, feed on algae by piercing algal cells with their toothy radula and sucking out the contents. While the cytoplasm of these cells is digested the chloroplasts are retained, intact and functional, within distinct branches of the digestive gland, here they may be stored from hours to months depending on the species. This phenomenon, which falls somewhere between endosymbiosis and predation, has been termed “kleptoplasty”. The chloroplasts within the slug continue to actively photosynthesise and so provide the slug with nutritional benefits. Experiments in the lab have shown that some Sacoglossan species can survive for an impressive ten months without food. Sacoglossan slugs are not the only animals that benefit directly from the photosynthesis of algae, most notably corals live in symbiosis with a type of algae called zooxanthellae which is retained within coral tissues providing them with nutrients in exchange for protection. However, only the Sacoglossan slugs are able to extract and use just the chloroplasts from algal cells while digesting the rest. This is cannot be called a symbiotic relationship since only the slug benefits.
It is interesting to consider how an adaptation like kleptoplasty could have evolved. Heike Wägele and Annette Klussmann-Kolb, writing in the journal Frontiers in Zoology, suggest that initially the uptake and storage of algal cells or chloroplasts, by turning the animals green, provided them with enhanced camouflage. This short-term storage of chloroplasts allowed for a continuation of photosynthesis within the slug and so also provided nutritional benefits. From these humble beginnings the evolution of photosynthesis as an adaptation began. Those animals that could retain chloroplasts in a functional state for extended periods of time would have had the advantage of being able to survive for longer without food. This trait would have been favoured by natural selection and so, once the process had started, evolution would have continued along the path to greater and greater efficiency at photosynthesis.
More recently Katharina Händeler and her colleagues, writing in the same journal, suggested that kleptoplasty evolved in two steps. First was the loss of the ability to rapidly digest chloroplasts, this benefited the slugs in the short-term by providing them with nutrients from photosynthesis. In the second step the slugs evolved the ability to prolong the survival of their acquired chloroplasts by supplying them with the nutrients and enzymes they require to function. In most cases the genes needed to produce these nutrients are contained within the DNA of the algae but not that of the slug. However, in at least the species Elysia chlorotica, and possibly others, algal genes have been incorporated into the slug genome by horizontal gene transfer. The transfer of genes between distinct species is extremely rare amongst eukaryotes and especially so amongst animals. The only known case of horizontal gene transfer from a alga to an animal ocurred in the Sacoglossan lineage, this exceptionally rare event gave species such as E. chlorotica the ability to substantially prolong the life of the chloroplasts it carries and so substantially enhance its fitness.
The trait that most defines the Opisthobranchs, or at least the Nudibranch clade, is undoubtedly their extraordinary colouration. This serves to warn would-be predators that these animals are toxic and should not be eaten. As an example of this point, there is a case, in 1937, of a 40 year old man who ate Aplysia kurodai and suffered from severe liver damage, almost certainly as a result of the toxins this species carries.
While some Opisthobranch species are able to synthesise toxic compounds de novo, most acquire them from their diet of other toxic species such as sponges, algae, jellyfish and tiny animals known as bryozoans to which they are immune. These toxins are then stored in specialized glands which surround the mantle called mantle dermal formations, or MDFs. As is so often the case in evolution, a structure that once served a different purpose has been modified to serve a new one. MDFs, it is thought, evolved from former excretory and detoxification organs.
I hope I have convinced you that slugs, of the marine variety, are more interesting than you thought they were. In case I have not there is one more point to consider. The Opisthobranchs are far mor diverse in terms of their colouration and defensive and foraging strategies than almost any other lineage of gastropod. Why should this be so? Well, if Wägele and Klussmann-Kolb are correct it is the reduction and loss of the shell that is responsible for the incredible diversity of Opisthobranch species.
Shells unquestionably provide substantial protection against predators, their loss in the Opisthobranch lineage must then have conferred even greater benefits or this trait would not have evolved. One possibility is that without having to carry a cumbersome and heavy shell around Opisthobranchs were able to exploit new, previously inaccessible, habitats. For example slugs in the Clade Aeolidoidea are able to feed on fragile hydrozoans, a food source that is inaccessible to most other invertebrates. New lifestyles also became accessible for example, with no shell to block the light Sacoglossan slugs were able to take up chloroplasts and use them to photosynthesise.
Without a shell for defense Opisthobranchs have had to evolve novel defensive strategies. Amongst gastropods defensive structures such as the storage of Cnidocytes and toxins are found only in shell-less slugs. These structures are likely to have evolved as a direct result of, and in synchrony with, the reduction of the shell over evolutionary time. This in turn led to the evolution of bright warning colouration such as that seen in Hypselodoris tricolor shown above.
Whatever the exact causes leading to shell loss were, it appears to have resulted in an adaptive radiation as a multitude of new niches suddenly became available. The loss of shells led ultimately to the evolution of a diverse and beautiful group of animals, many of which possess traits which are unique in the animal kingdom. I hope you will agree, that is interesting!
Haber M., Cerfeda S., Carbone M. et al. (2010). Coloration and Defense in the Nudibranch Gastropod Hypselodoris fontandraui, Biological Bulletin, 218 (2) 181-188. PMID: 20413794 Händeler K., Grzymbowski Y.P., Krug P.J. & Wägele H. (2009). Functional chloroplasts in metazoan cells – a unique evolutionary strategy in animal life, Frontiers in Zoology, 6 (1) 28. DOI: 10.1186/1742-9994-6-28 Wägele H. & Klussmann-Kolb A. (2005). Opisthobranchia (Mollusca, Gastropoda)–more than just slimy slugs. Shell reduction and its implications on defence and foraging, Frontiers in Zoology, 2 (3) DOI: 10.1186/1742-9994
Dolphins, porpoises and whales together comprise the order Cetacea. This order can be further split into two suborders; the odontocetes (“toothed whales” such as orca), and the mysticetes (“baleen whales” such as the blue whale). Baleen whales do not possess teeth, instead they have plates of baleen, a tough bristly substance which hangs from the upper jaw and is used in filter feeding. Unlike teeth, baleen is not made from dentine and enamel but from keratin, the same substance as your fingernails.
During feeding baleen whales open their mouths to take in massive quantities of water containing zooplankton such as small fish and crustaceans, they then close their mouths and, using their tongue, expel the water through the baleen plates trapping their prey in the process. Using this feeding technique the blue whale (Balaenoptera musculus) may take in and expel more than 70 tons of water in one go!
Despite their planktonivorous diet baleen whales can grow to be extremely large. At 180 tons the blue whale is the largest animal ever to have lived.
Baleen does not fossilise well, however, the ancestry of mysticete whales can be deduced from other skeletal features and from DNA. What we now know is that although all modern day mysticetes possess baleen, ancestral species, such as Janjucetus, did not, they possessed teeth as modern day odontocetes do. This means that at some point in the evolution of mysticete whales teeth were replaced by baleen. How this transition occurred is the subject of a 2008 paper by Thomas Deméré et al.
The paper provides two lines of evidence to show that baleen whales did indeed evolve from toothed ancestors. The first is palaeontological evidence based on a fossil whale, Aetiocetus weltoni, that has been dated to be 24-28 million years old. The second line of evidence is molecular, based on DNA sequences.
But first the palaeontological evidence. Deméré and his colleagues examined the fossilised skull of the mysticete whale Aetiocetus weltoni, which, unlike modern day species, possessed teeth. This was a species that existed before teeth had been replaced by baleen. However, upon closer examination of the jaw, small grooves known as “nutrient foramina” were found. These grooves are also found in modern baleen whales and conduct the passage of nerves and arteries to the epithelium of the palate to provide nourishment for the baleen which grows continuously. These slits are not found in any odontocete whales.
Nutrient foramina were also found in two other fossil mysticete whales of a similar age, Aetiocetus cotylalveus and Chonecetus goedertorum. What this suggests is that these species had both teeth and baleen. They are transitional forms.
The second line of evidence is molecular and based on DNA sequences. Given the hypothesis that toothless mysticetes evolved from toothed ancestors, it was predicted that the genes for making teeth (the secretory calcium binding phosphoprotein (SCPP) gene family) would persist in the genomes of modern day baleen whales but in a non-functional state. In other words, once released from the selective constraints of natural selection (due to disuse) random mutations acquired over time should have rendered these genes inactive but still be clearly recognisable to biologists as SCPP genes.
This is exactly what was found. DNA sequences taken from 12 species of modern day baleen whales showed the presence of enamel specific SCPP genes (AMBN, ENAM and DMP1). In mysticetes two of these genes had become completely non-functional due to frame-shift mutations, DNA insertions and deletions and premature stop codons which cause the synthesis of proteins to terminate too early. As a result of mutations the authors suggest that the genes AMBN and ENAM are now decaying pseudogenes.
One final piece of compelling evidence for the toothed ancestry of mysticete whales comes from development. Below is an image of a fin whale foetus with a section of the jaw dissected away. Fin whales do not have teeth, however, what this image shows is that in the embryo baleen whales do develop tooth buds. These develop from cells called odontoblasts which secrete dentin, but, unlike in odontocetes, this is never covered with enamel to form complete teeth. In mysticetes the teeth buds do not break the gumline but are reabsorbed before birth. Tooth buds no longer serve any function to mysticete whales but do provide strong evidence as to their ancestry.
So what does this all mean? We now know that around 30 million years ago the transition from teeth to baleen was underway in the mysticete lineage. At this time species such as Aetiocetus weltoni would have had both teeth and baleen. By acting as a sieve baleen would have enabled these species to catch smaller prey and thereby have access to a greater range of food sources. They may have been better fed enabling them to produce more offspring and so increase the proportion of animals carrying genes for baleen in the next generation. As filter feeding proved to be a highly successful strategy it gradually came to replace teeth entirely.
The fantastic image below is by Carl Buell and shows what Aetiocetus weltoni may have looked like with both teeth and baleen.
Demere T.A., McGowen M.R., Berta A. & Gatesy J. (2008). Morphological and Molecular Evidence for a Stepwise Evolutionary Transition from Teeth to Baleen in Mysticete Whales, Systematic Biology, 57 (1) 15-37. DOI: 10.1080/10635150701884632