Living fossils are evolving

Charles Darwin coined the term living fossil in On the Origin of Species. He didn’t use it the same way that it has come to be used. He suggested that living fossils are modern species that can be used to link to groups in the same way that fossils can. One of the examples he gave was the platypus, which lactates and lays eggs, which is evidence that mammals and reptiles share a common ancestor. I don't think he meant it to mean an unchanged relict, as some people interpret his words.

Today, a living fossil is a species that retains many features of their fossil ancestor so that it is recognisably closely related. There are some stunning examples of this, such as orb-weaving spiders. In 2011 a 165 million year old spider fossil was described by Seldon et al., which shared so many features with modern Nephila spiders that it was placed within the same genus. Interestingly, I have never heard of web building spiders being referred to as living fossils despite there being amazing conservation of traits in many groups.

The orb-weaving spiders Nephila clavipes (left) and N. jurassica (right) are separated by 165 million years, but placed within the same genus (image of N. clavipes from Wikipedia and N. jurassica from Seldon et al. 2011).

Unfortunately, living fossil has become synonymous with a species, or group of species, displaying no evolutionary change or very slow change. This is completely wrong. Although the conservation of morphology in Nephila is remarkable, there are more than 150 known species in the genus. Clearly there has been evolutionary diversification within the group. Indeed, whenever living fossils are examined in more than superficial detail it becomes difficult to see them as organisms that evolution forgot.

Horseshoe crabs are one of the most iconic living fossils. There are four living species in three genera. They are placed within the subphylum chelicerata, which makes them more closely related to spiders and scorpions than they are to true crabs, which are placed within the subphylum crustacea. Although there are fewer species of horseshoe crabs than Nephila, that fact that there are four species that are all different from fossil species is a strong indication that evolution hasn't stopped for them.

The Atlantic horseshoe crab, Limulus polyphemus, mating (photo Wikipedia)

The general shape of modern horseshoe crabs is strikingly similar to the fossils that date from about 450 million years ago. Close examination, though, shows that parts of their shape, their legs in particular, have changed over time. Briggs et al. 2012 looked at a fossil horseshoe crab from 425 million years ago, which is relatively early in their evolution. They found that modern horseshoe crabs are missing an entire set of limbs that were present in their ancestors.

All modern chelicerates, including living horseshoe crabs, have unbranched limbs; each limb is a single series of segments. Most crustaceans have limbs that branch at the base into two series of segments. Branched limbs, like those in crustaceans, are the ancestral condition and unbranched limbs are thought to have evolved several times among the arthropods. Indeed, Briggs et al. found that the fossil horseshoe crab had branched limbs, which have been lost in their descendents. 

Like horseshoe crabs, tadpole shrimp have a broad semi-circular carapace protecting their heads and are considered living fossils. There are 11 recognised species in two genera, Lepidurus and Triops. The two genera probably diverged about 180 million years ago, but there are fossil tadpole shrimp dating from about 250 million years ago. That's not as long as the really iconic living fossils, like horseshoe crabs and the coelacanths, but it is still an impressive amount of time to retain enough features to be easily recognised as related.

The tadpole shrimp, Lepidurus apus (photo Wikipedia)

The problem with relying on features that preserve in the fossil record is that it underestimates the actual amount of evolutionary change because generally only hard parts are preserved. A recent study of tadpole shrimp highlights this point. Mathers et al. 2013 used genetic analyses to construct the evolutionary relationships among the 11 species of tadpole shrimp. They found that there are actually 38 species and that these species arose relatively recently. This shows that rather than evolutionary stasis, there is likely to be high species turnover in the group.

There are many reasons why some features may be conserved over long periods of time. None of these have to do with natural selection taking a break. In fact, if natural selection did cease we should expect to see features wander under random genetic drift, as has been hypothesised for eyes in cave dwelling animals. Conserved features are much more likely to be the result of developmental constraints or stabilising selection.

References:

Briggs, D., Siveter, D., Siveter, D., Sutton, M., Garwood, R., & Legg, D. (2012). Silurian horseshoe crab illuminates the evolution of arthropod limbs Proceedings of the National Academy of Sciences, 109 (39), 15702-15705 DOI: 10.1073/pnas.1205875109 

Mathers, T., Hammond, R., Jenner, R., Hänfling, B., & Gómez, A. (2013). Multiple global radiations in tadpole shrimps challenge the concept of ‘living fossils’ PeerJ, 1 DOI: 10.7717/peerj.62

Selden, P., Shih, C., & Ren, D. (2011). A golden orb-weaver spider (Araneae: Nephilidae: Nephila) from the Middle Jurassic of China Biology Letters, 7 (5), 775-778 DOI: 10.1098/rsbl.2011.0228

In the cave of the blind, the no-eyed crab is king

Cave dwelling creatures are often blind. The prevailing view is that, in such species, mutations in the visual system have little or no effect on fitness and vision is lost as these mutations gradually accumulate. There are several other types of characters that we can be reasonably confident are adaptations to life in caves, such as elaboration of structures for touch or smell. However, it is often hard identify which population cave adapted species are descended from and, therefore, how long ago they invaded caves. Without this information it has been hard to test ideas about the evolution of traits associated with life in the dark.

A cave form of the fish, Astyanax mexicanus, which is eyeless and unpigmented, traits typical in caves. It is a commonly used model species in studies of adaptation to cave environments (photo Wikimedia Commons).

Sebastian Klaus and colleagues from the National University of Singapore and Goethe University examined five species of freshwater crab in the genus Sundathelphusa, which occur on Bohol Island in the Philippines. Four species are only found in caves and the other has established several populations in caves. The repeated invasion of caves by the crabs has led to varying degrees of adaptation to life in the dark within the group. 

Freshwater crabs in the genus Sundathelphusa from Bohol Island. Thy are arranged from least cave adapted (top) to most cave adapted (bottom). From top to bottom the species are Sundathelphusa boex, S. vedeniki, S. urichi, S. sottoae and S. cavernicola (from Klaus et al. 2013).

The team used genetic data to estimate the time at which each species and population last shared a common ancestor. They then compared several features of cave-adapted crabs with their closest terrestrial relatives. Reductions in the visual system were just as pronounced as changes in cave-adapted features, indicating that evolution occurs at similar rates. The authors argue that this is a clear sign that eye loss is under directional selection because changes should appear more slowly if they are a result of selectively neutral mutations. 

They don’t speculate at all about what might favour eye-loss in the Bohol crabs, but hint in the introduction that it could be due to trade-offs between vision and other sensory systems. Trade-offs occur where increasing one aspect of fitness necessarily requires the reduction of fitness in another. If eyes are energetically costly to build and maintain then retaining functional eyes might prevent greater investment in other senses. Trade-offs are ubiquitous in biology and have been implicated in the loss of eyes in other cave dwelling species.

While I was doing research on this study I came across several creationist websites that argue cave adapted creatures are strong evidence that evolution is false because a trait is lost. According to them this shows evolution progressing in the wrong direction to what is predicted. They argue that evolution should progress towards more information and greater complexity. This is incorrect and shows, yet again, that creationists typically have a poor understanding of evolutionary theory.

The 'logic' of this argument is similar to the idea of a "Great Chain of Being", which pervaded early thinking about biology. This type of thinking is where we get several antiquated, but persistent terms, such as "missing link" and "highly evolved". It continues to dog evolution in the way that evolutionary information is often presented, such as the placement of organisms more closely related to us at the right or top of phylogenetic trees and at the end of textbooks.

The phylogeny of primates with humans at the top and less related groups at the bottom (from Wikipedia).

Linear descent was never part of Darwin's theory, nor was an increase in information ever a necessary assumption on which evolutionary theory rests. When you look at an evolutionary tree (like the primate tree above), all of the living species at the branch tips have an equally long evolutionary history. They are not descended from each other, they are descended from a common ancestor. You could say that they are equally evolved.

The first evolutionary tree drawn by Darwin over 20 years before the publication of On the Origin of Species.

Evolution doesn't prevent information from increasing, but contrary to the creationist claims it does predict that there will be strong limits on it. Both single trait and multi-trait trade-offs are thought to prevent organisms from becoming perfectly adapted. Single trait trade-offs occur where elaboration of a structure increases fitness in one environment, but reduces it in others. Multi-trait trade-offs occur where two or more structures are dependent on a shared finite resource.

Blind crabs are not evolving in the wrong direction. There is no wrong direction, they're just evolving under the constraint of trade-offs. Eye reduction and loss of pigmentation are not the only evolutionary changes that are occurring either. Other traits are becoming more elaborated, such as the length of their legs and the hairs on their claws, suggesting a multi-trait trade-off. This result is not only consistent with evolutionary theory, but expected.

An abbreviated version of this post is published on the Australasian Evolution Society website in the Research Highlights section.

Reference

Klaus, S., Mendoza, J., Liew, J., Plath, M., Meier, R., & Yeo, D. (2013). Rapid evolution of troglomorphic characters suggests selection rather than neutral mutation as a driver of eye reduction in cave crabs Biology Letters, 9 (2), 20121098-20121098 DOI: 10.1098/rsbl.2012.1098

Evolution, climate change and coral

Increased carbon dioxide in the atmosphere poses several problems for organisms living in the marine environment. Increases in temperature and ocean acidification are the two best known and most worrying. In order to predict how climate change and ocean acidification will affect marine species, we need to know how they respond to these conditions. The effect of climate change on corals has attracted a lot of attention because of their importance for biodiversity.

We can't just expose corals to predicted conditions because corals of the future won't be naive to these environments and are likely to have evolved. We know that evolution can be extremely rapid, often within decades. Ignoring the potential for evolution to influence the effects of climate change on marine organisms could lead to inaccurate projections of the effects of climate change on extinction risk. Yet many authors are ignoring the effects of evolution and acclimation in making their predictions. 

The three-spine stickleback, Gasterosteus aculeatus has been documented adapting to freshwater conditions from saltwater ancestors in just 13 generations (photo Wikipedia)

In their 2007 paper, Hoegh-Guldberg, et al. dismiss the importance of evolution because "reef-building corals have relatively long generation times and low genetic diversity, making for slow rates of adaptation". But, long generation times are not present in all coral species and the response of corals to climate change is going to depend partly on their algal symbionts, which have short generation times. 

Unfortunately, the rates of evolution in corals and their symbionts are extremely poorly known. In terrestrial systems though, genetic variation for traits related to thermal performance is common and evolutionary responses to changing climate are typical. For instance, changes in allele frequencies consistent with responses to global warming have been documented in a number of insects, such as fruit flies and mosquitoes (e.g. Bradshaw & Holzapfel 2001, Umina et al. 2005).

Acclimation, or phenotypic plasticity, will also affect the way that corals respond to climate change. Plastic responses to the environment can occur within generations and across them. For instance, Donelson et al. (2011) looked at the tropical damselfish, Acanthochromis polyacanthus, and found that their offspring could completely compensate for the negative effects of higher temperatures. But, this only occurred when both they and their parents where reared at the same temperature.

The tropical damselfish, Acanthochromis polycanthus (photo Wikipedia)

There are indications that some acclimation is occurring in corals too. Under stress, corals expel their algal symbionts, which gives them the appearance of having been bleached. Coral reefs that experience greater variability in sea surface temperature and those that have recently been subjected to bleaching are less susceptible to bleaching. This greater resilience suggests that some acclimation to climate change is possible within short time-frames. 

A bleached coral in the foreground with an unbleached coral of the same species behind (photo Wikipedia)

We need a better understanding of how evolution and acclimation may influence the response of corals to climate change so that our predictions are accurate. But, we already know which direction things are probably going to go. John Pandolfi's work has shown that under historical climate change, diversity on corals reefs has declined and populations have moved to higher latitudes (e.g. Pandolfi et al. 2011, Kiessling et al 2012). 

Climate change is currently more rapid than previous episodes and this will limit the amount of adaptation that can occur. Corals are also already under significant pressure from other anthropogenic sources of stress that have resulted in substantial declines and changes in population composition. These pressures, too, will decrease the ability of corals to cope with the effects of climate change. By removing these pressures, we will give corals the best chance possible to adapt to a warmer and more acidic ocean.

Hoegh-Guldberg, O., Mumby, P., Hooten, A., Steneck, R., Greenfield, P., Gomez, E., Harvell, C., Sale, P., Edwards, A., Caldeira, K., Knowlton, N., Eakin, C., Iglesias-Prieto, R., Muthiga, N., Bradbury, R., Dubi, A., & Hatziolos, M. (2007). Coral Reefs Under Rapid Climate Change and Ocean Acidification Science, 318 (5857), 1737-1742 DOI: 10.1126/science.1152509  

Bradshaw, W., & Holzapfel, C. (2001). Genetic shift in photoperiodic response correlated with global warming Proceedings of the National Academy of Sciences, 98 (25), 14509-14511 DOI: 10.1073/pnas.241391498

Umina, P., Weeks, A. R., Kearney, M. R., McKechnie, S. W., & Hoffmann, A. A. (2005). A Rapid Shift in a Classic Clinal Pattern in Drosophila Reflecting Climate Change Science, 308 (5722), 691-693 DOI: 10.1126/science.1109523

Donelson, J., Munday, P., McCormick, M., & Nilsson, G. (2011). Acclimation to predicted ocean warming through developmental plasticity in a tropical reef fish Global Change Biology, 17 (4), 1712-1719 DOI: 10.1111/j.1365-2486.2010.02339.x

Pandolfi, J., Connolly, S., Marshall, D., & Cohen, A. (2011). Projecting Coral Reef Futures Under Global Warming and Ocean Acidification Science, 333 (6041), 418-422 DOI: 10.1126/science.1204794 

Kiessling, W., Simpson, C., Beck, B., Mewis, H., & Pandolfi, J. (2012). Equatorial decline of reef corals during the last Pleistocene interglacial Proceedings of the National Academy of Sciences, 109 (52), 21378-21383 DOI: 10.1073/pnas.1214037110

Online nature sound archive

Cornell University has just released their full archive, the World's largest, of bird calls and other animal sounds. It comprises nearly 150,000 recordings dating back to 1929. The Cornell press release says that they are trying to make the sound recordings as accessible as possible, to the broadest audience possible. If you're interested go check it out.

It's Yoda, but not as you know him

A new species of acorn worm has been named after Jedi Master Yoda, the best character in the Star Wars trilogy*. Acorn worms are not true worms. They are more closely related to echinoderms (starfish, sea urchins, sea cucumbers, etc.) than they are to worms. They were once placed as a subphylum of the chordata (i.e. our own phylum), but are now placed within their own phylum, the hemichordata.

Yoda purpurata, the newly described species of acorn worm

The paper described three new species of deep-sea acorn worms in the family Torquaratoridae. Two of which, Allapasus isidis and Tergivelum cinnabarinum, were from previously known genera. But, Yoda purpurata is a new genus and species. It's named after Yoda because the appendages at the head end of the animal are reminiscent of Yoda's ears. All three species were found at about 2.5 kilometers deep on the mid-Atlantic ridge.

*To count as a true Star Wars film, it can't just carry the name. You also have to be able to sit through it without wanting to punch George Lucas. This caveat leaves just three films that can be considered part of the Star Wars canon. And these three films are the originals, not the remakes. 

Priede, I G, Osborn, K J, Gebruk, A V, Jones, D, Shale, D, Rogacheva, A, & Holland, N D (2012). Observations on torquaratorid acorn worms (Hemichordata, Enteropneusta) from the North Atlantic with descriptions of a new genus and three new species Invertebrate Biology, 131 (3), 244-257 DOI: 10.1111/j.1744-7410.2012.00266.x

Evolution, the most fascinating science

There are many amazing evolutionary adaptations. Here is a short video documenting just one.

Rapid speciation in starfish

Australian waters are extremely rich in starfish species. Indeed, a little over 15% of all known species of starfish occur in Australia. For at least two of these starfish, speciation occurred extraordinarily fast. At most, they became separated about 22 thousand years ago, but the best estimate for the timing of the split is about 6 thousand years ago.

We know that evolution can be very rapid (e.g. sticklebacks) and that sometimes this leads to speciation (e.g. cichlids). But, in these cases selection is probably acting on a small number of alleles that are already present in the population. What makes the starfish study so breathtaking is that there has been profound changes to life history in the two species, which likely involved selection on many morphological and physiological traits.

Puritz et al. looked at Cryptasterina pentagona and its sister species C. hystria. Like most starfish, C. pentagona has separate sexes and reproduces by 'broadcasting' sperm and eggs into the water column where fertilisation occurs. In stark contrast, C. hystria produces both sperm and eggs simultaneously, and it exclusively self-fertilises within its own body cavity. The embryos of C. pentagona develop in the plankton, while C. hystria broods its offspring within the gonad until they are ready to emerge as small starfish.

It takes an expert to distinguish Cryptasterina hystria (top) and C. pentagona (bottom) in the wild. In fact I've seen the bottom picture shown as C. hystria and C. pentagona, but I think I got it right (photo Jon Puritz).

Puritz et al. speculate that water temperature may have provided the selective pressure that favoured the evolution of the C. hystria life history. Viviparity, like that seen in C. hystria, has been documented in a number of other starfish species. And it is consistently associated with species that occur in colder water. The two Cryptasterina species are separated by about 375 kilometers, with C. pentagona in the warmer north and C. hystria to the cooler south.

The authors also argue that small population size may have selected for self-fertilisation. If there are so few individuals in the population that your gametes are unlikely to meet another individual's, it's better to fertilise your own than to not reproduce at all. It's expected that genetic variation in a population that self-fertilises should be very low. But, genetic variation in C. hystria is so low it suggests the whole species derived from very few individuals, perhaps just a single one.


The transition from broadcast spawning with planktonic larval development to self-fertilisation with larvae brooded within the gonad has occurred in another Cryptasterina species, C. pacifica. In the closely related genus Parvulastra, a similar transition has occurred too, but probably over 500 thousand years. This suggests that the genetic variation required for the dramatic shift in life history is widely present in the group of starfish to which the genera Cryptasterina and Parvulastra belong. But, the speed at which evolution has occurred is truly astonishing.

Parvulastra exigua, note its similarity to the Cryptasterina species (photo Museum Victoria).

Puritz JB, Keever CC, Addison JA, Byrne M, Hart MW, Grosberg RK, & Toonen RJ (2012). Extraordinarily rapid life-history divergence between Cryptasterina sea star species. Proceedings. Biological sciences / The Royal Society, 279 (1744), 3914-3922 PMID: 22810427

An unusual crustacean meets its parents

Many animals living in the ocean have complex life histories where the young look nothing like the adults and often occupy different habitats. Frogs, with their early tadpole stage, are classic examples of animals with complex life histories. But, tadpoles look far more like frogs than the larvae of other animals resemble their adult forms. Different species of distantly related crustacean larvae, for instance, can look far more like each other than they resemble the adults of their own species.

The nuaplius larvae stage (left) of a cylopoid copepod (top) and penaeid shrimp (bottom) and their adult forms (right). These distantly related crustaceans appear similar as larvae, but not as adults (images Wikipedia)

Because larvae look so different from the adult form, identifying the species that larvae belong to can be tricky. Often it requires the larvae to be carefully reared in the laboratory to see what they eventually turn into, but this isn't always possible. In some cases, it is possible to place larvae within a species using genetic techniques, but this requires a DNA sequence from the adult to compare to.

One type of crustacean larva that has been difficult to assign to an adult form are the Cerataspids. There are three known species that have been placed in two genera, Cerataspis monstrosa, C. petiti and Cerataspides longiremus. Like many unusual crustacean larvae, the first to be discovered (C. monstrosa in 1828) was thought to be an adult of the crustacean order Leptostraca. However, it later became apparent that they were probably larvae of shrimp within the Penaeoid superfamily, possibly from the family Aristeidae.

The crustacean larva Cerataspis monstrosa (image from Bracken-Grissom et al. 2012)

Through a combination of skill and luck, Bracken-Grissom et al. were able to resolve the adult identity of C. monstrosa. The luck involved getting their hands on a specimen of the larva that was suitable for DNA analysis. Almost all we know about C. monstrosa comes from examining specimens in the gut contents of its predators, like skipjack tuna. But, Bracken-Grissom et al. unexpectedly obtained a single specimen from a trawl at a depth of 420 meters in the Gulf of Mexico.

Bracken-Grissom et al. collected DNA sequence data from the specimen and compared it to sequences of crustaceans available from a database of genetic sequences. Because C. monstrosa had been liked with Penaeoid shrimp in the family Aristeidae, they concentrated their analysis within those taxonomic groups. And they hit pay dirt. The DNA from the C. monstrosa specimen was a near perfect match with a deep-sea Aristeid shrimp Plesiopenaeus armatus.

The Aristeid shrimp Plesiopenaeus armatus (image from Bracken-Grissom et al. 2012)

Plesiopenaeus armatus has a similar geographic distribution to C. monstrosa, but it is known from deeper water. The contrast between the larval habitat and  adult habitat is not unusual for organisms with complex life histories. Indeed, many complex life histories involve much more dramatic habitat shifts. However, the transition from mid-water pelagic larvae to abyssal adults is not known in many species.

The findings of Bracken-Grissom et al. have implications for the other species of Cerataspis larvae that haven't been linked to their adult form. They suggest that C. petiti is the larva of the only other known species in the genus Plesiopenaeus, P. coruscans. Further they suggest that Cerataspides longiremus is the laval stage of a closely related Aristeid shrimp, possibly an unidentified representative of the same genus.

Bracken-Grissom HD, Felder DL, Vollmer NL, Martin JW, & Crandall KA (2012). Phylogenetics links monster larva to deep-sea shrimp Ecology and Evolution DOI: 10.1002/ece3.347

Australia's new marine parks revisited

Last month I wrote about the release of a draft plan for a set of marine parks in Australian Federal waters. I expressed concern that the large area and their distance from shore would make enforcement of the fully protected areas difficult. It seems I'm not alone in this concern. Three marine scientists in Western Australia have written an article in The Conversation expressing similar concerns and others about the design of the marine parks, including the under-representation of some habitats within the parks.

Shark week

Apparently it's the 25th year of the Discovery Channel's shark week. So, you can listen to deep, manly voices leaving dramatic pauses between words as you watch sharks all this week. Well, assuming you have a Discovery Channel subscription...

Perhaps in celebration, but more likely as coincidence, Ed Yong has an interesting piece on discovering the past shark biodiversity of a Central Pacific island by examining cultural artifacts.

Flowering plants in the sea part 2 - Sex

A while ago I wrote about seagrass and some of the interesting adaptations that they have to the low light levels in the sea. Another important part of their biology that had to adapt to conditions in the marine environment were their flowers. Because seagrasses moved into the sea on multiple occasions there are several strategies that they use for pollination.

The most recent entrants to the sea, in the genus Enhalus, have flowers that are pollinated in air. Male flowers break away from the plant and fertilise the female flowers, which are attached to the plant by long coiled tendrils. Unlike many other types of seagrass, the female flowers are easily recognised as flowers.

The floating female flower of Enhalus acroides. The small white polystyrene bead-like objects are the male flowers (photo The Tide Chaser). 

A close up of the male flowers of E. acroides (photo Urban Forrest).

Pollination is important in terrestrial plant populations, but was thought not to be terribly important in seagrass populations because they are mostly clonal and populations expand by vegetative growth. Indeed, expansion via the rhizomes can produce large seagrass meadows that contain genetically very similar plants.

The female flower of surfgrass, Phyllospadix torreyi  (photo Carol Blanchette).

The flower of eelgrass, Zostera marina (photo Jan Holmes)

Seagrasses also invest a lot of energy into sexual reproduction. This is curious because it takes energy away from vegetative growth, which is primarily how meadows are maintained and recover from damage. Indeed, sex seems paradoxical in species, like segrasses, that can produce offspring without sex.

One potential reason for the large investment in sexual reproduction is seagrasses is long range dispersal. Vegetative growth expands meadows, and can do so quite rapidly, but it can't jump large gaps and establish new populations or spread genes to new locations. Dispersing pollen and seeds may be able to accomplish this.

The pollen of seagrasses is large and elongate (relative to other flowering plants), and consequently, poorly suited to long distance travel. Although estimates are rare, some studies suggest that pollen may only be able to travel a few tens of meters. Pollination, therefore, is likely to occur at local scales in most seagrasses.

The fruit of eelgrass, Zostera marina. Each fruit contains a single seed (photo Jan Holmes).

The seeds of seagrasses have the potential to disperse genes much further than the pollen and establish new meadows. Seagrass seeds, or the structures that carry the seeds (e.g. fruit) have a variety of adaptations for dispersal that effect how far the seeds will travel before they start a new population. Probably the most important factor determining dispersal distance.

The seeds themselves are usually neutrally or negatively buoyant because they must eventually reach the sediment to grow into an adult plant. The structures that carry the seeds, however, are often floating and can transport the seeds considerable distances (up to several hundred kilometers). Some seagrasses are even able to transport their seeds in the insides of herbivores like dugongs and turtles.

Dugongs are mostly interesting because they transport seagrass seeds.

Another reason that sex is important for seagrasses is the resilience of populations to disturbance. We know that communities with a greater diversity of species often have an enhanced ability to resist and recover from disturbances. Interestingly, seagrass patches that have higher genetic diversity show a greater resistance to damage by herbivores and recover faster after damage. Although, the faster recovery may be due to the lower levels of damage in more diverse patches than faster rates of vegetative growth.


Further reading:


1 G. A. Kendrick et al. (2012). The Central Role of Dispersal in the Maintenance and Persistence of Seagrass Populations BioScience, vol 62(1): 56-65



2 J. D. Ackerman (2006). Sexual Reproduction of Seagrasses: Pollination in the Marine Context. In: Seagrasses: Biology, Ecology and Conservation (A. W. D. Larkum, R. J. Orth, C. Duarte Eds.). 89 - 109


3 R. J. Orth et al. (2006) Ecology of Seagrass Seeds and Dispersal Strategies. In: Seagrasses: Biology, Ecology and Conservation (A. W. D. Larkum, R. J. Orth, C. Duarte Eds.). 111 - 133



4 A. R. Hughes and J. J. Stachowicz (2004). Genetic diversity enhances the resistance of a seagrass ecosystem to disturbance. PNAS vol 101(24): 8998 - 9002.
  

An ocean of plastic

There are five major oceans in the world. There's the Arctic Ocean, the Atlantic Ocean, the Pacific Ocean, the Indian Ocean and the Southern Ocean. In the Pacific, the Atlantic and Indian oceans there are huge circular currents called gyres. The Indian Ocean has a single gyre, while the Atlantic and the Pacific have two, one in the northern hemisphere and one in the southern hemisphere.

The five great oceanic gyres showing the direction of rotation

The northern hemisphere gyres rotate in a clockwise direction, while the southern hemisphere gyres rotate in an anti-clockwise direction. The direction of rotation has to do with the Coriolis effect, which is what people joke about when the say that water goes down plug-holes in different directions in Europe compared to Australia. The Coriolis effect doesn't matter too much for water going down plug-holes (other forces are far more important), but operating over long time periods and over large distances it produces gyres.

Because the gyres rotate they are good at accumulating floating items in their centres. Waste material is drawn into the gyres from the countries that surround the gyre. When the waste reaches concentrations that are significantly higher than the rest of the World's oceans that area of ocean is termed a garbage patch. So far surveys have found garbage patches in the North Pacific, North Atlantic and Indian Ocean gyres. Garbage patches also form in other places, but the oceanic gyres form the biggest patches.

Of all the garbage patches the North Pacific gyre is the largest by a considerable margin. Mainland Australia has an area of 7.69 million square kilometres and estimates of the size of the North Pacific Garbage Patch are as high as 15 million square kilometres. So, basically there's a patch of garbage that could cover an area almost twice the size of Australia floating in the North Pacific. It should be noted, however, that other estimates are considerably smaller. Estimates vary largely because different studies use different densities of debris to define what a garbage patch is.

Plastic particles hanging underwater in the North Pacific garbage patch (photo Scripps Oceanography).

The garbage patches collect a huge array of debris and chemical waste. A lot of it, about 80%, comes from land-based sources. Natural disasters, such as a tsunami or a hurricane can lead to large amounts of waste entering the sea. However, the most common route is through storm water and waste water inputs. The other 20% of waste is lost or deliberately dumped from ships at sea. Although it has been illegal to dump waste at sea for the last 20 or so years, the law is almost impossible to enforce.

By far the most common thing found in the garbage patches is plastic. Mostly it's small particles of plastic, but sometimes very large items like fishing nets that are kilometers long can be found. The fact that it is mostly plastic is pretty amazing seeing as plastic has only become common since the Second World War. But the plastic is able to accumulate because, unlike many other type of rubbish that finds its way into the sea, there are very few organisms that can break it down.

A ghost net floating in the North Pacific garbage patch (photo Scripps Oceanography).

Plastic has a number of negative effects on marine animals. Probably the effect that most people would be familiar with is that large items of plastic, like ropes, fishing line and fishing nets can entangle marine animals. This can cause them to drown, if they breath air, it can inhibit their movements making them more vulnerable to predators and it can cause them injuries as they try to struggle free.

A beached whale's tail entangled with ropes (photo Mike Baird).

Another effect is that marine animals can consume the plastic because it looks to them like a tasty piece of food. At its most minor the animal has simply wasted its time and effort catching the plastic. But, if an animal eats enough plastic it can clog their digestive tracts making it hard for them to eat and digest real food. And it is not just the larger animals like whales, turtles and sea birds that are at risk from ingesting plastic. We know that there are some very small, even microscopic animals that are eating plastics.

Plastic bag fragments found in the contents of a turtle's stomach (photo Victoria González Carman).

Plastics have also been reported to accumulate toxic chemicals on their surface in high concentrations. And if marine animals eat the plastics the chemicals can be released during digestion and become incorporated into their tissues. So even if an animal eats plastic rarely, it can acquire a toxic dose of some chemicals that enter its system via the plastic. The research on toxic plastic is controversial and not yet widely accepted.

So plastic waste is a huge problem for life in the ocean. In fact, one researcher looking at plastics in the ocean has argued in a recent book that the biggest effect on the marine environment this century won't be climate change, it'll be plastic waste.