Mangrove Plants 1: Getting to the root of things

Often, I get besieged by botanists (including closet botanists and ornithologists) due to my botanical inaptitude. So here I am atoning for my zoological bias and aversion to all things from kingdom Plantae.

To start off, the swampy mangrove is a good place to introduce and highlight how highly adapted plants can be since there are less than 30 true mangrove plant species in Singapore and many have developed unique adaptations for the habitat.

Here, conditions on the ground is mostly muddy and unstable with low oxygen and high salt content from the sea. This poses various challenges to plants - imagine living in salty quick sand!

Hence, from ground up, plants that have evolved roots for support and coping with low oxygen and salt immediately gain a competitive advantage over those that do not.

One such group is Rhizophora. If asked how does one increase stability? Rhizophora's answer would be to grow more legs.

Members in this group develop prop or stilt roots that branch and loop from the trunk and branches. The result is a network of charmingly grotesque gothic pillars at the bottom of the tree.

When exposed during low tide, these long roots also help with oxygen intake.

These roots also have an additional trick that blocks salt from seawater entering the roots in a process called ultrafiltration.

In Bruguiera and Ceriops, the trees send their roots out far and wide, and increases anchorage by having bent, kneed roots at intervals that resemble the legs of people doing sit-ups on the ground.

This curious structures not only adds stability by increasing surface area of attachment and binding more sediment, but the parts that stick out also helps obtain oxygen.

In Bruguiera, the roots are also said to be able to perform ultrafiltration to remove salt.

Avicennia and Sonneratia also spread their roots far and wide for stability, but have spike-like breathing roots in place of kneed roots.

In spy or adventure movies, characters sometimes hide underwater and breathe using straws or tubes poking out of the surface. Breathing roots, or pneumatophores, work in a similar way.

The roots of Sonneratia are capable of ultrafiltration, but not in Avicennia. The latter, has other tactics which will be discussed in a future post.

But what is most interesting is how unrelated (or only distantly related) groups have somehow acquired similar adaptations at root level for mangrove living. Rhizophora, Bruguiera and Sonneratia can exclude salt when taking in seawater (ultrafiltration), while Avicennia and Sonneratia solve oxygen shortage with spike-like breathing roots.

This phenomenon is what scientists term as convergent evolution. Another example would be how dolphins (order Cetacea) and manatees (order Sirenia) have separately evolved fluked tails for swimming. Oops, mentioned mammals in a plant post.

Further reading:

1. Ng, P. K. L. and Sivasothi. N. 1991. How plants cope in the mangroves. A Guide to the Mangroves of Singapore Volume 1. Singapore Science Centre.

Life Saving 123: Marine Animal Defenses at Pulau Semakau

The rich habitat along the sea shore provides lots of feeding opportunities for animals. However, this comes at a price. Compared to animals on land or in the sea, inhabitants here are vulnerable to predators coming both from the sea and land. Consequently, many have evolved anti-predator defences so as not to end up at someone else's dinner table. Examples of these could be found during our exploratory walk at Pulau Semakau today.

The most basic anti-predator defence has got to be fleeing. If you can't catch me, you can't eat me. Those that do that exceedingly well all eluded my capture on camera. The focus today will thus be on those that could not get away fast enough.

If you cannot get away fast enough, then better hope that your enemies cannot see you.

The hairy crab (Pilumnus sp.) is covered with long hair that breaks up the crab's outline and binds sediments to help it blend with its surroundings. If that does not work, it will dash into the closest crevice for cover.

Taking camouflage one step further, cuttlefish and squids such as this big-finned reef squid (Sepioteuthis sp.) can change their colour to match their surroundings. Another trick they may use is to release a cloud of ink to confuse predators while they make their escape.

If you are as slow as a snail, more extreme tactics will have to be used. A turban snail (Turbo sp.) has a hard shell which it can retract fully into to protect its soft body.

On its foot is a trap-door-like operculum that seals up the

opening. The thick and tough operculum has a rounded surface and protects the snail from the prying pincers of crabs and other predators.

The shape and colour of the operculum resemble cat's eyes and is sometimes made into jewelry.

Sometimes, best defence is offense.

The flower crab (Portunus pelagicus) packs a nasty pinch from its sharp pincers. When threatened, the aggressive crab faces its aggressor head on, pincers ready. On top of that, it is heavily armoured with a shell packed with numerous spikes to discourage predators from swallowing it.

The hell's fire sea anemone (Actinodendron sp.) defends itself from any animal that dares to touch it with a painful sting. This comes from stinging cells tipped with venom and they look like little harpoons under the microscope.

If you are a small shrimp with minimal defensive powers, then it is best to find a powerful ally.

Apparently the stinging cells of sea anemones are so effective that the anemone shrimp (Periclimenes brevicarpalis) have found a way to take advantage of them by living among them for protection. But as if free protection is not enough, this species has been observed feeding on the tentacles of its host anemone!

Today's exploratory walk at Pulau Semakau also saw new records of 3 sea cucumber species for the island's shore. Even though they look helpless, sea cucumbers are able to protect themselves too. Some contain toxins or taste downright foul to predators, others try to confuse enemies by vomiting their guts and other organs out while some produce sticky threads to bind attackers in a gooey mess. Very neat.

From left: Ball sea cucumber (Phyllophorus sp.), Holothuria notabilis, and brown sandfish sea cucumber (Bohadschia vitiensis).

At the end of this trip, I still wonder how people squat over ankle-high water without getting their backsides wet. Must acquire this veritable skill.

Further reading:

1. Bruce, A. J. and Svoboda, A. 1983. Observations upon some pontoniine shrimps from Aqaba, Jordan. Zoologische Verhandelingen 205: 3-44.

Pulau Hantu Recce Trip

The maiden post on the first field trip on the first day of the year is also the first time for me on Pulau Hantu.

After we arrived, a heavy downpour sent the intrepid explorers scuttling for shelter.

We eventually headed out along the northern coast though the small mangrove patch within the lagoon where it was mostly soft and sandy.

Sponges as well as algae dotted the area and we found quite a few golf ball sponges (Cinachyrella australiensis) along a stretch. These multi-celled animals have small dimples all over giving them a golf ball-like appearance.

The golf ball sponge produce spike-like spicules that act like the skeleton of the sponge.

Sponges are filter feeders that obtaining food by trapping bits of particles in the water. A larger body that is supported by spicules will therefore enable the sponge to trap more food.

The yellow internal skeleton can be seen radiating out from the centre of the sponge. Some of these needle-sharp spicules stick out of the surface and can cause great discomfort if they pierce and break in human skin when handled.

Passing the rock walls, many nerite snails (family Neritidae) were seen on and along the rocky surface facing the sea. These gastropods feed on algae which they scrape off using their rasp-like radula, an action which is similar to licking ice-cream!

Further out, a seasonal Sargassum bloom blanketed most of the reef and we had to thread gingerly to avoid stepping on unseen organisms. These algal blooms supposedly tend to occur between December to February which coincides with heavy rain and temperature changes. Over at the northern shore, algal blooms involving a different group of algae (dinoflagellates) have been blamed for the mass death of fish and other marine organisms a few days before this.

A tiger-tailed sea horse (Hippocampus comes) was found hiding among the coral reef. The sea horse is in fact a rather odd-looking fish. It is a rather weak swimmer but it makes up for it by having a prehensile tail that can grasp seaweed or rocks to prevent itself from being swept away by strong currents.

This grumpy looking red egg crab (Atergatis integerrimus) was sitting in a tide pool but moved away as we approached.

Red egg crabs are reef dwellers and can grow to a width of 10 cm.

Although they look very attractive, the bright colours actually serve to warn predators that it is poisonous.

The toxins they contain are not broken down even after cooking and they should not be eaten.

Lying in a tidal pool among the Sargassum was a Bohol nudibranch (Discodoris boholiensis). This sponge-eating mollusc has the ability to break off non-vital parts of itself when attacked while the main body makes its escape. The lost parts will then regenerate later.

One of the last animals of the day literally warmed the cockles of my heart. A heart cockle (Corculum cardissa) spotted by Peiya.

This mollusc has a shell made up of two parts - one convex side and a slightly flatter side. They have been reported to host photosynthetic algae in its body in a relationship known as symbiosis. The cockle benefits from food produced by its tenant, while the algae is sheltered and protected from predators.

Soon, it was time to go and we had to leave our heart-shaped cockle behind. Cool weather, great company and lots to discover. I could really enjoy this!

Thanks to Ron for the organism ID.

Further reading:

1. Farmer, M. A., Fitt, W. K. and R. K. Trench. 2001. Morphology of the Symbiosis Between Corculum cardissa (Mollusca: Bivalvia) and Symbiodinium corculorum (Dinophyceae). The Biological Bulletin 200: 336-343.