It is that time of the day again. The soft and sweet sounds of the waves flowing seaward permeate the air and thousands of creatures emerge as the shore is left uncovered by the blanket of seawater.
Welcome to the intertidal zone – the part of the shore that is exposed twice daily when it is low tide, but submerged when it is high tide.
Let me introduce myself: I am Harold the hermit crab. You’ve probably seen some of my kind before while walking along our shores – those seashells that ‘crawl’ about – yes, that’s us. I am pretty sure they call me ‘hermit crab’ because I reside alone in empty mollusc shells for my lifetime.
But here’s a secret – I’m not actually a true crab.
True crabs have four pairs of walking legs but I only have two; and they have hard chitinous exoskeletons or an ‘outer shell’ while I have to live in an empty shell forever as protection. Don’t tell the rest though!
The intertidal zone being exposed is a prime time for us intertidal creatures to roam the shores and scavenge for food (I’m not picky, I eat almost anything from algae to dead creatures). Along the way to my usual dining rock, I spotted a few of my kind gathering around something in the distance. I crawled over to take a look, stepping into a tide pool as the cool waters rejuvenated my sun-bathed body. I realised that they were scavenging on a dead snail. Hmm.. maybe later, I thought. As I walked away, I heard something gliding across the sandy shore. I turned around and peered over – behold, a beautiful moon snail.
Its beautiful shell radiated in the sunlight. A magnificent large, white mass protruded from the shell opening, exerting its dominance. This white mass is its muscular foot, which it uses to burrow into sand or glide across the shore. This foot can be really scary – it can wrap around a clam, and the snail’s radula (something like a tongue) would emerge to drill a perfectly circular hole within the clam shell so that it can digest the clam (you won’t want to mess with these guys!).
So next time you see perfectly circular small holes in a seashell, perhaps a moon snail is looming around! Oh and one more thing – these snails secrete mucus over layers of sand to create spiral sand collars along the shore where they lay eggs, so be careful not to step on these!
Alright, enough about the moon snail. I’m going to scuttle over to see what’s in the seagrass.
Ah, the seagrass is a sanctuary for a plethora of marine creatures as they provide food and habitats for them. In fact, some of our seagrasses are food for dugongs that visit our shores once in a while!
In the distance, I noticed a couple of sea cucumbers lying amongst the seagrass – the usual suspects: the pink warty sea cucumber and the thorny sea cucumber. Did you know that sea cucumbers are actually animals and not plants? They belong to the same ‘group’ as starfishes, and have organs such as a mouth, anus and respiratory tract. Also, they are not sedentary all the time and can move slowly via little rows of tube feet on their bottom side.
They have a pretty impressive defense mechanism too. When they sense danger or feel threatened, they are able to expel their intestines and organs to form a white sticky mess outside their body in a process known as evisceration to deter predators. However, this comes at the expense of organs, and it takes time to regenerate. These sea cucumbers are also really important to our ecosystem due to their roles in recycling nutrients and ‘cleaning up’ our seabed!
Just as I was about to walk off, a large shadow appeared over me.
Oh no – is that what I am thinking of? I turned around.
They have arrived.
This article is Part 1 of a two-part series about the biodiversity of the intertidal zone. Want to find out what’s going to happen to Harold the hermit crab? View part 2 here.