Corner of Natural Curiosities

Saturday 22 March 2014

How and What Do Sea Urchins Eat?

In most sea urchins (such as the regular sea urchins), the mouth is on the bottom side of the body, whereas the anus is on the upper surface. This is an excellent adaptation to living on the seafloor, so that the mouth is in contact with the substrate, enabling it to graze on algae and detritus. Some species, such as the European Edible Sea Urchin (Echinus esculentus), a North Atlantic species, are omnivorous and feed on both sponges and bryozoa, in addition to seaweed.

In these regular sea urchins, the mouth consists of a complex feeding mechanism (known as Aristotle's lantern) which consists of five strong jaws, each with a single tooth of the mineral calcite. After food has been grasped by the apparatus, it is then passed up through a complexly coiled gut, and the waste is passed upward through the anus.

Above: a simplified vertical cross-section through a regular sea urchin, showing the digestive organs. (M = madreporite; GP = genital pore).

In heart urchins and sand dollars (irregular sea urchins) there is a distinct front and back (due to their bilateral symmetry). The mouth is still on the lower surface, though it is nearer the front, and the anus is nearer the back end.

Above: specimen of heart urchin (Echinocardium), showing its lower surface, including mouth and anus (Courtesy of UCL, Grant Museum of Zoology_specimen NON2391)

Sand dollars have short spines which move the sand and organic debris over their upper surfaces and into their mouths — allowing the sea urchins to 'sieve' through the substrate surface for food.

Heart urchins, living 10 cm or deeper within burrows, beat tiny hairs (cilia) on their spines to draw in currents of water, containing small particles of food. These currents also help to wash the faecal waste along a specialised sanitary pipe, at the back of the burrow.

Above: simplified cross-section through a burrow of Echinocardium. Note the long tube feet above and behind the urchin which are used to construct parts of the burrow. Smaller sticky tube feet, at the front of the urchin, catch food particles and pass them to the mouth.

Saturday 15 March 2014

Where Do Sea Urchins Live?

Like other echinoderms (which also include starfish, brittle stars, sea cucumbers and sea lilies), sea urchins are marine invertebrates.

They occur in oceans, mainly in shallow coastal waters. Lacking the ability to osmoregulate (control the balance of salt and water), they rarely venture into brackish waters (where there is a freshwater content).

Most sea urchins (particularly the regular sea urchins) live on harder ocean substrates, clinging onto rocks in turbulent zones using their tube feet. The irregular sea urchins (including sand dollars and heart urchins) prefer more sandy substrates. Sand dollars live just beneath the surface, whereas heart urchins live deeper (10 cm or more) within burrows.

Thursday 6 March 2014

How Do Sea Urchins Reproduce and Grow?

The sexes are separate in sea urchins. At breeding time, normally in the summer, the gonads (containing eggs or sperm) swell considerably within the sea urchins' plated skeletons.

Spawning (releasing of eggs and sperm) takes place on a massive scale, at the same time within members of the same population. These products are released from the sea urchins via the genital plates, at the top of the organisms.

Above: a simplified vertical cross-section through a regular sea urchin, showing the gonads. (M = madreporite; GP = genital pore)

External fertilisation then takes place with new sea urchins starting their lives as swimming larvae (echinopluteus). The swimming larvae, unlike most sea urchin adults, are bilaterally symmetrical (one imaginary plane divides them into two equal halves).

These larvae also have arms, unlike all sea urchin adults, which they use to trap plankton for food. After several weeks, a part of each larva (the rudiment) starts to develop into an adult sea urchin and structures such as the arms are resorbed (dissolved and assimilated).

The newly developed sea urchins — those that have not been eaten by predators — then sink to the seafloor. Most of these will have no front or back end (being radially symmetrical) and will be able to move equally easily in all directions.

Some species of sea urchin produce eggs which are brooded by females and develop directly into adult sea urchins (there is no planktonic stage). In these species, the females can be more easily distinguished from the males as they have specialised brood pouches for their eggs. Species which adopt this life-cycle strategy are usually confined to polar waters.

Monday 30 September 2013

“Sea Urchins Feed through Their Bottoms”. True or false?

"Sea urchins feed through their bottoms"... I have heard several times, including a similar remark from the film, Johnny English [DVD](upon hearing this, in a sushi bar, Rowan Atkinson's character then proceeds to spit out the sea urchin dish, in disgust).

This myth stems from the remarkably different anatomy, or body plan, of sea urchins (echinoids).

Sea urchins consist of a somewhat spherical body, covered in spines. In most species, the mouth is on the lower or 'bottom' surface (known as the 'oral surface') — whereas its ‘true’ bottom or anus is on the opposite side of the organism, on its upper surface (‘aboral surface’).

Thus, sea urchins feed through their ‘bottom’ surface, but this includes their mouth, not their anus. So Rowan Atkinson’s character need not be so disgusted…

This unusual body plan (at least compared to our anatomy) appears to be an adaptation of the sea urchin to living on the ocean floor, so that its mouth is in contact with the substrate, allowing it to feed on algae or detritus. As the anus is situated on its upper surface, waste does not therefore become mixed up in its food.

Read more about sea urchins...

Thursday 26 September 2013

Peacock Caterpillars in September?

These strange alien-looking black creatures are, believe it or not, caterpillars of the butterfly species, the Peacock (Inachis io). They can be seen here (Sevenoaks, UK) eating their favourite food, the unpopular stinging nettle (Urtica dioica). They shelter, here, underneath their communal web built of silk — an organic, wispy tent also incorporating their droppings and the skin which they periodically shed in order to grow. Note the spines, which provide these caterpillars with a source of protection from predators, particularly when they wriggle their body from side to side, or roll up in a ball.

However, the most unusual fact about the sighting, is that they were spotted in September — several months after the single brood of caterpillars normally emerges.

Typically, the adult female normally leaves 200 or more small eggs in May, on the leaves of nettles. The caterpillars normally emerge 2 to 4 weeks later in late May or June and form their chrysalis in early July. In late July and August, the adults emerge and begin to stock up on nectar for their hibernation during the cold British winter.

So, then Peacock caterpillars in September? This must be a second brood, a rare occurrence for the species but occurs during 'good years' . And, indeed, this year we've had a good, warm summer. The Big Butterfly Count, run by the charity Butterfly Conservation, has recorded four times as many butterflies this year, compared to 2012 — a real surge — with an increase in Peacocks of 3,500%! Read more...

However, will this second batch survive? By the time they have transformed into adults, later in the autumn, they will face much greater difficulties surviving the cold (being reliant on the heat of the sun for temperature regulation) and also gathering nectar from fewer sources around. Read more about butterflies...

Below: an adult Peacock feeding in August on a buddleia