What is the first thing you think of when you hear the term, “sea sponge?” Sponge Bob, right?! Well, you wouldn’t be the only one who seems to have a picture of Sponge Bob living under the sea. Although both sponges, major differences exist between the two, the most obvious being that Sponge Bob is a fictional children’s character. Sea sponges are real animals living under the sea and definitely not in pineapples. This article will give an insight into what sea sponges are and how best to spot them on your next dive with Cape RADD.
Sea sponges are amongst the most bizarre marine animals. The reason for this is because they do not consist of the typical animal body parts, in fact, their classification into the animal kingdom is based on their physiology. It is their plant-like appearance that results in their misidentification by divers and by those snorkelling. Sea sponges have a cellular level of organisation. This means that they consist only of cells specialised to perform different life functions and have no organs present. A sea sponge’s body structure consists of the following, osculum, a spongocoel, and ostia (pores) which are lined with chaonocytes. An osculum is an opening through which water is expelled. A spongocoel is a large central cavity in which water enters via the ostia. Chaonocytes are very small finger-like projections that function to trap nutrients or food particles.
There are three general groupings of sea sponges, according to body shape. They are further classified according to the material that makes up their skeleton (spicules).
The three general groups that sponges belong to are:
• Ascon: These have a typical vase-like shape. They are considered to be the simplest of the groups in terms of structure and make-up of the water-current system. Ascon sponges, referred to as asconoids, have a thin body wall that has short, straight pores that lead directly into the spongocoel. The pores are lined with choanocytes.
• Sycon: These have a distinct tubular body shape. Unlike asconoids, sycon sponges, referred to as syconoids, consist of two types of canals. These are namely radial canals and incurrent canals. They make up the water-current system and make syconoids more complex than asconoids. Syconoids have thicker walls which fold into radial canals. Unlike asconoids, choanocytes do not line all of the ostia, they line only the radial canal.
• Leucon: These are the most varied in terms of body shape as well as have the most complex water-current system of the three groups. Leuconoids consist of a body of thick and dense mesenchyme. Mesenchyme, which is also referred to as mesoglea, is a jelly-like substance that makes sponges feel squishy. Leuconoids are most complex because they have numerous and complex branching canals. Choanocytes are found lining all the small spherical flagellate chambers in leuconoids.
As previously mentioned, sea sponges are classified into the animal kingdom based on their physiology – how they carry out critical life functions. So one may wonder how it is that they feed, reproduce and grow if they don’t have even the most basic morphological structures shared across the animal kingdom.
Sea sponges are sessile filter feeders. What this simply means is that they settle on a substrate for the entirety of their lifespan and filter nutrients as water enters and exits through their bodies. Water enters the spongocoel via ostia. To obtain nutrients, choanocytes filter the water and digestion occurs. Digestion takes place in the cells. After digestion is complete, waste is carried out with the water that exits via the excurrent canals (osculum for asconoids).
Sea sponges are capable of both sexual and asexual reproduction. Although having no true gonads, or sexual reproduction organs (testes and ovaries), specialised cells are capable of producing sperm and eggs.
The first image below depicts the process of sexual reproduction. Sexual reproduction produces free-living embryos which later settle on a substrate to grow into sessile adults.
The second image depicts the most common form of asexual reproduction in sponges, budding.
What is budding? Budding is the process during which an adult sponge experiences an outgrowth of cells which eventually separate to form a different sponge.
They may not be as entertaining as Sponge Bob, but here are 5 cool facts about sea sponges:
1. They have been inhabiting the oceans for approximately 600 million years! Sponge Bob can’t beat that!
2. Sponge Bob may have arms and legs, however, sea sponges have spicules (sharp structures made of either silica or calcium carbonate) that they can use as a protection mechanism.
3. The size of the sponge is dependent on the rate of water flow through the body.
4. Sponges play a significant role in the oceans as they can help improve water quality as well as reduce the amount of carbon in the water through nutrient uptake.
5. You can consider sponges to be inspirational to your personal growth. Imagine surviving the tough ocean environment with just cells, at their most basic level, at your disposal!
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For further reading on sea sponge skeletons, visit Britannica.