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Dorsal view

The world is full of animals that have diets and feeding styles remarkably different from my own. Today’s is a relative of the earthworm and is often sanguivorous, meaning it lives on blood - it is a leech. Many leeches are both sanguivorous and predatory and feed on aquatic macroinvertebrates (i.e., insect larvae, nymphs, and naiads).

Before we go further, let me cover a couple of helpful mnemonics. In biology we use anterior and posterior and my aides-mémoires is A comes before P – thus making it easy to recall anterior is the front and the posterior is the rear. Next, are the words dorsal and ventral and the cue is D comes before V; dorsal is topside and ventral is underside – top then bottom.

Ventral view showing young in cocoon of parent.

A large sucker at the posterior of a leech and its smaller anterior sucker are one of its most well-known and prominent features. The large posterior sucker is the location of the anus and the smaller anterior one has the mouth. The mouth can be located centrally on the anterior sucker or be a slit at the edge along the edge.

Leeches are somewhat distant relatives of the earthworm and share some common characteristics. For example, like many earthworms, they may have a thin and slippery exterior and many breathe through their skin.

Ventral view showing mouth at anterior end.

Like many Annelids, it is hermaphrodite, having both male and female reproductive features for reproduction. Although it is inviting to imagine they self-fertilize, they rarely do so. Rather, they engage in reciprocal fertilization by exchanging spermatophores with another leech. The reason for this is that self-fertilization increases the likelihood of amplifying genetic anomalies that result in an abnormality, something most organisms try to avoid. 

Ventral view showing sucker at posterior end.

Leeches have a wide variety of reproductive strategies but all include producing fertilized eggs and then placing them in a cocoon. Many affix the cocoon to substrates such as logs, rock, and plants. Others, as the one featured here today produce a cocoon that attaches to the ventral side of the body. Eggs that are in cocoons attached to a substrate have a large yolk rich in nutrients that sustain the developing young. After the hatching, the young in the ventrally located cocoon of this leech attach themselves with the posterior sucker. Food then passes through the parent’s skin into the intestinal tract through the anus. Because this particular leech is predatory it does not need to attach to a host to get food – something that is not possible while it is carrying its young for several weeks or even months.

A ventral view showing where the cocoon was located on the parent of the species Placobdella ornata.  This species lives in the folds of skin on the legs of freshwater turtles (i.e., the Western pond turtle).

The leech you see here is Placobdella ornata, and Kate found it near a turtle basking site. This particular leech occurs in the folds of skin around the legs of the Western pond turtle (Emys marmorata). Because it does attach itself to a turtle that spends a particularly long time out of water it has a robust outer covering that keeps it from desiccating (drying out) when it is out of the water. It is predatory and sanguivorous because the young develop in a cocoon on its ventral side. 

Newly hatched Placobdella ornata found in a cocoon on the ventral side of a parent. Note the well developed posterior sucker that attaches to the parent's modified ventral tissue so nutrients can flow into the hatchling.

Consider this possible interesting evolutionary story. Overtime a turtle develops that can spend a great deal of time out of water warming itself in the sun to digest its food and get heat for other body processes. Because water is the “stuff of life”, this particular turtle develops a hardy skin, which keeps the water in. Think about it a second, the better an organism can conserve water the more able it is to live away from water. One of the best ways to keep the water in is to have a thick watertight skin. For example, frogs and salamanders have thin skin and need to live near water so they do not dry out. In contrast, tortoises and lizards that can tolerate the desert have a think dry skin. Then there are freshwater and saltwater crocodiles that need to keep the water out and they have a thick skin too. And, all the while the turtle is evolving, so is the leech and its skin and way of life change as well. I have oversimplified it but you get the idea! When I come up with these kinds of stories – it makes biology and what I do so COOL!