“Dock Spider” - Friend or Foe?

I think it is a pretty safe bet to say that anyone who has spent a significant amount of time in and around the Thousand Islands has had at least one memorable encounter with the subject of this story, the “Dolomedes Tenebrosus”, or as it is more commonly known, the “Dock Spider”.

Whether it’s finding one of these hairy creatures nestled up in your towel, being met on the swim ladder by one as you are about to climb out of the water, or just having one pop up unexpectedly from between the dock boards as you walk to the boat, the dock spider has the uncanny ability to strike fear into the hearts of all but the hardiest of souls.

However, if we cast our arachnophobic tendencies aside for a moment and look at these animals from a purely scientific perspective, I think you will agree that they are pretty amazing in the ways that they have developed, adapted to their environment, and go about their daily life. 

As any entomologist worth their weight in salt can tell you, every living organism can be sorted into specific groupings by following the rules of taxonomy, which is the science of classifying plants and animals into various categories based on physical characteristics and appearances.

As Table 1 illustrates, the dock spider, along with being a member of the “Animal” kingdom, is part of the “Arthropoda” phylum. The generic characteristics that the members of this group share include a segmented body, three or more pairs of jointed legs, and bilateral symmetry. They also possess a rigid, tough external covering called an exoskeleton whose major component is chitin, a structural carbohydrate polymer.

As we move down the classification chart the specifics of the “Class”, “Order” and “Infraorder” categories narrow down the physical attributes of the creature even more. In the case of our dock spider, the defining physical traits are two body parts, the cephalothorax and the abdomen, and four pairs of legs that are all attached to the cephalothorax. Also, these spiders have eight eyes, all roughly the same size and arranged in two rows, and they do not have any antennae. They have a pair of fangs that are used to inject their venom into prey to paralyze it but, rest assured, it is not harmful to humans unless there is a severe allergic reaction to it. Females have been known, on rare occasions, to strike at people when harassed and their large fangs are capable of penetrating human skin if they choose to bite, but generally they are not aggressive and will run away if confronted.

Figure 1, below, illustrates the general anatomy of a spider.

The “Pisauridae” family of spiders is also known as “Nursery Web Spiders”. They do not spin webs to catch their prey but they do spin an egg sac out of spider silk to lay their eggs in. The female carries this egg sac everywhere she goes using her jaws and pedipalps, rather than attaching it to her spinnerets. When the eggs are ready to hatch the mother spider will build a nursery “web” in a protected location to place her egg sac in. She then guards this nest until the spiderlings hatch and venture into the outside world.

Dock spiders exhibit the typical arachnid characteristic of sexual dimorphism where the female of the species is much larger than the male, quite often up to twice the size or more. It also isn’t uncommon for the female to devour the male after mating is completed, and in light of this fact, the male has been known to bring along a sacrificial fly as a gift for his lady in an effort to take the edge off her hunger before starting the courtship rituals. Unfortunately for the male the presentation of gifts doesn’t always save him from the potentially nasty fate that awaits him as the female may eat him anyway to provide nourishment for her brood. These lady spiders certainly give credence to the expression, “large and in charge”!

There can be well over a thousand spiderlings in the nursery web when all of the eggs have hatched. The hatchlings typically stay in the protective confines of their nest for some time after birth as it usually takes them more than one season to fully mature and be ready to reproduce. It is believed that these spiders can produce two or three egg sacs in their lifetime.

Perhaps the most interesting fact about the “Dolomedes” family of spiders, other than being the largest spiders found in Ontario, growing to over 90 mm. in size, is that they are also known as “fishing” spiders. As previously mentioned, these spiders do not spin a web to catch their prey. Instead, they wander around to stalk their prey and capture it using their front legs, which are equipped with several small claw-like appendages. This is the primary reason that their vision has to be so sharp.

These spiders typically, but not always, reside near water and will hunt their prey along the edge of rivers, lakes and streams. They will rest on the edge of the water with their front legs submerged to sense vibrations in the water caused by approaching prey. When the targeted meal is within reach the spider lunges forward to capture it with its’ front legs and fangs. At this point the venom is injected into the prey to paralyze it and to also aid in the spider’s digestive process. Typical menu items for the “Dolomedes” spider include waterborne or floating insects, small minnows and tadpoles. They will eat other small insects, such as slugs, if their preferred diet is not available.

Not only do these spiders fish from the edge of the shore, they have the ability to run across the surface of the water in pursuit of prey. In addition to this remarkable feat, they can also jump straight up in the air, from the surface of the water, to avoid swimming predators, such as bass. These amazing aquatic feats are possible due to the fact that the spider’s legs have a waxy surface that is hydrophobic, or resistant to water, and do not become wet when in contact with water.

A Scientific American article entitled, “How is it possible for insects and spiders to walk on water or walls?” [2] states, “Because the legs are not wetted by the water, the animal does not become submerged until the downward pull of gravity (the animal's weight) exceeds the opposing vertical component of the water's surface tension.” The article goes on to say, “Fortunately, the weight of the animal, when supported by the water's surface tension, pushes the points of contact downward, creating dimples in the water surface. When the legs are moved backward to thrust the body forward, it is the legs with their accompanying dimples that move backwards, and it is the drag of the water moving past this leg/dimple that gives the spider something to push against.” Thus, the spider is able to propel itself across the surface of the water.

If that isn’t enough to impress you, the “Dolomedes” spider is also capable of submerging itself for up to half an hour, either to hide from a predator or to pursue prey, by capturing air bubbles in the hairs on its legs and using that trapped air to breath while under water. A Wikipedia article on the Dolomedes spider [3] states, “They can also climb beneath the water, and then air becomes trapped in the body hairs and forms a thin film over the whole surface of the body and legs, giving them the appearance of fine polished silver. Like other spiders, Dolomedes breathe with book lungs beneath their abdomens, and these open into the air film, allowing the spiders to breathe while submerged. The trapped air makes them very buoyant and if they do not hold onto a rock or a plant stem they float to the surface where they pop onto the surface film, completely dry.”

The Dolomedes Tenebrosus that is indigenous to the Thousand Islands region is also known as the “dark fishing spider” due to its’ dark grey appearance. There are several distinguishing markings that help to identify this species of spider, including alternating light and dark rings on the legs and three black W-shaped marks on the abdomen. A close cousin of the “Tenebrosus” that is also native to the area is the “Scriptus” which is very similar in appearance but has more white markings on its body.

Although the dock spider, sometimes also called the wharf spider, usually lives near water they have been known to move inland, inhabiting rock piles and wooded areas. As the colder weather approaches they may also seek shelter in more protected areas such as boat houses, cottages or even homes in order to hibernate. During their active periods they tend to hide in corners and crevices during the daylight hours and come out at night to do their hunting. This is their primary defense mechanism against their main predators; birds, snakes, fish, and frogs. Being a cold blooded invertebrate they also enjoy sunning themselves on logs, rock, or docks on warm sunny days but are very skittish and will usually run away or hide when approached.

So the next time you come across one of these gentle giants rather than going with your first instinct to whack it with a broom or crush it with your shoe let it be and observe it for a while. I think you will gain a new found respect for these creatures and will be entertained at the same time.

Acknowledgements: I would like to thank the three photographers that graciously allowed me to use their terrific pictures of Dolomedes Tenebrosus spiders in this article. I found the close-up photograph of the large dock spider in jaimejouelapiano’s on- line gallery (http://jaimejouelapiano.deviantart.com/gallery/40721641?offset=24). The photograph of the mother spider guarding her nest was found on Michael J. DeLuca’s on-line blog (http://mossyskull.com/visions/fishing-spider/). The photograph of the female spider and her babies was supplied by Bill and Judy Munro. Bill is a previous contributor to Thousand Islands Life and other examples of his beautiful nature photography can be found at /BackIssues/Archive/tabid/393/articleType/ArticleView/articleId/1160/NatureOutdoor-Photographyhellip-may-be-just-what-yoursquove-been-looking-for.aspx

References:

[1] Bug Guide  http://bugguide.net/node/view/15740

[2] Scientific American On-line http://www.scientificamerican.com/article.cfm?id=how-is-it-possible-for-in

[3] Wikipedia – Dolomedes , http://en.wikipedia.org/wiki/Dolomedes

[4] YouTube video of Dolomedes Tenebrosus spiders mating  (Viewer Discretion Advised!) http://www.youtube.com/watch?v=PUtWvbi2kTI

University of Arkansas – Department of Entomology http://www.uark.edu/ua/arthmuse//dolomede.html

Bow Narrows Camp Blog http://bownarrows.blogspot.ca/2012/01/dock-or-fishing-spiders-are-nothing-to.html

Frontenac News On-line http://www.frontenacnews.ca/2009/09-25_jun_25/outdoors_lol_09-25.html

By Tom King

Tom King and his wife Marion, have lived in Milton, Ontario for the past twenty-five years, where they both worked and raised their family of three children; Kris, Mike and Becca. Tom still has a strong attachment to the Thousand Islands, having grown up in Gananoque and being a “river rat” from a very early age. The family tries to return to the islands every summer and for the past few years have been renting a cottage on Sampson (a.k.a. Heritage) Island, just out from Gananoque.

This is Tom’s 9th TI Life article.  Click here to see past articles.