Crustaceans, Spiders, Insects [Arthropoda]

These animals all have jointed legs and hard exoskeletons composed mainly of protein and chitin, a type of carbohydrate. This rigid outer cuticle provides structural support, protective armor for the animal’s insides, and places of attachment for its muscles. The drawback to the stiffness of an exoskeleton is that it does not expand as the animal grows. This is why an arthropod must periodically shed its old cuticle and grow a new one through a process called molting.

The bodies of most arthropods are divided into three sections: head, thorax and abdomen. An inner cavity houses an open circulatory system with a poruous, tubular heart and blood.

horseshoe crabs, spiders, ticks
[Chelicertata]
These arthropods have only two body sections, since the head and thorax are fused together. They lack antennae and mandibles (jaws). Instead they have chelicerae (cheli, meaning claws), on their first set of appendages for grinding food before passing it to the mouth. The other four pairs are used for walking.

Atlantic Horseshoe Crab
Limulus polyphemus
Again, not a true crab, since the arrangement of its body segments and appendages has landed it in the same phylum as spiders, ticks and mites. It uses its legs to forage in soft substrates for worms and mollusks. The legs help chew the food before putting it into its mouth, located at the base of the legs. It breathes through gills, which look like pages of a book on its underside. Though scary looking, the horseshoe crab doesn't pinch, and uses its dagger-shaped tail only to right, propel and steer itself.

Groups of them spawn along the shores of Great Bay in the first weeks of June. A good viewing area is the boat ramp at the Sandy Point Discovery Center in Stratham. Just watch out for the eggs. Take a closer look at them in the picture at right.

spiders, ticks, mites
[Arachnida]
These have six pairs of legs: the first for piercing and sucking, the second for sensory perception, the last four for locomotion.

In ticks and mites, the abdomen is fused with the head and thorax. These feed on vertebrate blood, plants and fungi. Though few grazers are able to consume the tough plant material growing in the marshes, mites are able to suck their sap.

Spiders have an abdomen that is usually larger than their fused head and thorax, and linked by a thin pedicel. The abdomen has no legs. Spiders survive on the body fluids of their prey. They capture prey with the help of silk webs, which they excrete from glands in their abdomen, through appendages called spinnerets on the rear of the abdomen.

There are lots of spiders in the tidal marshes. Good places to spot their webs are at the edges of pools and pannes.

Sea Spiders
[Pycnogonids]
While exploring intertidal habitats, you may come across teeny, tiny sea spiders. At about one-quarter inch long they are tough to spot, but they may be found under stones or feeding on hydroids or anemones. Though they are not true arachnids, their bodies appear spider-like, with three segments and four pairs of legs. They also have on their head a proboscis, sometimes flanked by a wee pair of pincher-like appendages.

insects, millipedes
[Mandibulata]
Mandibles (from the mandere, meaning chew) are the pair of appendages by the mouth of these arthropods that bite and chew food—basically the jaws. Since they are easier to spot and identify, only the insects are covered here.

Their three body segments include: a head with a pair of antennae and compound eyes; a thorax with three segments, a pair of legs on each; and an abdomen with 12 segments.

Insects breathe through a network of tubes leading from pores in their cuticle to gas exchange sites in the cells of their body tissues. A cavity inside the insect body contains a kind of blood that, among other things, delivers nutrients to cells.

Some insect eggs hatch into larva resembling the adult, through a process called incomplete metamorphosis. Other insects hatch and the larvae go through a resting stage, the pupa, in which there is a complete reorganization of their tissue. This is called complete metamorphosis.

Some of the insects living in estuary habitats include:

Common Backswimmer
Notonecta glauca
This little bug swims on its back, using two of its legs as oars and breathing from a film of air held in place by fine hairs. When it senses vibrations, it dives in search of prey that may include another insect, or even a tadpole or a small fish. After stabbing with its feeding tube, the backswimmer injects its toxic saliva into the prey and sucks out their body fluids. Be careful with these since they give a nasty, if harmless, bite. This is a freshwater bug, and so can be found in marshes where there are freshwater flows. Dozens of these were collected from a marsh panne of the upper Squamscott River in one scoop of a net.

Springtail
Anurida maritime
This wingless insect is about one-eighth inch long and bluish-gray. If you pull back a pile of seaweed in intertidal areas of rocky shore or marsh, you may see bunches of them floating on the surface of little pools of water.

Praying Mantises
These insects sit quietly among leaves and stems, with their front legs up as if in prayer, waiting to eat. When prey passes within range, the mantis clamps it with its legs, then eats it alive. Mantis legs have spines to help them maintain a grip on their prey. In the marshes around the estuary, there is plenty of food for mantises, including spiders, beetles, butterflies, mosquitoes, crickets and grasshoppers. The mantises are food for birds and bats. They are actually equipped with an inner ear especially tuned to the frequencies of bat echolocation.

Depending on the species, most mantises in this area can range in size up to several inches long. They are usually green or brown. They may pinch you with their legs if you pick them up.

Dragonflies of various species
The photo shows an unidentified species of Meadowhawk dragonfly, a member of the skimmer family. There are several Meadowhawk species that fly around marshes, bogs, sluggish streams and various other wetlands in much of north America, usually from summer through early fall. In general, dragonflies feed on a wide range of prey including ants, flies, butterflies and even other dragonflies and damselflies. Their life span is up to one month, during which time birds, frogs, spiders and other predators may consume them.

Mosquito

lobsters, shrimp, crabs, barnacles
[Crustacea]
What do a lobster and a barnacle have in common? An obvious feature is their tough exoskeleton. But all arthropods have those. What makes these and other crustaceans unique are their two pairs of antennae.

Most species of this group share another trait, the form they take in their first phase of life. Crustaceans break out of the egg as nauplius larvae, or free-swimming, microscopic organisms with three pairs of appendages and usually a single eye. After a series of molts, they take very different adult forms. This larval phase is only one of the criteria for classifying crustaceans. Amphipods and isopods never go through it, and they are still considered part of the phylum because of their structure and form. (Their ancestors are thought to have had a nauplius larval phase.)

The empty shells of crustaceans, especially those of crabs, are common at the shore. But these are not necessarily a result of death. In order to grow, crustaceans periodically shed their old skeletons and grow roomier ones in a process called molting. They detach their muscles from the old skeleton and begin secreting a fresh one. As the old skeleton cracks open, they move out and go off to hide their vulnerable condition until the new skeleton hardens. Meanwhile they pump themselves full of water to ensuring the hardening chitin is large enough for the next phase of growth.

Most crustaceans are aquatic (the pillbug is not) and breathe through gills. But those capable of travel on land, thanks to their jointed legs, can still survive until the tide returns as long as they have a cool, moist refuge. Crabs often hide in the shelter of seaweed piles.

One helpful way to understand crustaceans is to classify them in one of two big groups: either as decapods or non-decapods.

DECAPODS
Lobsters, crabs, shrimp are examples of decapods. Their head and thorax is fused together and covered by a carapace; they have stalked eyes; and of course, they have five pairs of legs (deca—10, pods—legs).

American Lobster
Homarus americanus
It is easy to recognize this crustacean with its long carapace, four sets of legs and cutter and crusher claws. This lobster can be found locally in the Gulf of Maine and up into Great Bay Estuary, where it prefers sandy substrate sheltered by rocks or other objects.

Though they eat a variety of benthic (bottom dwelling) organisms including periwinkles, urchins, worms, sea stars, fish and algae, they prefer crabs. Large fish like Atlantic cod eat adult lobsters. Striped bass and black sea bass sometimes prey on juveniles.

Lobsters generally head into estuarine waters to where the salinity is at least 20 to 25‰. This is why they move into Little Bay and Great Bay in late summer. That is when the water salinity rises to those levels.

Harris mud crab, white-finger mud crab
Rhithropanopeus harrisii
This is one of a couple of mud crabs found in coastal areas north of Cape Cod. This Harris mud crab, which grows to three-quarters of an inch, was found walking in shallow water of the sandy beach at Frost Point in Ordiorne State Park, where the jetty protects Little Harbor. Gosner wrote that they are usually found in brackish water under stones and among algae on dock pilings. As tiny as they are, mud crab claws are strong enough to break open small bivalves and barnacles.

Green Crab
Carcinus maenus
Unknown in the northeast before the late 1800's, this voracious crab is thought to have made its way from Europe as larvae in the ballast water of ships.

Certain traits have helped the green crab become a dominant predator since arriving in New Hampshire in the early 1900's. It is very adaptable, surviving temperatures, salinities and periods of exposure that are intolerable to other crabs. As an omnivore, the green crab has a very flexible appetite, feeding on algae, marsh vegetation, worms, snails, mussels, crabs, oysters, sea stars, urchins, fish and even other crabs.

Some impacts it has had on the ecology of shallow subtidal and intertidal habitats include out-competing the rock and Jonah crabs, reducing clam and oyster populations by preying on juveniles, and interfering with eelgrass habitat restoration efforts by digging up eelgrass shoots while foraging and burrowing.

You can find them in subtidal areas along the shore (if you snorkel), or under seaweed canopies, in rock crevices and among marsh vegetation in intertidal areas. You may also occasionally see one being preyed on by a gull. Though they are greenish above, the males are yellowish below and the females are reddish-orange below.

Rock Crab & Jonah Crab
Cancer irroratus & Cancer borealis
The rock crab is an ivory yellow color, covered with dense patches of tiny reddish spots. Its carapace grows to just over five inches, with small rounded indentations on the front edge. It prefers sandy bottoms and eats worms, bivalves and echinoderms.

The Jonah crab has a larger, dull brown and purple carapace, and reddish orange legs. The indentations on the front edge of its carapace are more jagged. It prefers rocky bottoms, and eats blue mussels and tunicates. They can be found in intertidal areas in more marine salinities.

Long-Clawed Hermit Crab
Pagurus longicarpus
This common hermit crab species is found in intertidal and subtidal waters with at least 18‰ salinity. For their armor they usually choose the shells of these gastropods: periwinkle, mud dog whelk, or oyster drill.

Grass Shrimp & Sand Shrimp
Palaemonetes spp. & Crangon septimspinosa
Grass shrimp are transparent and grow one to two inches long. They feed on detritus, associated phytoplankton and zooplankton, tiny worms and other fauna living between the grains of sediment. Though grass shrimp tolerate marine salinities, they prefer brackish water. They are also abundant water that is turbid, like Great Bay. They are usually found in shallow areas seeking shelter and food among stands of seaweed or oyster beds. Forage fishes like mummichogs prey them on.

Sand Shrimp have translucent bodies with brown patches, and grow to nearly three inches long. They can be found on sandy bottoms and in eelgrass beds in more marine water salinities. They eat the same kinds of things as grass shrimp.

NON-DECAPODS
Many of the members of this group are important food sources for larger animals in the food web. They include the large contingent of plankters, called copepods, which have elongated microscopic bodies and large antennae. Larger shrimp-like plankters, called krill, are set apart from decapods because they have more than 10 legs. Amphipods and isopods have many noticeable body segments with legs on each.

Amphipods
These tiny crustaceans tend to be flattened side to side with an arched back and have seven pairs of legs. Scientists have identified several species in Great Bay Estuary (nearly 7,000 species have been identified worldwide). They are usually found in shallow water or intertidal areas of marshes, eelgrass beds, rocky shores and tidal flats. Amphipods are also found under stones and among marsh vegetation or eelgrass beds.

Though they seem insignificant, amphipods play an important role in the ecosystem. Some burrow tubes in the sediments feeding on microscopic algae and other particles, while others devour piles of vegetation cast ashore by the tide. They are an important food source for fish swimming in the estuary, including mummichogs, killifish, Atlantic silversides, Atlantic tomcods, black sea bass, rainbow smelt and striped bass.

Dozens of amphipod species have been identified in North Atlantic shore habitats, but distinguishing among the delicate variations of their tiny frames is difficult—even for those equipped with a microscope, references and expertise.

Barnacles
Northern Rock Barnacle & Ivory Barnacle
Balanus balanoides & Balanus eburneus
Thousands of these little white crustaceans are found attached to rocks, shells and other hard surfaces on the shore. When covered by the tide, barnacles extend feathery limbs, called cirri, into the water to strain out zooplankton, which are plentiful in the spring and fall. Water flow is important to these creatures, since it carries their food supply. When there is no current, they move the water by beating their cirri.

Barnacles fertilize neighboring barnacles in the fall. The eggs are held until they develop into larvae, which are released in late winter. After metamorphosing for six to eight weeks in the water column, some of the larvae will survive long enough to cement themselves to hard surfaces. They secrete a protective housing in the form of overlapping calcareous plates, which they open while submerged to feed and breathe. They close them to escape predators and dehydration. Barnacles compete with each other, some seaweed species, and other creatures like blue mussels, for limited attachment space. Schools of juvenile fish are among the predators of barnacle larvae. Sea stars and the carnivorous dog whelk snail prey on adults. Sponges also overtake them by smothering them.

The northern rock barnacles, pictured above, are found where salinity is high in the lower estuary. As salinity drops in the upper estuary, they are displaced by the larger ivory barnacle.

Copyright © 2006 Barbara Driscoll.