One of the community science volunteer projects I oversee in the Pensacola area is the Florida Horseshoe Crab Watch. The first objective of this project is to determine whether horseshoe cabs exist in your bay – FYI, they do exist in Pensacola Bay. The second objective is to determine where they are nesting – we have not found that yet, but we have one location that looks promising. One of the things my volunteers frequently find are the molts of the horseshoe crabs. Many keep them and I have quite a few in my office as well. One volunteer was particularly interested in the fact that they even molted and that they could leave this amazing empty shell behind and yet still be crawling around out there. So, I decided to write an article explaining the process in a little more depth than I typically do.
Horseshoe crab molts found on the beach near Big Sabine.
Photo: Holly Forrester.
I titled the article “The Molting of Crabs” but it could be the molting of any member of the Phylum Arthropoda – they all do this. The Phylum Arthropoda is the largest, most diverse, and successful group of animals on the planet. There are at least 750,000 species of them. This is three times the number of all other animal species combined. One thing unique to this group is the presence of an exoskeleton.
The exoskeleton is made of chiton and is secreted by the animal’s hypodermis in two layers. It provides the protection that the calcium carbonate shells of mollusk do but is much lighter in weight and allows for much more movement. Arthropods have jointed legs, hence their name “arthropod – jointed foot”, to enhance this movement even more. The entire body is covered by this exoskeleton.
The outer layer is thin and called the epicuticle. It is composed of proteins and, in many arthropods, wax. The inner layer is the thicker procuticle. The procuticle consists of an outer exocuticle and an inner endocuticle. These are composed of chiton and protein bound to form a complex glycoprotein. The exocuticle is absent at joints in the legs and along lines where the shell will rupture during molting. In the marine arthropods the procuticle includes salts and minerals. Where the epicuticle is not waxy and is thin, gases and water can pass into the animal’s body. The cuticle also has small pores that allow the release of compounds produced by glands within the animal. Not all of the cuticle is produced on the outside of the body. Some portions of it are produced around internal organs.
The colors of the crabs and other arthropods are produced by concentrations of brown, yellow, orange, and red melanin pigments within the cuticle. Iridescent greens, purples, and other colors are produced by striations of the epicuticle refracting light.
One disadvantage of the protective exoskeleton is the fact that it does not grow as fast as the interior soft tissue. They have solved this problem by periodic shedding, or molting, of the shell. Science calls this ecdysis, but we will continue to call it “molting”.
Step one is the detachment of the hypodermis from the skeleton. The hypodermis now secretes a new epicuticle. Step two, the hypodermis releases enzymes which pass through the new epicuticle and begin to erode the untanned endocuticle of the old skeleton. During this process the muscles and nerves are not affected and the animal can continue to move and feed. Step three, the old endocuticle is now completely digested. With the new procuticle produced by the hypodermis, the animal is now encased by both the old and new skeleton. Step four, the old skeleton now splits along predetermined lines, and the animal pulls out of the old skeleton. The new exoskeleton is soft – hence, the “soft-shelled blue crab” – and can be stretched to cover the increased size of the new animal. This stretching occurs due to tissue growth during steps 1-3, and from the uptake of air and water. The hardening of the new skeleton occurs due to the tanning of the new cuticle.
Stages between molts become longer as the animal grows older. Thus, there are numerous molts when the animal is young and as they age, they become fewer and farther between. Most insects have a finite number of molts they will go through. The marine arthropods seem to molt throughout their lives, though some species of crabs cease molting once they reach sexual maturity.
Molting is under hormonal control. Ecdyisone is secreted by certain endocrine glands, circulated through the blood stream, and acts directly on the epidermal cells. There are hormones that, if secreted, will inhibit the molting process. These are usually released if the animal senses trouble and that is not a good time.
During the period when the old shell is being digested many of the salts and minerals are absorbed by the tissue of the animal. Some people can eat crab but have allergic reactions when consuming soft shell crabs – most likely due to the increased salts and minerals in the tissue at this time. During step 3, many crustaceans will seek shelter and will remain there for a period of time after molting allowing the new shell to harden. The regeneration of lost limbs occurs during the molting process as well.
Molts of many species are hard to find because the “soft-shelled” animal can consume the molt to increase needed salts and minerals – or other marine animals may do so for the same reason. But horseshoe crab molts are pretty common and cool to collect. Another common molt found is that of the cicadids in the pine forest areas of our panhandle. The entire process is pretty amazing.
Reference
Barnes, R.D. 1980. Invertebrate Zoology. Saunders College Publishing. Philadelphia PA. pp. 1089.
In terms of diversity and abundance, the Phylum Arthropoda is the most successful in the Animal Kingdom. Between them all, there are over one million species. They can be found in all habitats, from the deepest part of the ocean to the highest places in the mountains, from the polar region to the most extreme deserts. Most are insects, but there are also arachnids, centipedes, millipedes, and the ones most common to the marine environment – the crustaceans. With the numerous species within this group, and new ones being discovered all the time, the classification of arthropods is constantly changing. Currently Crustacea is considered a subphylum and there are about 30,000 species within.
Insects are one of the most abundant forms of life on the planet. Photo: Princeton University
There are several keys to the success of arthropods. Number one, their shell. It was seen with the mollusk that having a hardshell to protect your soft body was a winner. However, mollusk make their shells from heavy calcium carbonate. Though this provides excellent protection against most predators, it did slow them down considerably making it much easier for predators to catch them. It is understood that in the world of defense, speed is important. The arthropods make their shells from a strong, but much lighter material called chitin. This material is strong but serves as the creatures’ exoskeleton and must be shed periodically as the animal grows.
Number two, their legs. The name “arthropod” means jointed foot and one glance at the legs of any of these, you will see why scientists call them this. To increase speed animals, need to break contact with the surface of the ground. Birds are the best, lifting off and flying – the fastest form of location there is. The slug-like mollusks have their entire bodies in contact with the sediment, as the “slug” along the bottom of the sea. Many creatures have developed legs and walk, this is the case of the arthropods. Some hop great distances, like the flea. Others can actually swim, like blue crabs. And many of the insects have wings and can fly. But these jointed legs, along with a lighter shell, have been very effective defense for these creatures.
Number three, their sense organs and brain. Though not as intelligent as octopus and squid, arthropods are very aware of their environment and very quick to respond to trouble or a food source. Drop a piece of cheese during a picnic and see just how fast the ants find it. Heck don’t drop the piece of cheese and see how quickly they find it! These animals have a series of hairs, bristles, and setae connected to their shell that can detect movement and pressure changes in the environment. There are canals, slits, pits, or other openings in the shell that can detect odors. And then they have their compound eyes. Compound in the sense there are more than one lens. Each lens does provide an image of the target (in other words, they do not see 100 images of you) but rather each provides a level of light intensity sort of like individual pixels in a computer image, or the image we see when the camera displays “squares” of light so that you cannot read someone’s license plate, or the logo on their t-shirt – we see this on TV news and shows often. Compound eyes do not produce as clear an image as our eyes, but they are MUCH better at detecting motion, and there is an advantage to this. Try stomping on a cockroach, or swatting a fly, and you will see what I mean.
And number four, a high reproductive rate. You see this in many of the “prey” type species. Most arthropods are dioecious (males and females) and can produce millions of offspring at rates that you could never consume them all. So, they survive and can quickly support gene flow and adaptation. These are well designed animals.
Blue crabs are one of the few crabs with swimming appendages.
Photo: Molly O’Connor
In the crustacean world we find several groups. They differ from their arthropod cousins in that they have 10 jointed legs, and two sets of antenna (one set long, the other short). They include at least 30,000 species including the shrimplike cephalocarids, the shrimplike branchipods, the shrimplike ostracods (common in the deep sea), the roachlike copepod (part of the plankton we spoke about in another article in this series), the mystacocards, branchiurans, the familiar barnacles, krill, the roachlike isopods, flea like amphipods, and the most familiar of the group – the decapods – which includes the crabs, shrimps, and lobsters. It is this last group we will focus on.
There are three basic body parts to an arthropod. The head, thorax, and abdomen. In crustaceans the head and thorax are fused into one segment called the cephalothorax (the head of a shrimp or crawfish). The abdomen is what most call the shrimp and crawfish tail (the part we usually eat).
Insect body parts.
Crabs are the ones we most often encounter when exploring the seagrass beds. Not that the others are not abundant, they are, they are just not seen. They differ in that their abdomen is curled beneath their cephalothorax. The most commonly encountered is the famous blue crab (Callinectes sapidus). Most crabs have modified two of their 10 legs into chelipeds (claws) and most folks seeking crabs for dinner are aware of these claws. The blue crab belongs to a group called the protunid crabs which have modified two additional legs into swimming paddles – they can swim. They are often found crawling around the edges, and within, the seagrass searching for food. When detected over sand, they quickly bury themselves and sometimes people step on them not knowing they are there. When spooked they often will emerge with chelipeds extended and when the time is right, will scurry off running sideways with one cheliped pointed at you. They can get quite large and are a popular fishing target for both recreational and commercial fishermen. The males (the ones with the long then telson on their curled tailed) are more common in the upper estuaries. The females (the ones with the more round telson) frequent the lower bay. During breeding season, the males will move to the lower estuary to find a female. Once found he will crawl on her back and “ride” for a couple of days in what commercial fishermen call “doublers”. At some point the male will provide a tube filled with sperm called a spermatophore to the female. He then moves on. The female will store the spermatophore until she feels it is time to fertilize the eggs, then does so. The eggs begin to develop beneath her abdomen in a spongy looking mass. Early in development the mass is an orange color. Closure to hatching it is brown. Females carrying this spongy mass are called gravid and are illegal to harvest in Florida. The larva will be released in the millions as tiny plankton and go through several life stages before becoming young crabs and starting the whole story again. These popular crabs live for about five years.
Male and female blue crabs.
Photo:
Another crab found in the grassbeds is the spider crab (Libinia dubia). This crab does resemble a spider, is slow moving, and very hard to see. It has small chelipeds and feeds on debris and organic material collected by the grasses. They too can get quite large and resemble the king crabs harvested in Alaska.
Stone crabs (Menippe mercenaria) are more often associated with rocky bottoms, or artificial reefs, but they have been found in burrows and crevices within grassbeds. The have wide-stocky chelipeds, which is a favorite with some seafood lovers. Those in the grassbeds do not get as large as those found around the reefs of south Florida, where they support a large commercial and recreational fishery.
The stone crab has been a popular seafood target in Florida for decades.
The hermit crab is a common resident of grassbeds. The most frequently encountered is the striped hermit (Clibanarius vittatus). Like all hermit crabs, they lack an external shell covering their abdomen and must cover their tail with an empty mollusk shell. Their curled abdomen can grab and wrap around the columella within the mollusk shell and carry around their new home. These hermits have been found in a variety of mollusk shells and are found roaming the beaches at low tide feeding on organic debris and cleaning the grassbeds.
A room with a view: a stripped hermit crab sizes up a potential residence
One crab that is often associated with seagrasses is not actually a crab at all. The horseshoe crab (Limulus polyphemus) lacks antenna and is more closely related to the arachnids. This ancient mariner has been plowing the bottoms of estuaries for over 400 million years. They resemble stingrays with their elongated telson and feed on a variety of small invertebrates both in the grassbeds, and in other estuarine habitats. They are quite common in the eastern panhandle and seem to be making a recovery in the western end.
A large horseshoe crab found in Little Sabine.
Photo: Amanda Mattair
Though rarely seen, shrimp are very prolific in our seagrass beds. Pulling a seine or dip net through the grass will expose their presence, usually in high numbers. The most commonly collected species are those known as grass shrimp (Palaemonetes sp.). There are a few species, and all are small and mostly translucent, though one is a brilliant green. Feeding on organic debris within the grassbed these little guys are an important food source for the larger members of the community.
The more famous of the shrimp group are the brown and white shrimp. These are the species we find on our dinner plates and are one of the most popular commercial species in the country. Brown shrimp (Farfantepenaeus aztectus) are also known as bay shrimp and “brownie”. They are a darker brown than the white shrimp and their uropod (the fan on the tail of the shrimp) is lined in a red color. They do not get as large as the whites and are very popular for fried and steamed dishes. The white shrimp (Litopenaeus setiferus) is a larger shrimp, is lighter in color (“white”), and their uropod is lined with a neon green color. Both of these commercially important species spend their juvenile and young adult days in the grassbeds of our estuaries. Later in the fall the adults move into the nearshore waters of the Gulf where they spawn and die. The planktonic larva drift back into the estuary with the incoming tide, finding the grassbeds and the cycle begins again.
The famous Gulf Coast shrimp.
Photo: Mississippi State University
The large diversity of crustaceans within the grassbeds speaks to the importance of this habitat to all marine life. Many are commercially important to the local economy and depend on a healthy ecosystem to survive. All the more reason to protect our grassbeds.
Most of us in the Florida panhandle realize how important seagrasses are to the ecology of our estuaries. Not only do they provide habitat for commercially important finfish and shellfish, but they also help trap sediments, remove nitrogen from the system, and slow coastal erosion. But seagrasses throughout Florida have suffered over the last 50-60 years from environmental stressors created by humans. There has been a large effort by local municipalities to reduce these stressors, and surveys indicate that these have been successful in many locations, but there is more to do – and there are things you can do to help.
Reduce Stormwater Run-off
Stormwater run-off may be the number one problem our seagrass beds are facing. With the increased development along the panhandle, there is a need to move stormwater off properties and roads to reduce flooding of such. Older communities may still have historic drain systems where rainwater is directed into gutters, which lead to drainpipes that discharge directly into the estuary. This rainwater is freshwater and can lower the salinity in seagrass beds near the discharge to levels the seagrasses cannot tolerate, thus killing them. This stormwater also includes sediments from the neighborhood and businesses that can bury grass near the discharge site and cloud the water over much of the system to levels where needed sunlight cannot reach the grasses. Again, killing the grass.
Most would say that this is an issue for the county or city to address. They should be redesigning their stormwater drainage to reduce this problem. And many municipalities have, but there are things the private homeowner or business can do as well.
One thing is to modify your property so that the majority of the rainwater falling on it remains there and does not run off. Much of the rainwater falling on your property falls on impervious surfaces and “stands” creating flooding issues. You can choose to use pervious surfaces instead. For larger businesses, you might consider a green roof. These are roofs that literally grow plants and the rainwater will irrigate these systems with less running into the street. There is a green roof at the Escambia County Central Office Complex building in Pensacola. To learn more about this project, or visit it, contact Carrie Stevenson at the Escambia County Extension Office.
The green roof on top of the Escambia County Central Office Complex in Pensacola.
For those buildings that cannot support a green roof, you can install gutters and a rain barrel system. This moves rainwater into a barrel (or series of barrels) which can then lead to an irrigation system for your lawn or garden. All of which reduces the amount entering the streets.
rain barrels can be used to capture rainwater and avoid run-off.
Finally, you can use pervious materials for your sidewalks, driveways, and patios. There are a number of different products that provide strength for your use but allow much of the rainwater to percolate into the groundwater, thus recharging the groundwater (our source of drinking water) and reducing what reaches the street.
Plant Living Shorelines
Coastal erosion is an issue for many who live along our waterfronts. The historic method of dealing with it is to build a seawall, or some other hardened structure. These structures enhance the wave energy near the shoreline by refracting waves back towards open water where they meet incoming waves increasing the net energy of the system. Something seagrasses do not like. There are many studies showing that when seawalls are built, the nearby seagrass begins to retreat. This increased energy also begins to undermine the wall, which eventually begins to lean seaward and collapse. Placement and maintenance of these hardened structures can be expensive.
FDEP planting a living shoreline on Bayou Texar in Pensacola.
Photo: FDEP
Another option is a softer structure – plants. The shorelines of many of our estuaries once held large areas of salt marsh which provide habitat for fish and wildlife, reduce erosion, and actually remove sediments (and now pollutants) from upland run-off. But when humans moved to the shorelines, these were replaced by turf lawns and, eventually, seawalls. Returning these to living shorelines can help reduce erosion and the negative impacts of seawalls on seagrasses. Actually, several living shoreline projects enhanced seagrasses in the areas near the projects. Not all shorelines along our estuaries historically supported salt marshes, and your location may not either. It is recommended that you have your shoreline assessed by a consultant, or a county extension agent, to determine whether a living shoreline will work for you. But if it works, we encourage you to consider planting one. In some cases, they can be planted in front of existing seawalls as well.
Avoid Prop Scarring While Boating
Seagrasses are true grasses and posses the same things our lawn grasses have – roots, stems, leaves, and even small flowers – but they exist underwater. Like many forms of lawn grass, the roots and stems are below ground forming what we call “runners” extending horizontally across the landscape. If a boat propeller cuts through them form a trench it causes a real problem. The stems and roots only grow horizontally and, if there is a trench, they cannot grow across – not until the trench fills in with sediment, which could be a decade in some cases. Thus “prop scars” can be detrimental to seagrass meadows creating fragmentation and reducing the area in which the grasses exist. Aerial photos show that the prop scarring issue is a real problem in many parts of Florida, including the panhandle.
The scarring of seagrass but a propeller. These can remain “open wounds” for years.
Photo: Rick O’Connor.
The answer…
When heading towards shore and shallow water, raise your motor. If you need to reach the beach you can drift, pole, or paddle to do so. This not only protects the grass, it protects your propeller – and new ones can be quite expensive.
If Florida residents (and boating visitors) adopt some of these management practices, we can help protect the seagrasses we have and maybe, increase the area of coverage naturally. All will be good.
If you have any questions concerning local seagrasses, contact your local Extension Office.
In Part 5 of this series, we looked at a group of invertebrates that few people see, and no one is looking for – worms. But in this article, we will be looking at a group that seagrass explorers see frequently and some, like the bay scallop, we are actually looking for – these are the mollusks.
With over 80,000 species, mollusk are one of the more successful groups of animals on the planet. Most fall into the group we call “seashells” and shell collection has been popular for centuries. There is an amazing diversity of shapes, sizes, and colors with the snail and clam shells found in coastal areas worldwide. As snorkelers explore the seagrass beds it is hard to miss the many varieties that exist there.
Seashells have been collected by humans for centuries.
Photo: Florida Sea Grant
One group are the snails. These typically have a single shell that is coiled either to the right or left around a columella. Some are long and thin with a extended shell covering their siphon (a tube used by the animal to draw water into the body for breathing). Others are more round and ball-shaped. Each has an opening known as the aperture where the animal can extend its large fleshy foot and crawl across the bottom of the bay. They can also extend their head which has an active brain and eyes. Snails lack teeth as we know them, but many do have a single tooth-like structure called a radula embedded in their tongue. They can use this radula to scrape algae off of rocks, shells, and even grass blades. Others will use it as a drill and literally drill into other mollusk shells to feed on the soft flesh beneath.
In the Pensacola area, the crown conch (XXX) is one of the more common snails found in the grasses. This is a predator moving throughout the meadow seeking prey they can capture and consume. Lighting whelks, tulip shells, and horse conchs are other large snails that can be found here. You can often find their egg cases wrapped around grass blades. These look like long chains, or clusters, of disks, or tubes, that feel like plastic but are filled with hundreds of developing offspring.
The white spines along the whorl give this snail its common name – crown conch.
Photo: Rick O’Connor
A close cousin of the snail are the sea slugs and there is one that frequent our grassed called the “sea hare”. This large (6-7 inch) blob colored a mottled green/gray color, moves throughout the grass seeking vegetation to feed on. When approached, or handled, by a snorkeler, they will release a purple dye as a “smoke screen” to avoid detection. Snails secrete a calcium carbonate shell from a thin piece of tissue covering their skin called a mantle. The genetics of the species determines what this shell will look like, but they are serve as a very effective against most predators. Most… some fish and others have developed ways to get past this defense. But the slugs lack this shell and have had to develop other means of defense – such as toxins and ink.
This green blob is actually a sea slug known as a sea hare. It was returned to the water.
Photo: Rick O’Connor
A separate class of mollusk are the bivalves. These do not move as well as their snail cousins but there are NO access points to the soft body when the shell is completely closed – other than drilling through. One creature who is good at opening them are starfish. Seabirds are known to drop these on roads and buildings trying to crack them open. But for the most part, it is a pretty good defense.
Bivalves possess two siphons, one drawing water in, the other expelling it, and use this not only for breathing but for collecting food – all bivalves are filter feeders. They will, at times, inhale sand particles that they cannot expel. The tend to secrete nacre (mother of pearl – shell material) over these sand grains forming pearls. Most of these are not round and are of little value to humans. But occasionally…
The pen clam is a common bivalve found in grassbeds.
Photo: Victoria College.
Oysters may be one of the more famous of the bivalves, but they are not as common in seagrass beds as other species. Most of our seagrass species require higher salinities which support both oyster predators and disease, thus we do not see as many in the grasses. Clams are different. They do quite well here, though we do not see them often because they bury within the substrate. We more often see the remaining shells after they have been consumed, or otherwise died. The southern quahog, pen shell, and razor clam are clams common to our grassbeds.
The one group sought after are the bay scallops. Scallops differ from their bivalve cousins in that they have small blue eyes at the end of each ridge on the shell that can detect predators and have the ability to swim to get away. They usually sit on top of the grasses and require them for their young (spat) to settle out. They are a very popular recreational fishery in the Big Bend area where thousands come very year to get their quota of this sweet tasting seafood product.
Bay Scallop.
Photo: FWC
There is another group of mollusk that are – at times – encountered in the seagrass beds… the cephalopods. These are mollusk that have lost their external calcium carbonate shells and use other means to defend themselves. This includes speed (they are very fast), color change (they have cells called chromatophores that allow them to do this), literally changing the texture of their skin to look and feel like the environment they are in at the moment, and expelling ink like some of the slugs. This includes the octopus and squid. Both are more active at night but have been seen during daylight hours.
The chromatophores allow the cephalopods to change colors and patterns to blend in.
Photo: California Sea Grant
As mentioned, shell collecting is very popular and finding mollusk shells in the grassbeds is something many explorers get excited about. You should understand that taking a shell with a living organism still within is not good. Some areas, including state parks, do not allow the removal of empty ones either. You should check before removing.
The word “jellyfish” tends to initiate a similar response in most people – “scream”, “run”, “this is going to hurt”. Being stung by a jellyfish is not pleasant and is something most would prefer to avoid. Our beaches warn us when they are out by flying a purple flag.
When exploring the seagrasses, this is not the first animal people thing they will encounter. Few associate jellyfish with the seagrass community. But within any community there are those we call residents (they reside here) and those we call transients (just passing through). It is the second group that we can place most jellyfish, at least the ones we are concerned about.
The sea nettle.
Photo: University of California at Berkley.
Jellyfish are animals, but not your typical ones. They are obviously invertebrates but differ from most others by having radial symmetry (having a distinct top and bottom, but no head nor tail). They possess ectoderm and endoderm (so, they have a skin layer and some internal organs) but they lack the mesoderm that generates systems such as the skeletal, circulatory, and endocrine. Though they do not have a brain, they do have a simple nervous system made up of basic neurons and some packets of nerve cells called ganglia. They seem to know when they are not in the upright position and know when they have stung something – which initiates the feeding behavior. But they are pretty basic creatures.
When you view a jellyfish the first thing you see is the “bell” and the tentacles – we always see the tentacles. The bell is usually round (radial), could be bell-shaped, or could be flat. It is made of a flexible plastic-like jelly material called mesoglea. Most of the mesoglea is actually water. When you place most jellyfish on the dock and come back in a few hours there may be nothing but a “stain” of where it was. It completely evaporated. There are some exceptions to this, like the moon jelly and the cannonball jelly, who leave thick masses of mesoglea for long periods of time.
Image: Wikipedia.
If you look closer at the “bell” you will see shapes within the mesoglea. Some are stripes, and may have color to them, others look like a clover leaf. These are the gonads of the animal. Jellyfish are hermaphroditic (the gonads can produce both sperm and egg), and they reproduce by releasing their gametes into the water column when triggered by some environmental clue to do so.
Around the edge of the “bell” many have a thin piece of tissue called the velum that can undulate back and forth and allow the jellyfish to swim. Swimming can involve moving up or down in the water column, or turning around, but the swimming action is not very strong and the tide and current actually plays a larger role in where the animals go – like pushing them through a seagrass bed.
Under the “bell” is a single opening, the mouth, that leads into a simple gut (the gastrovascular cavity). This serves as the stomach of the creature. But there is no anus, when the jellyfish has digested its food, the waste is expelled through the same opening – the mouth. This is called an incomplete digestive system.
Jellyfish are predators and hunt small creatures such as baitfish. Though they know whether they are upside down or not, and may be able to detect light, most have no true eyes and cannot see their prey. Some species may be able to detect scent in the water and undulate their velum to try and move towards potential food, but most drift in the water and hope the tide carries them to dinner. To kill their prey, they extend tentacles into the water. These tentacles are armed with stinging cells known as nematocysts. Each nematocyst holds a coiled harpoon with a drop of venom at the tip. They are encased in a cell membrane and are triggered when an object, hopefully food, bumps an external trigger hair that will fire the harpoon. This will then trigger the release of many nematocysts and the potential prey will be “stung” by many drops of venom. The venom can either kill or paralyze the prey at which time the tentacles bring it to the mouth. Many jellyfish have venom that is painful to humans, like the sea nettle and moon jelly, others have a mild venom that we do not even notice. Some have a very strong venom and can be quite painful, like the Portuguese man-of-war which has put some in the hospital. The famous box jelly of Australia has actually killed humans. We do have box jellies in the Gulf of Mexico, but they are not the same species.
This box jellyfish was found near NAS Pensacola in November of 2015.
Photo: Brad Peterman
As the tide pushes these transients through the seagrass meadows, their tentacles are extended and small baitfish like juvenile pinfish, croakers, and snapper become prey. But there are resident jellyfish as well.
With the Phylum Cnidaria (the stinging jellyfish) there are three classes. Class Scyphozoa includes the bell-like jellyfish that drift in the water column with extended tentacles – what are referred to as medusa jellyfish. But there are two other classes that include benthic (bottom dwelling) jellyfish called polyps.
Polyp jellyfish resemble flowers. The “bell” part is a stalk that is stuck to a rock, pier, or seagrass blade. Their tentacles extend upwards into the water column giving the creature the look of a flower. Instead of drifting and dragging their tentacles, they hope to attract prey by looking like a hiding place or other habitat. The sea anemone is a famous one, and a good example of the polyp form. But it also includes corals and small polyps known as Hydra. Hydra are tiny polyps that are usually colorless and can easily attach to a blade of turtle grass. Here they extend their tentacles into the water column trying to paralyze small invertebrates that are swimming by or grazing on the epiphytes found on the grass blades.
The polyp known as Hydra.
Photo: Harvard University.
Another jellyfish that drifts in the current is Beroe, what some call the “football jellyfish” or “sea walnut”. This a relatively small blob of jelly that lacks tentacles but rather has eight rows of cilia/hair (ctenes) along its side that move quickly and move this animal through the water. But like their medusa cousins, not against the tide or current. These jellyfish do not sting, they lack nematocysts, and hence are in a different phylum known as Ctenophora. Kids often find and play with them when they are present, and they are luminescent at night. These stingless jellyfish feed on small plankton and each other and are another transient in the seagrass community.
The non stinging comb jelly.
Florida Sea Grant
There are certainly species of jellyfish to be aware of and avoid. But as you look deeper into this group there are harmless and fascinating members as well. Most of these Hydra are very small and hard to see while snorkeling, but they are there. Another creature to try and find while you are exploring and play “seagrass species bingo”. Have fun and stay safe.
I recent took my granddaughter on a dolphin tour out of Pensacola Beach. It was amazing. It was a cool October morning, not a cloud in the sky, the winds were calm, the water crystal clear due to the lack of rain over the past few weeks, and the dolphins were out.
They are amazing animals and always seem to grab your attention no matter how many times you see them. I was a student at Dauphin Island Sea Lab from 1980-81 and taught there from 1985-1990. No matter how many times we heard “dolphins” when out on one of the research vessels, everyone had to run over to look. People do enjoy seeing dolphins. There is just something about them.
A group of small dolphin leap from the ocean.
Photo: NOAA
During the tour at one location, we saw a group of them (a pod) feeding on fish in the shallow water. They would roll and chase, you could see the sand being kicked up from the bottom as they did. At another location we saw them in breeding mode. Slower moving, caressing, fluke slapping as they turned all around in the water near us. The tour guide told us all sorts of dolphin facts, and some great jokes to go along with them. It was a good program, and my granddaughter was loving it.
She looked over at me at one point and said, “dolphins use to walk on land”. I responded that actually their ancestors did. Dolphins, as we know them, were very much aquatic animals. This led to thoughts on other dolphin questions I have heard over the years.
What is the difference between a dolphin and a whale?
Size… and in some cases teeth.
All whales and dolphins are in the mammalian order Cetacea. Mammalian orders are divided based on the type of teeth they have. Cetaceans are homodonts, meaning they have only one type of tooth. For the toothed whales, these are canines, they lack the molars and incisors that many other mammals have. But some have no teeth rather a specialized fibrous material called baleen, similar to the bristles of a broom, with which they can filter plankton from the water.
There are over 90 species of cetaceans in the world’s oceans, 21 of those are known from the Gulf of Mexico. In a recent published survey by the National Marine Fisheries Service, most of the cetaceans in the Gulf of Mexico are of the toothed whale variety and most occur beyond the continental shelf (which is between 60 and 140 miles south of Pensacola). The only baleen whale in their report was the Byrde’s Whale (Balanopatera edeni). They estimate about 33 of these whales based on their transect surveys and all of these were found beyond the continental shelf between Pensacola and Apalachicola Florida. The largest of the toothed whales reported was the sperm whale, which can reach over 60 feet. They estimate 763 sperm whale in the Gulf, and they were found across the basin beyond the continental shelf.
But it is the bottlenose dolphin (Tursiops truncatus) that we see on the dolphin tours. This is a relatively small toothed whale, reaching lengths of 13 feet, though most in the Gulf region are less than 10 feet. They are the most abundant and most frequently encountered cetacean near shore and within the estuaries and seem to prefer these shallower waters to the open Gulf beyond the shelf. The National Marine Fisheries Service divides them into stocks based on their geographic distribution. They report 37 different stocks of bottlenose dolphins in the northern Gulf. These are divided into western, eastern, and northern stocks, and then subdivided into estuarine stocks. There are separate stocks for the Perdido Bay and Pensacola Bay groups. This report indicated the stock size for the Pensacola and Perdido Bay dolphins was unknown, though our tour guide indicated there were about 250 in the Pensacola Bay stock. The National Marine Fisheries Service did report about 179 dolphins in the Choctawhatchee Bay stock. The reports estimated over 51,000 individuals for the northern Gulf.
Though not listed as endangered or threatened by the Endangered Species Act, there is some concern on the smaller estuarine stocks and so they have been labeled as “strategic”. There has been fishery related mortality with these dolphins in our waters, primarily with longlining and otter trawl operations, but losses are less than four animals/year and do not seem to be impacting their populations.
What is the difference between a dolphin and a porpoise?
Though many associate the long beak as a dolphin, there are dolphins with short snouts. Killer whales are actually large dolphins. The answer goes back to the teeth, as it always does when classifying mammals. Dolphins have conical shaped teeth where porpoise have more spade shaped ones.
How smart are dolphins?
As everyone knows these are highly intelligent animals. They use an audible form of communication that includes squeaks, clicks, and whistles, to keep the pod together. Researchers have discovered that these audible sounds have a sort of “accent” to them that tells dolphins which pod the dolphin communicating is from. This appears to be very important being that dolphins from one social pod may not accept others from different one. I remember in 1993 when a group of five pantropical spotted dolphins stranded on Pensacola Beach. There were four adults and one 3-month year old in the group. After failed attempts to return the dolphins back to the Gulf, it was decided to transport them to a quarantine area near the EPA lab on Pensacola Beach. There was a virus spreading through some European populations and they did not want to risk taking them to the Gulfarium. In route three of the four adults passed away. The remaining adult was named Mango and the juvenile was named Kiwi. After a period of time in quarantine Mango passed away leaving on the young Kiwi. There was a move to return Kiwi to the wild but some of the dolphin experts on scene told me the likely hood of a different pod accepting Kiwi was a risk, and finding her original pod was very unlikely. After determining the dolphin did not have the virus of concern, they decided to move her to the Gulfarium in Ft. Walton Beach, where she lived the rest of her life.
How does dolphin echolocation work?
Echolocation is different than communication, in that it is inaudible. As with communication, the sounds are produced by expelling air through the blowhole. In the case of communication, there is a muscle that partially closes the opening of the blowhole producing the sounds we hear. In echolocation this is completely closed, and the sound waves are moved through a fat filled melon near the head. The shape and density of the melon can be changed by the animal to produce different frequencies of sound but all inaudible to our ears. These sounds are emitted through the melon into the environment, where they contact something and “echo” back to the dolphin. These echoes are received in a fat filled cavity of the lower jaw and transferred to the brain – where the animal is then made aware of the object out in front of them. Some studies suggest that it may be more than knowing there is an object, they may be able to distinguish different kinds of fish. Though it is most effective within 600 feet, studies show their range may be up to 2000 feet. Studies have also shown that some species of toothed whales can alter the frequency of these echolocated sounds to stun their prey making them easier to catch.
Dolphins are amazing animals.
They live between 30 and 50 years in the wild. During this time, they form tight social groups, feed on a variety of prey, and produce new members every 2-3 years. There is so much more to the biology, ecology, and social life of these animals and we recommend you read more. Once you understand them better, we also recommend you take a dolphin tour to view these amazing creatures.
