Defenses
 
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Invertebrates hot buttons for invertebrate defenses part of BCCR hot button for behavioral defenses part of invertebrate defenses in BCCR hot button for toxic chemicals part of invertebrate defenses in BCCR hot button for structure part of invertebrate defenses in BCCR
The topic considered here reating to defenses of invertebrates deals with behaviour. Other invertebrate defenses are accessible via the icons.
 
 

Invertebrate defenses: behaviour

hot buttons for behaviour part of BCCR

Behavioural defenses in Caribbean invertebrates are many and varied, but are generally encompassed in the icon-topics shown here.

This part of behavioral defenses deals with escape, mostly accomplished by fast swimming and/or quick withdrawal into shelter.

 
 

Invertebrate defenses: behaviour: escape

 
 

seahorse dive leader for Biology of Caribbean Coral Reefs website

photograph of a spiny lobster Panulirus argus taken from a video

"Watch out for lobsters. They can pull away quickly and have lots of sharp spines on their body, and especially the antennae." - St. John 2007

NOTE Panulirus argus

 
 

photograph of spiny lobster Panulirus argus restingphotograph of swimming crab Portunus sebae
Fast escape-swimming or quick withdrawal into the safety of a burrow, tube, or crevice is the first and sometimes only line of defense for many reef invertebrates.

The hind legs of swimming crabs Portunus sebae are flat and paddle-like, enabling quick escape from predators 0.5X

 

For fast, backwards escape the Caribbean spiny lobster
Panulirus argus powerfully flexes its abdomen forwards
and the broad, splayed-out tail fan acts as a paddle 0.3X

 

photograph of Loligo opalescens in Barbados courtesy George Lilly, NewfoundlandOctopuses, squids, and other cephalopods use jet-propulsion to escape from danger. Over the first few seconds of swimming a squid's velocity can surpass that of a salmon but, unlike a salmon, a squid tires quickly...within just a few moments. Photograph courtesy George Lilly, Newfoundland.

 

Loligo opalescens in Barbados



The process of jet propulsion is explained in the panels below:

 
drawing 1 in a series of 4 to illustrate mechanism of jet propulsion in a squid drawing 2 in a series of 4 to illustrate mechanism of jet propulsion in a squid
This view shows the spacious mantle cavity of a squid. Water flow for general gas exchange, and removal of urine and feces enters on either side of the siphon.
Features of interest within the cavity are the gills (ctenidia) and the anus.
The inflow is created by rapid contraction of special muscles in the mantle wall that expands the volume of the mantle cavity. The seawater flows past 2 valves on either side of the siphon, the motion of the water causing the valves to be pushed open.
drawing 3 in a series of 4 to illustrate mechanism of jet propulsion in a squid drawing 4 in a series of 4 to illustrate mechanism of jet propulsion in a squid
The seawater flows to back of the mantle cavity, turns upwards, and then moves frontward, irrigating the gills, and picking up urine from the nephridiopores
(not shown), feces from the anus, and ink from the anus if it is being released.
The mantle walls now contract powerfully, forcing the water out the siphon in a strong jet. Notice in the drawing that the outgoing flow forces the 2 valves closed, directing all the flow out the siphon . Ink, if present, is forced out as well in a dark cloud.
 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of ventral surface of Humboldt squid Dosidicus gigas taken from a video

"Here's a graphic illustration of how the jet works, this time in a stranded Humboldt squid. Oops! Not too good for the camera!" - Baja de los Angeles, Mexico 2005

NOTE Dosidicus gigas

 
 

photograph of several sea hares Aplysia brasiliana stranded on a beach
There are about 5 species of sea hares in the Caribbean Sea, at least 2 of which are confirmed swimmers. Sea hares have several other lines of (chemical) defense and have only a few predators so, while swimming could serve as a means of escape, it more than likely serves some other as yet unknown function. Photograph courtesy Anne Dupont, Florida.

 

 

 

 

Stranded sea hares Aplysia brasiliana on a beach in Florida, possibly after
a bout of swimming. This species is a stong swimmer, often engaging in
swims in late afternoon and early evening for as yet unknown reason 0.6X



 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of Aplysia brasiliana swimming, taken from a video Aplysia brasiliana is found commonly throughout the Gulf of Mexico and coasts of Florida. This video was taken during research experiments on its swimming energetics. Note the bobbing motion of the head, discussed below - University of Texas Marine Science Institute, Port Aransas 2002
 
 

list of main features of the daily life of a Caribbean sea hare Aplysia brasiliana
Scientists are divided in their opinions on the function of swimming in sea hares Aplysia. Let's ask the professor for a summary of the daily activities of A. brasiliana to give us some background on what its function may be.

Correspondent: "Professor, please take a moment to sum up the life of this species of sea hare."

Professor: "This species reaches about 20cm maximum length and lives for a single year. Individuals have voracious appetites, consuming huge volumes of seaweeds each day, and growth is fast. When reproductively mature, which is at a relatively small size, they spend long periods each day copulating and depositing eggs. In the Gulf of Mexico they seem to do most of their swimming at night, beginning in late afternoon/dusk. Swimming is accomplished by flapping broad mantle flaps known as parapodia. The mode of propulsion is not fully understood, but may involve sculling, jetting, or hydrodynamic lift, or perhaps a combination. They commonly swim at the sea surface and rhythmically bob their heads out of the water in time with the cycle of swimming beats."

With that as a background, the Professor now asks that you give some thought to how swimming might have evolved in Aplysia brasiliana and other species of sea hares, whose habits of life are similar to those of A. brasiliana. So, do this and let's see what ideas you can come up with. Afterwards, CLICK HERE for some possible hypotheses and explanations.

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of Aplysia pulmonica swimming, taken from a video

Of 36 or so species of sea hares worldwide, only some 6 or so are known to swim. This individual Aplysia pulmonica is one of a large swimming group that arrived on the shores of Oahu, Hawai'i in the 1980s. It was filmed swimming after being captured and held overnight in a laboratory aquarium. At the time the video was made it had been swimming continuously for 18h. The occurrence of this species on the beaches of Oahu was a unique event, as it has never been recorded since - Kewalo Marine Laboratory, University of Hawaii 1980. Video courtesy Dan Dunlop, Honolulu.

NOTE a survey of the scientific literature reveals more swimming species than this. Some of the confusion originates from incorrect identifications, multiple naming of the same swimming species, and the propensity of many Aplysia species to flap their parapodia when re-immersed in seawater after being confined in a collecting bucket. Sea hares do this to aerate their gas-exhanging ctenidium. The situation is compounded by other authors repeating these same erroneous observations

 
 

Some possible hypotheses with explanations (in no particular order):

1. if you propose that they swim to find food, then you may be right. A locomoting sea hare will stop dead in its tracks on contact with an edible seaweed, whether it is swimming in open water or crawling on the sea bottom. There is also a suggestion that sea hares with full guts are less likely to swim than ones with empty guts. This may be in response to feedback signals saying "it's now time to digest", or perhaps they are just too heavy to swim (sometimes considerable sand is ingested with the seaweed food).

2. if your idea is that swimming is just a handy means to get around or that it replaces crawling as a locomotory means, then you may be on to something. This species is a notably poor crawler in comparison with other sea hares.

3. could swimming help to find a mate in open water? Tests on Aplysia brasiliana show that 2 individuals that meet when swimming just pass one another by . As for one individual using swimming to locate a crawling individual for use as a copulatory matel, it could happen, but keep in mind that swimming in A. brasiliana seems to be mostly at the sea surface.

4. if you think that sea hares may swim to locate and join large breeding aggregations, then your idea is good. They routinely form such aggregations by crawling from downstream locations along gradients of pheromones, but they could do the same by swimming.

5. if you think that swimming could lead to colonisation of new habitats, then it's a good idea, but for the following: 1) it has never been observed, 2) swim durations are probably too short to make this an effective strategy, and 3) the life cycle of sea hares includes a pelagic larva that floats in the plankton for several weeks and has the potential to be transported long distances by ocean currents; hence, is more likely to be the chief distributive phase in the life cycle, not the adult.

6. if you've hypothesised that they swim to escape from predators, then it is a plausible notion, even though it has never been observed in either the field or laboratory. Sea hares tend to have few predators, possibly owing to the toxic nature of their flesh and ink secretions. Still, it's a good idea and would be testable.

7. if you think that sea hares swim to get exercise, you had better think again!

8. if you think that sea hares swim to get to the sea surface to enable celestial navigation, well, it's an idea, but without convincing evidence. The video above shows Aplysia brasiliana swimming at the surface and they usually swim at night, but more research would need to be done to confirm that they are actually use celestial cues for naviagation...in any case, navigation for what purpose? Nonetheless, at least one scientist believes that the bobbing enables use of celestial cues (sun, moon, stars, or whatever) for navigation. However, keep in mind that their eyes are simple, non-image-resolving, and are probably useful for light-dark discrimination only.

NOTE these are chemicals released from one individual of a species that affect other members of that same species, often in a reproductive way. As an example, human male sweat carries pheromones that affect the estrus cycles in human females

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