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Chris Krause

Prof. P. Lynch

MB20 #2107

5 Oct. 2005

(Figure 1 – the Loligo in apparent motion)

Exploring the Loligo (Plei) Squid

Introduction

The sea is home to many enigmatic and fascinating organisms but none is more

dynamic and celebrated then the squid. The best way to learn about the squid is to get

inside of one, but before we do that we must understand some basic fundamental

information about it. Under the microscope today is the Loligo Plei.

The Slender Inshore Squid, also known as the Arrow Squid (Loligo), is a

medium-sized squid belonging to the family Loliginidae. It occurs abundantly in coastal

waters of the Atlantic Ocean, from Argentina northward to North Carolina.

Kingdom: Animalia

Phylum: Mollusca

Class: Cephalopoda

Subclass: Coleoidea

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Order: Teuthida

Suborder: Myopsina

Family: Loliginidae

Genus: Loligo

Species: L. plei

Physical Description

As both common names imply, these squid have elongate, cylindrical bodies with

a length to width ratio of 7:1. The arms are in contrast short and weak; the two tentacles

are somewhat less than the mantle length. The rhombus-shaped fins are large, up to

about 50 percent of the mantle length. Running the ventral length of the mantle is a

noticeable ridge. The squid are a reddish orange color with a large compliment of

chromatophores.

The suckers of the arms possess blunt teeth. On the meaty ends (clubs) of the

tentacles, there are four rows of suckers; the inner two (mesial) rows are three times as

large as the outer two (marginal) rows. The larger suckers have horny rings with up to

45 teeth.

These squid reach a maximum mantle length of 33 centimeters in males and 22

centimeters in females. Sexual dimorphism is apparent, with the left ventral arm in

males modified into a hectocotylus; this is used to facilitate fertilization during mating.

Males also have a number of purple stripes running lengthwise on the ventral side of the

mantle; along with other visual cues produced by chromatophores, these stripes are

used in elaborate courtship displays.

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Ecology

Due to their abundance, Slender Inshore Squid are important to both commercial

and subsistence fisheries. During the annual summer mating season, the squid

congregate in large numbers near shore. During the austral summer in waters off Brazil,

annual catches may reach 763 metric tonnes.

While there may be safety in their numbers, the squid are an important prey item

for large fish such as tuna and sharks and a number of cetaceans: Pygmy Killer

Whales, Orcas, Atlantic Spotted Dolphins, Rough-toothed Dolphins, and Bottlenose

Dolphins are all known predators of Slender Inshore Squid. Other predators include

South African and Antarctic Fur Seals.

The squid prey upon several species of estuarine fish, including: killifish,

mosquito fish and mollies. Small crustaceans such as grass shrimp are also taken.

(cited in its entirety but modified for spelling and grammar from wikipedia.org)

Our goal has now become to better understand such an animal, inside and

out. The most effective means of comprehensively understanding the Loligo is to

dissect it. Because the Loligo is so advanced we can expect to find a number of

developed organs in the lab but for now we will focus on four of its most

prevalent features we are bound to discover upon further examination: the

caecum, tentacle(s), pen and mantle (skin).

Materials

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Lab Goggles – Equipped at all times to ensure the eyes are protected from trace liquids

still present within the Loligo carcass and also accidental scalpel slashes.

Dissecting Scalpel – Utilized to part open the surface skin and cut free attached organs

within the squid.

Lab Apron – Equipped to protect the skin and clothes from trace liquids and the scalpel

blade.

Dissecting Tray – The dissection platform; the wax backing allows for easy pinning and

manipulation of the squid and the raised metal sides allow for easy collection of any

possible trace liquids and organic material.

Loligo Carcass – The squid we are dissecting should be freshly caught or suspended in

formaldehyde or other preserving agent. Note: The squid MUST be dead before

dissection is to be commenced.

Pincers – Used to grip, manipulate and control the dissection process when attached to

the Loligo carcass, also used to extract and sort organic materials.

Powdered Latex Glove (2) – Used to protect the hands and wrist area from liquids as

well as offering primitive protection from the scalpel blade.

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Dissection Pins (4+) – Deployed to keep a dissected squid spread apart and/or to

isolate specific areas for examination.

Method

Initially the lab was prepped for dissection. This was done by donning the lab

apron and securely tying it at the midsection, equipping powdered latex gloves on both

the left and right hands and finally snugly fitting the lab goggles to the skull. Next the

dissection tray was cleaned thoroughly in cold water and dried in the lab heater so that

no trace liquids could be observed on the wax surface. The scalpel, pins and pincers

were then purified in similar fashion.

Next the Loligo carcass was removed from its isolated refrigeration and placed

upon the dissecting tray and finally placed upon the lab table. The squid was then

positioned in such a fashion so that a vertical cut down the face side of the mantle

(funnel) was possible (see attached “Squid Anatomy 1”).

Dissecting pins were then inserted into the squid so that the cut could be

performed with as little downward force as possible to minimize the damage done to the

soft internal features. The scalpel was then utilized to completely cut the squid open

from the top down to right above the eyes, effectively exposing an internal cavity. The

skin of the mantle being effectively parted was now rolled over with the pincers so that

the whole of the cavity could now be examined. The squid was now effectively entirely

accessible to us (the head area excluded).

The basic dissection was now concluded, next would be isolation and

examination of individual areas of the Loligo. To perform this action the tissues around

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the organ intended for extraction must be secured by redeploying the dissection pins

and then the scalpel must be used to isolate said organ, while the pincer is used to

extract it. For example, to examine the mantel a patch of it must be isolated in a square

pattern using the dissection pins and then be cut free using the scalpel, at which point

the skin can finally be extracted for examination. This method was used for all four

subjects: the caecum, tentacle(s), pen and mantle (skin).

Properties were recorded for each of these organs and their image sketched for

further research and consideration. When the experiment was completed the organic

remains of the Loligo were discarded and all tools thoroughly cleaned using the method

outlined in paragraph 1 of this section.

Results

Just as suspected all four organs (the caecum, tentacle(s), pen and mantle

(skin)) were located, isolated and examined. Several advanced organs were also

observed such as exceptionally developed eyes and nervous system when compared to

other invertebrates such as a sea anemone or other cnidarian.

For a comprehensive representation of a properly dissected squid please see the

attached “Squid Anatomy 1” which effectively displays not only a proper dissection but

also all major parts of the Loligo’s structure. In dissecting and subsequently researching

our findings on such a squid we have indeed had our understanding of the animal

expanded ten fold and can begin to draw conclusions on why such an advanced

nervous and optical system is necessary for its survival and feeding.

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(Figure 2 – A properly dissected squid emphasizing the caecum, tentacles and mantle)

(Figure 3 – The extracted pen of a squid – note this is from a larger squid and not the Loligo)

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(Figure 4 – The pen when dipped into the squid’s ink sac can be used to write like a normal ballpoint pen!)

(Figure 5 – Detail of the suckers which are attached to the tentacles and arms)

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(Figure 6 – Real life detail of the tentacle and suckers – note that this particular example is not the Loligo)

Discussion

Caecum

The caecum (See figure 2) is basically analogous to the human caecum – it is

essentially the organ where absorption and digestion occurs (Wheeler).

To better understand the purpose of the caecum let’s briefly review the digestive

processes of Loligo. Food is torn apart into smaller pieces by the jaws on the head and

the food particles, once swallowed, pass along the long esophagus to the stomach. The

muscular stomach mixes the food with digestive enzymes. As digestion proceeds, food

is gradually transferred to the caecum where the food is mixed with more digestive

enzymes. Unlike most other mollusks which complete digestion intracellular in the

digestive gland, squids absorb nutrients into the blood flowing through vessels in the

wall of the caecum. Indigestible wastes in the caecum are carried by ciliated ridges to

the intestine, the wastes then passing to the rectum where they are finally voided

through the anus into the exhalant water current. (cited in its entirety but modified for

spelling and grammar from Randell)

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It is surprising that a squid or any invertebrate would develop an organ so similar

to that of humans. As an aside, the squid also shares similar organs such as a stomach

and anus – suggesting universality amongst advanced organisms to cater to the

requirement of dealing with waste. With two creatures which share little to no

evolutionary connection it is baffling as to why such similar organs would develop – and

there is no apparent reason for this other then analogous development.

Tentacle

The squid’s tentacle is surrounded in teethed circular suckers which are intended

to tear into and stick to a target so that the prey can be drawn inward. Once the prey

approaches the beak it is attacked by the arms (similar to tentacles but smaller and

surrounding the beak) and eventually crushed by the beak itself once immobilized. This

method of attacking prey is not only devastating but entirely effective and represents

perhaps the most lethal method in all of the sea. Combine this with the squid’s

trademark speed and maneuverability and you have a truly fearsome opponent. The

Loligo’s larger brother the giant squid can even defend itself against sperm whales!

Most ancient cultures even have epic stories of squid attacks and of ships being pulled

down into the abyss, it is apparent that like spiders and snakes, they are a primeval fear

in the back of every human being’s mind and we cannot help but momentarily recoil at

the sight of one, even if isolated in a tank with 15’ thick walls.

Pen

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The pen represents the evolutionary remains of the squid’s shelled ancestors. All

mollusks must have this structure somewhere on them, whether it is a shell surrounding

them or a hidden and transparent lattice of a pen on the interior of the mantle. The pen

can literally be used as a modern pen if care is taken to puncture the ink sac and dip the

tip into it. Applying this combination to paper can result in pretty good looking writing –

the ink is permanent and stains clothes just like synthetic ink. I managed to write

“Marine Bio is my god” before the weak pen of the Loligo disintegrated. On larger squid

specimens the pen is much more pronounced, being up to a foot long or more and

having sufficient weight and durability to it. I suspect that these were once used as

writing implements by the ancients and we have gotten our word for pens from it.

Mantle (Skin)

Last but not least the most spectacular organ of the Loligo is the mantle and

more specifically, the chromatophore. A squid or octopus much like a chameleon can

instantly transform his skin pigmentation to match his surroundings. Since squids are

predators, this mechanism is just as much as a tool used to stalk and hunt prey as it is

for defense.

Chromatophores or pigment cells are color changing cells. A chromatophore is

composed of a single chromatophore cell and numerous muscle, nerve, glial and sheath

cells. These cells are contractile and contain vesicles that contain three different liquid

pigments. To change their color the cells distort their form or size stretching or

contracting their outer covering thus changing its translucency or opacity. Octopuses

can operate each individual chromatophore resulting in a wide variety of color schemes.

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These cells are highly developed and numerous in the cephalopods. (cited in its entirety

but modified for spelling and grammar from wikipedia.org)

Although the squid is an exceptionally intelligent creature for an invertebrate we

must deduce that it’s remarkably pronounced nervous system is dedicated to this

mechanism and this is why the brain is so large. It most likely requires a great deal of

processing power to simultaneously change hundreds of cell’s colors in tandem and this

most likely explains the elaborate nervous system.

Final Word

We have only scratched the surface of squid anatomy and examination; there are

volumes to be written on this exquisite and complex creature. In studying squid and

other advanced invertebrates such as octopuses we can better learn where evolution is

headed and where the root of all life stems from. Perhaps in the future we will

completely understand some of the squid’s eccentricities and intelligence but today we

can only look and deduce from what we are exposed. In only examining four structures

of the Loligo we have brushed the most fascinating of all sea creatures but there is still

an empire more to learn, and a universe more to attain.

Bibliography

(Figure 1) Sarabel, Chino, Otto E. Wieghardt. (2005, September, 5). "Pescados tipicos

en LANZAROTE" (Illustration: Loligo) LANZAROTE Isla Mitica. (Online), Available:

http://www.webdelanzarote.com/pescados.htm

http://www.webdelanzarote.com/pesca12.jpg

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Attributed. (2005, September, 5). "Slender Inshore Squid" wikipedia.org. (Online),

Available:

http://en.wikipedia.org/wiki/Slender_Inshore_Squid

Attributed. (2005, September, 5). "Chromatophore" wikipedia.org. (Online), Available:

http://en.wikipedia.org/wiki/Chromatophore

(Figure 2) Wheeler, K. and D. Fautin. 2001. "Cephalopoda" (On-line), Animal Diversity

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http://animaldiversity.ummz.umich.edu/site/resources/biodidac/ceph011b1.gif/

medium.jpg?XTHEME=POPUP

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http://giantsquid.msstate.edu/LessonList/dissection.html

http://giantsquid.msstate.edu/Images/GS11.jpg

http://giantsquid.msstate.edu/Images/GS17.jpg

(Figure 6) Attributed. 2003. "Evolutionary Biology" (On-line), iMedia. Accessed October

05, 2005 at

http://imedia.buffalo.edu/galevbio.html

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Accessed October 05, 2005 at

http://www.usask.ca/biology/randell/203/mollusca/Page17.htm

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Aldrich, F. A., and Steele, V. J. 1967. "The Giant Axon of the Giant Squid" Architeuthis

dux , Steenstrup, with observations made on the animal's mode of locomotion.

(unpublished).

Aldrich, F. A., and Bradbury, H. E. 1970. "Locomotion of Giant Squids of the Family

Architeuthidae." (unpublished).