Prof. P. Lynch
5 Oct. 2005
(Figure 1 – the Loligo in apparent motion)
Exploring the Loligo (Plei) Squid
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.
Species: L. plei
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.
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).
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.
Dissection Pins (4+) – Deployed to keep a dissected squid spread apart and/or to isolate specific areas for examination.
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 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.
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.
(Figure 2 – A properly dissected squid emphasizing the caecum, tentacles and mantle)