Things we do in class - Fish dissection

Figure 1 - The Tilapia (Osteichthyes: Actinopterygii: Teleostei: Perciformes: Cichlidae: Tilapiine), before dissection. Instruments of the trade

Today marks the first time dissecting an animal during a practical session in "Topics in Aquatic Biodversity". Have watched people do it, and have heard people do it but have never done it myself before because it was never compulsory during biology classes in Junior College and in university, until now. It was quite an interesting experience, albeit frightening at first because it was fish but it really was not so bad.

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Fishes, a general introduction

Fishes are a diverse group of animals, that both live in marine and freshwater environments.

They are generally split into these extant groups:

• Hagfishes (Myxiniformes)
• Lampreys (Pteromyzontiformes)
• Jawed Fishes
  - Cartilagenous fishes
  - Bony fishes [ray finned - split into Sacropterygii and Actinopetrygii and lobe finned] 

Note: the Sacropterygii are further split into lungfishes, Tetrapods and Coelacanth; Actinopterygii further split into Neoptergyii and Chondrostei (bichirs, surgeons: which have some what ossification but are generally cartilagenous). Neopterygii is further split into gars (swim bladders function as lungs; heteroceral tail, ganoid scales), bowfins (similar to gars; which are actually able to breathe air, storing the air in its swim bladder connected to its digestive tract) and Teleostei

They are, however, paraphyletic, because any animal that "has a cranium, gill-bearing and lack limbs with digits" are lumped to this clade whereas the 'Tetrapods' are nested within this clade. Thus, it is not monophyletic.

Though, that is, besides the point. Most would be interested in fish as a commodity because it is important for food, which leads to problems of overfishing and is popular in the pet trade, which can lead to the proliferation of invasive animals, in the context of Singapore.

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Morphology: the exterior and interior

note: the information presented here is learnt during lessons by Dr Tan Heok Hui

Just looking at the morphology of a fish can yield lots of interesting information. One can never divorce morphology from taxonomy and classification of fishes.

By looking at its exterior morphology (Figure 2), one can infer life history traits, habitat and food preferences about the fish based on certain generalizations -- but of course, there are always exceptions! Further, there are specialised structures, indicating some form of adaptations (e.g. barbels for sensory purposes).

1. Position of mouth: superior, terminal, inferior (and a whole suite of in-betweens)

2. Shape of body: torpedo, streamlined like the Tuna; oval shaped/ compressed laterally like the Mola Mola

3. Dentition: the type of teeth/ teeth plates (hooked, grazer-type, flesh-cutting, grinding; fused that is beak-like), the arrangement of the teeth (cartilagenous fishes have rows of teeth, unlike bony fishes), presence of pharyngeal teeth (to act as grips, not masticatory) which is located in the skull

4. Fins: shape of the five fins (ventral fins: pelvic, anal; dorsal fin(s); caudal fin, pectoral fins), whether it is fused or not; caudal fins can be truncate, emarginate, forked, deeply forked, rounded; pelvic fins can be fused; there can be one or two pectoral fins
5. Lateral line: can be complete or incomplete, from the top of the operculum to the (mid-) end of the peduncle; hyped-up lateral line systems with strong/weak voltage, the former for defense/finding prey and the latter for navigation.
6. Buccal cavity adaptations: in mudskippers it is used to store water, projectile expulsion in archer fishes, egg brooding in Osphronemidae/ Cichlidae/ Apogonidae/ Ariidae
7. Scales: Ctenoid/Cycloid (usually for higher fishes like the Teleostei; Ctenoid have a toothed outer edge, fishes with spiny rays as opposed to soft rays); Ganoid (garfishes, bichirs); Placoid (dermal/epidermal origins for Cartilagenous fishes)

Further, dissection to look at the internal morphology of the fish can further confirm the inferences (based on external morphology) such as the diet (gut content, length of gut), gender (male or female, by the gonads) and age (from otoliths in temperate fishes). Also, one can look at the gill rakers.

Figure 2 - Anatomy of a female fish. In a male fish, the ovary is replaced by the testicles.The bones are dorsal to the organs.
Source: http://en.wikipedia.org/wiki/Fish_anatomy

Figure 3 - Bamboo sharks (Chondrichthyes: Orectolobiformes: Hemiscylliidae), a male and a female. The males have a pair of claspers at the anal fins.

Figure 4 - Xin Rui's neat dissection area. She found two little crabs and one fish in the stomach of the predatory fish (second fish from the left).