Diapositive 1 - uliege

Comparison of the adductor mandibulae and jaw
lever mechanisms between three damselfishes
(Pomacentridae)
Damien OLIVIER, Bruno FREDERICH and Eric PARMENTIER
Laboratoire de Morphologie Evolutive et Fonctionnelle, Institut de Chimie (B6c) Universit de Lige, 4000 Lige, Belgique.
Corresponding author: [email protected]

Introduction
Feeding ability is determined by functional morphology of the trophic apparatus because it constrains the nature of consumed
food. One way to understand the structural bases of the generation and transmission of force and motion during feeding is to
realize comparative studies between closely related species having different food habits. In this study we chose to make
comparisons of the adductor mandibulae, which is muscle that closes the mouth in all teleost, between three species of
damselfishes. We chose this family because of their great ecological and food diversity. The goal was to determine whether the
food diversity in this family was related to any mechanic and/or anatomical differences in the adductor mandibulae of the
different species.

Materials &
Methods

Abudefduf sordidus

Dissections were made on three species of
damselfishes (five adults/species) ; two herbivorous
that graze algae: Plectroglyphidodon lacrymatus and
Abudefduf sordidus ; and one zooplanktivorous:
Dascyllus trimaculatus. Each bundle of the adductor
mandibulae muscle (A1, A2 and A3, see Figure 1) was
weighed and their respective Mechanical Advantage
(MA) (Figure 2) values were calculated in each species.
The MA is the ratio between an in-lever and an outlever. The in-lever is the distance between the angularquadrate joint and where a bundle inserts on the jaws.
The out-lever is the distance between the angulararticulate joint and the tip of the lower jaw. High MAs
indicate high force transmission and low MAs indicate
high velocity transmission. In other fish families, it has
already been shown the MAs values were different
according to bundles of the adductor mandibulae.
Generally, A1 and A2 are considered as force
transmitter and A3 as a speed transmitter.

High MA Force transmitter

Low MA Speed transmitter

Figure 2: Illustration of the different Mechanical Advantages.
1: articulo-angular; 2: dentary; 3: premaxillary; 4: maxillary; 5: palatine .

Results & Discussion
Table 1: Comparison between herbivorous and planktivorous species

Herbivorous

Dascyllus trimaculatus

+

-

A2
A3

++
--

-++

A1

--

++

A2

+
++

--

A3

Conclusion

Zooplanktivorous

A1

System built to increase
force transmission

System built to increase
speed transmission

Figure 3: Relative importance of A1, A2 and A3 bundles .

Adductor A3 was the heaviest and A2 the lightest
bundle in each species (Figure 3).
A3 had the smallest MA value and A1 had the highest
MA value in each species (Figure 4).
Despite these similarities, we found out many
differences according to the trophic diversity as
summed up in table 1.

Plectroglyphidodon lacrymatus

Figure 1: Illustration of the bundles of the adductor
mandibulae (A1-A3) in a lateral view of the
cranium in the three species.

A3 was proportionally more developed and had lower
MA value in the zooplanktivorous D. trimaculatus than
in both herbivorous species: faster mouth closing is
required to capture efficiently evasive preys.
Conversely, herbivorous need forceful mouth closing
to graze benthic algae: their A1 and A2 are more
developed. However, conversely to A2, A1 had higher
MA value in the zooplanktivorous species, future
studies are necessary to investigate and understand
this result.

Figure 4: A1, A2,and A3 levers .

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