The Biology of Marine Fishes
by J. Charles Delbeek, B.Sc., M.Sc., B. Ed.

I have been asked to write an ongoing column on the biology of marine
fishes and how this relates to our attempts to keep them in a
closed-system environment. Hopefully, much of the information I will
be presenting in this column will be new to the majority of you (at
least that is what Scott has asked for), however, I'm sure that there
will be those of you for which this may not be true. In any event, I
would appreciate as much feedback from you as possible regarding the
content of these articles and anything you would like to see covered
in future columns. Of course, to completely cover the biology of
marine fish in such a limited series of columns would be impossible
but what I will attempt to cover is that information which will be of
the most concern regarding keeping fish in the marine aquarium.
Towards this end the following topics will be covered: fish form and
its relationship to their ecology, the effect of light on fish
behavior, coloration, osmoregulation, respiration, excretion,
breeding behavior and feeding behavior plus anything else you might
like covered (if space and time permit).

Fish Form and Ecology: Why Does That Fish Look That Way Daddy?
The great diversity of fish form is a direct reflection of their
ecological diversity. Fishes can be found in almost any body of
water, be it an underground cave, beneath the arctic ice, in the
deepest marine trench, in a fast moving mountain stream or in a
tropical reef. In each of these environments, fish have evolved body
forms, fin shapes and behavior to help them survive. Much can be
learned about the ecology of a fish simply by examining its
anatomical features or by watching it move through the water (Moyle
and Chech, 1982).

Most fishes can be classified into one of six broad categories of
body shape: rover-predator, lie-in- wait predator, surface oriented,
bottom dwelling fish, deep-bodied fish and eel-like fish. The rover-
predator type of body form is what comes to mind when one thinks of a
fish: a streamlined (fusiform) body with the fins well spaced along
the body providing stability and maneuverability. A fusiform body is
often indicative of a fish which is constantly on the move, searching
for prey which they capture through pursuit (Moyle and Chech, 1982).
An example of a rover-predator often found in marine aquariums are
wrasses (family Labridae) and members of the genus Pseudanthias (e.g.
Purple Queen). Lie-in-wait predators are usually piscivorous (fish
eating) in nature and their morphology is well suited for the capture
of fast swimming prey from ambush (Moyle and Chech, 1982). The body
is often elongated, the head flattened and the mouth large. The
dorsal and anal fins are typically placed far back on the body and
the caudal fin is large. This arrangement of fins, and their size,
gives the fish the large amount of forward thrust needed to capture
their prey. Their cryptic coloration and secretive behavior make
them also less visible to their intended victims. Examples commonly
found in the marine aquarium are the Serranidae (groupers) and
Apogonidae (cardinal fishes). Surface-oriented fish are usually
small in size with an upward pointing mouth, a dorsoventrally
flattened head with large eyes and a posteriorly placed dorsal fin
(Moyle and Chech, 1982). This morphology is well suited for the
capture of small fish and/or invertebrates which live just below the
water surface. I can't think of any marine aquarium fish that fall
into this category but some examples would be flying fishes and
halfbeaks (Exocoetidae) as well as the freshwater killifish (genus
Fundulus) and the four-eyed fish (Anableps anableps). Bottom
fishes come in a wide variety of shapes and sizes but the common
characteristic is an adaptation to a benthic (bottom) existence. The
swimbladder in bottom dwelling fish is generally reduced or absent.
Bottom fishes can be classified either as bottom rovers or bottom
clingers (Moyle and Chech, 1982). Bottom rovers have compressed
bodies in one plane or another, a humped back, large pectoral fins,
flattened heads and terminal or subterminal mouths. The sand tilefish
(Malacanthus plumeri) is an excellent example of a bottom rover as
are the goat fish (Pseudupeneus maculatus) and sharks (believe it
or not). Bottom dwellers are small fish with flattened heads, large
pectoral fins and modified pelvic fins that allow them to adhere to
the bottom. Examples of bottom fishes include the popular blennies
(family Blennidae), gobies (family Gobiidae) and the Mandarin
(Synchiropus splendidus; ever noticed the size of those blue
pectorals?!). Deep- bodied fishes are laterally flattened fish whose
body depth is at least one-third of their overall length (Moyle and
Chech, 1982). The dorsal and anal fins are typically long and the
pectoral fins are high on the body directly above the pelvic fins.
The mouth is usually small and protrusible, the eyes large and the
snout short (Moyle and Chech, 1982). Most deep- bodied fish have
developed stout spines in a bid to sacrifice speed for
maneuverability and protection from predators. Common marine
examples of deep- bodied fish include butterflies and angelfishes
(family Chaetodontidae). Eel-like fish have elongated bodies, blunt
or wedge shaped heads and tapered or rounded tails; pelvic fins are
either reduced or absent (Moyle and Chech, 1982). Eel-like fishes
are well adapted for entering small crevices and holes in the reef.
The most obvious examples commonly encountered in marine aquariums
are the moray eels (genus Gymnothorax).

Fin shape and placement, perhaps more than body shape, can tell you
an awful lot about the habitat in which a certain fish has
developed. Fin type can be placed into two main categories: paired
fins (pectorals and pelvics) and unpaired fins (dorsal, anal and
caudal). The pectoral fins are important in low speed maneuvering
and their size and position in a particular fish give an indication
as to their function in that species. Pectoral fins tent to be high
up on the sides of fish which depend on precise movements for
capturing prey such as the Long- nosed Butterfly (Forcipiger
flavissimus). Pectoral fin length has often been assumed to
increase with the importance of low speed maneuvering in the behavior
of a fish (Gatz, 1972). Therefore, the longer the pectoral fins, and
the more rounded they are, the more important low speed movement is
in the behavior of a fish. The same can be said of the overall size
of the pectoral fins. Often fish which spend a great deal of time on
or near the bottom, have large pectoral fins relative to the size of
the fish. Pectoral fins are often used by benthic species for
support and for moving amongst bottom debris such as in the
hawkfishes (i.e., Cirrhitichthys sp.) and Blennidae. In some fish
the pectoral fins have lost much of their role in swimming and have
taken on more of a behavioral role. For example the spreading of the
pectorals as a warning display exhibited by various species of
lionfish (family Scorpaenidae). Pectoral fin length is often
associated with the habitat preference of a fish species being longer
in rocky habitats and shorter in free swimming species. Pelvic fin
size is related also to the amount of braking they perform; the
larger the fin the more they are involved in braking and hovering
(Gatz, 1972). Again, in some fish, the pelvic fins have become
highly specialized. For example, in many of the Gobiidae, the pelvic
fins have become fused to form a suction device for clinging to rocks
and coral. The dorsal and anal fins are generally longer on
rover-predators and deep- bodied fishes, for stability while swimming
(Moyle and Chech, 1982). Short fins are often a trait of
bottom-dwelling or surface-oriented fish. In some fish species, the
dorsal fin has become more involved in display behavior than in
swimming (e.g. Blennidae and Gobiidae). The area and shape of the
caudal fin is directly proportional to the amount of swimming a fish
does (Gatz, 1972). The fastest swimming fishes, such as tuna and
marlin have a stiff, quarter-moon shaped fin attached to narrow
caudal peduncle (Moyle and Chech, 1982). Most rover-predator fish
requiring frequent sustained swimming have forked tails; the deeper
the fork, the more active the fish. Deep-bodied fish and most
surface- and bottom-oriented fishes have tails that are rounded,
square or only slightly forked.

Mouth shape, size and position are all good indicators of preferred
habitat and diet in fish. In general, bottom feeding fish have
mouths which point downward (subterminal) such as in the goat fish
and some blennies, while surface oriented fish have mouths which
point upwards (supraterminal) such as in Fundulus spp. The
majority of fish, however, have mouths situated at the anterior point
of the head (terminal mouths). Within the various types of coral
reef fish often kept in the home aquarium, there is a host of
different mouth shapes, ranging from the long tube-like mouths of
many Chaetodontidae to the large, gaping mouths of the Serranidae.
The size of the mouth is usually related to the size of prey consumed
by a fish. Therefore, fish which feed on small invertebrates by
suction often have a small, highly protrusible mouth, lined with many
small teeth (Moyle and Chech, 1982). Beyond these general mouth
types, fish exhibit a wide variety of mouth shapes which often
reflect specialized modes of feeding and nowhere is this more evident
than in reef fishes. The stability of the reef environment is often
cited as the mechanism which allows such a highly diversified
community to exist and to allow such highly specialized forms to
evolve. Examples range from the Long Nosed Butterfly with its
ridiculously elongated snout designed to reach into the smallest
crevice where it can find the microcrustaceans it requires to the
anglerfishes (order Lophiiformes) with their modified dorsal spine
which acts as a lure and its huge mouth capable of rapid movement (an
anglerfish capturing film is one of the fastest and shortest moments
in nature!).

This concludes the first article on fish biology. I hope that you
have gained a new appreciation for what highly specialized beasts
fish really are. When you next purchase a marine fish (or any fish
for that matter) try to apply what you've read in this column and you
won't be totally surprised when that new fish, which looked so nice
and happy and fat all by himself in the dealer's tank, mercifully
begins to dine on the inhabitants of your tank and soon begins to
look just as happy as before. . . , but fatter! Seriously though,
when purchasing a fish, try to take into account the native habitat
from which it came and its normal lifestyle. Imagine being a
marathon distance runner confined to 100 m track - would you be happy
running that tiny track for the rest of your life? Try to buy fish
best suited to the tank you have and don't expect a fish which
normally roams wide areas of the reef to totally prosper in a 30
gallon aquarium. It just isn't natural for the fish. Now its time
to get off the old soapbox and start thinking about the next
article: "The Effect of Light on the Behavior and Well Being of
Marine Fish: Who Shut Off the Lights?"