brought together by an environmental stimulus, not by
each other. An aggregation can be comprised of individuals of different sizes and even different species, and
the members of this type of group usually mill around
the mutually attractive stimulus and do not carry out
coordinated movements. The kind of stimuli that attract
aggregations include shelter sites, a cleaning station or
a localized food source. There are a number of nocturnal reef fishes that will aggregate in caves or under overhangs. (This is known as a resting aggregation.) There
can be interspecific differences in social status within an
aggregation, with certain fish being dominant over other
group members.
Many species do not live in these different types of
assemblages during their entire life cycles, but instead,
they group together during certain stages of their lives
or in specific social or feeding situations. For example,
the Striped Eel Catfish (Plotosus lineatus) forms large
schools when young, but as they get larger they live a
more solitary lifestyle. There are certain surgeonfishes
that form temporary groups in order to effectively invade
damsel territories, while some damsels only form groups
when feeding in the herrings and sardines (Clupeidae),
silversides (Atherinidae), and the tunas and mackerel
(Scombridae). Many of these fishes are pelagic and are
not reef residents. They do not seek shelter in the reef,
but rely on schooling as their primary form of defense.
AquArium ShoAlS
Unfortunately, replicating some of these natural social
units in the aquarium can be difficult. This may be a
function of lack of space, lack of predators, or lack of
feeding situations that trigger shoaling in the wild. In the
aquarium, where the selective pressures from predation
do not normally exist, fishes that naturally get along in
groups sometimes are less tolerant of each other. In some
cases, if the group is large enough, aggression is less of
a problem for the individual group members. By having more individuals in the group, the aggression of the
more dominant individual(s) is dispersed among a larger
group of subordinates so no one fish is harried to death.
This is often the case with anthias – you are better off
getting one male and five or six females, assuming that
you have a large enough tank to accommodate them all.
Below you will find some readily available fishes that can
be kept in groups in captivity. In most cases, it is a good
idea to introduce all members of a captive group at the
same time.
Some CAptive “ShoAlerS”
Striped Eel Catfish (Plotosus lineatus)
Chalk Bass (Serranus tortugarum)
Bartlett’s Anthias (Pseudanthias bartlettorum)
Banggai Cardinalfish (Pterapogon kauderni)
Lyretail Anthias (Pseudanthias squamipinnis)
Longfin Anthias (Pseudanthias ventralis)
Creole Fish (Paranthias furcifer) **
Blue Assessor (Assessor macneilli)
Threadfin Cardinalfish (Apogon leptacanthus)
Redspot Cardinalfish (Apogon parvulus)
Frostfin Cardinalfish (Apogon hoevenii)
Girdled Cardinalfish (Archamia zosterophora)
Pajama Cardinalfish (Sphaeramia nematoptera)
Golden Jack (Gnathanodon speciosus)
Monos (Monodactylus sebae, M. argenteus)
Schooling Bannerfish (Heniochus diphreutes)
Blackaxil Chromis (Chromis atripectoralis)
Bluegreen Chromis (Chromis viridis)
Vanderbilt’s Chromis (Chromis vanderbilti)
Fusilier Damsel (Lepidozygus tapeinosoma)
Creole Wrasse (Clepticus parrae)
Convict Blenny (Pholidichthys lecuotaenia) *
Zebras Dartfish (Ptereleotris zebra)
Green Dartfish (Ptereleotris microlepis)
* form groups when young
**individuals may occasionally behave aggressively toward
one another in captivity.
referenCeS
Lipton, James. 1993. An Exaltation of Larks: The Ultimate Edition
(Penguin, New York)
Michael, Scott W. 1998. Reef Fishes, Vol. 1 (Microcosm/ TFH,
Neptune, NJ)
ShoAling AS A defenSe AgAinSt predAtion
More than 25% of all known fish species live in shoals or
schools either obligatorily and throughout their lives, or temporarily or at different life stages (juvenile stages, reproductive cycles). The complex modes of behavior associated with
predator defense by shoals and schools works in a number
of ways:
1. The confusion effect: The presence of numerous individuals makes it more difficult to launch an attack on a single
one. It is not possible to locate and target a single individual.
2. Danger of harm to the predator: If the predator is not
right on target, it may collide with the close-packed members
of the shoal.
3. Discovery of predators: Many eyes see more than just
two. There exists a special mode of behavior, a so-called
“warning behavior,” that facilitates the recognition of predators. Birds and mammals use acoustic signals, while fishes
can deliver olfactory warnings.
There is a further strategy that can be used by the shoal,
a kind of confusion effect in reverse: An attacking predator
causes the shoal to split up, so the predator is left attacking
empty water. When it gives up, exhausted, the shoal re-forms.
Anyone who has seen a pack of Black-tipped Reef Sharks
hunting shoaling fishes will be familiar with this scenario.
