The diet of marine animals is usually determined by stomach content analysis. Although partially digested prey fragments can often be identified to species level, it is difficult to estimate the original mass of the prey organism. This information, however, is essential for calculating both the total food intake as well as the relative contribution of each prey item. In this study we present regression equations that can be used to estimate the original mass of 18 common South African crustaceans from various indigestible fragments such as the carapace (length and width), chelae (length and width of left and right dactylus) and eye (length and width)

Gibbons, M.J.

Isaacs, G.

Lamberts, C.

Moloney, C.L.

Richardson, A.J.

English

Aquatic Commons

FI
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Naga, The ICLARM Quarterly (Vol. 23, No. 2) April-June 2000 29
Introduction
Stomach content analysis is com-
monly used to determine the diet of
marine teleosts, chondricthyans,
cephalopods, crustaceans, seabirds
and mammals. It is often possible
to identify the prey remains to ac-
tual species, and this information
can provide insight into foraging
behavior. The results of stomach-
content analyses can also be used
to determine the trophic position of
a species within the ecosystem. Prey
fragments are usually separated into
taxonomic groups, weighed and
then used to calculate an index of
relative importance for each prey item
(Cortés 1997). Such “quantitative”
interpretations of stomach contents,
however, are biased by digestive
breakdown (Berg 1979).
To quantify the relative contribu-
tion of the various dietary compo-
nents, it is necessary to calculate the
ingested mass of each food item from
the fragments recovered in the stom-
ach contents. The procedure of cal-
culation relies on two pieces of
information. First, it must be pos-
sible to assign individual prey frag-
ments to individual prey species.
Hard body parts of cephalopods
(beaks), fish (otoliths) and most de-
capod crustaceans (carapaces,
chelae and eyes) are relatively resis-
tant to digestion and are sufficiently
distinctive to allow identification to
species or genus level. Second, there
must be an established relationship
between the size of the fragments
and the mass of the prey item. There
are some regression equations that
can be used to convert otolith length
to fish size, and beak length to
cephalopod size (Smale 1983). How-
ever, few such relationships exist for
crustaceans, even though their frag-
ments are common in teleosts
(Joubert and Hanekom 1980;
Pulfrich and Griffiths 1988; Punt et
al. 1992), chondrichthyans (Ebert et
al. 1996; Smale and Compagno
1997), cephalopods (Nixon 1987;
Sánchez and Obarti 1993; Quetglas
et al. 1998), marine mammals (David
1987) and other crustaceans
(Lawton and Lavalli 1995). In this
study, we attempt to redress this gap
in knowledge by providing regres-
sion equations that will allow the
calculation of ingested prey mass
from the size of conspicuous and
characteristic fragments for a num-
ber of common South African crus-
taceans.
Materials and
Methods
Crustaceans from intertidal, shal-
low and deepwater marine environ-
ments as well as estuaries were
collected from a variety of localities
along the west coast of South Af-
rica. The crustaceans examined in-
cluded crabs, hermit crabs, prawns
and mantis shrimps. Some of the
material was collected specifically
for the purpose of this study,
whereas other specimens (particu-
larly the benthic and benthopelagic
crustaceans that occupy the conti-
nental shelf ) were examined from
the collections at the South African
Museum. All material was pre-
served in 70% ethanol.
The conspicuous crustacean
fragments that were recovered from
the stomach include carapaces,
chelae and eyes. The following stan-
dardized measurements were re-
corded from each intact specimen
of non-natantian decapod and sto-
matopod: wet weight; carapace
width; carapace length; left dacty-
lus length; left dactylus width; right
dactylus length; right dactylus
width; and eye width. Eye length
was also measured when the eye
was ovoid. In the case of natantian
decapods (the true prawns and
shrimps), only the carapace, eye and
weight measurements were made.
The length and width of the dac-
tylus of the left and right chelae
were measured separately because
these limbs can show marked asym-
metry, with some species being ei-
ther right or left-“handed”. For example,
the hermit crab Parapagurus bouvieri
Length-Weight Relationships of Some
Important Forage Crustaceans
from South Africa
A.J. Richardson, C. Lamberts, G. Isaacs, C.L. Moloney and M.J. Gibbons
The diet of marine animals is usually determined by stomach content analysis. Although partially digested prey fragments can often
be identified to species level, it is difficult to estimate the original mass of the prey organism. This information, however, is essential for
calculating both the total food intake as well as the relative contribution of each prey item. In this study we present regression equations
that can be used to estimate the original mass of 18 common South African crustaceans from various indigestible fragments such as
the carapace (length and width), chelae (length and width of left and right dactylus) and eye (length and width).
Abstract
30 Naga, The ICLARM Quarterly (Vol. 23, No. 2) April-June 2000
Group Species Wet weight Variable a b r2 n Range
Mean (range) (g) (mm)
Crabs Cyclograpsus punctatus 2.47 (0.12-9.96) carapace length 0.9416 2.553 0.88*** 107 0.4-2.2
carapace width 0.5260 2.753 0.82*** 107 0.6-2.8
left chela length 1.4674 1.988 0.72*** 105 0.4-2.5
left chela width 5.3479 1.486 0.52*** 105 0.2-1.6
right chela length 1.4984 1.884 0.65*** 104 0.4-2.4
right chela width 4.4221 1.306 0.36*** 105 0.2-1.9
eye width 6.4741 2.199 0.43*** 107 0.2-0.8
Geryon chuni 91.58 (39.69-220.07) carapace length 0.0002 3.239 0.90*** 14 47-77
carapace width 0.0001 3.308 0.87*** 14 55-90
left chela length 0.0054 2.551 0.90*** 14 31-61
left chela width 0.2585 2.179 0.61*** 14 10-20
right chela length 0.0037 2.620 0.85*** 14 34-65
right chela width 2.6030 1.262 0.27+ 14 9-22
eye width 16.9443 0.797 0.22+ 14 3.75-11.25
Goneplax angulata 8.81 (3.13-20.24) carapace length 0.0359 1.829 0.61*** 62 14-33
carapace width 0.0061 2.160 0.63*** 62 18-43
left chela length 0.1016 1.225 0.70*** 62 21.0-59.1
left chela width 0.5082 1.315 0.62*** 62 5-19
right chela length 0.0643 1.348 0.78*** 50 21.3-61.0
right chela width 0.1583 1.823 0.82*** 50 6-16
eye width 5.1603 0.284 0.08* 62 0.83-8.75
eye length               0.20n.s. 10 2.08-5.00
Hymenosoma orbiculare 1.77 (0.06-4.35) carapace length 0.0005 2.787 0.97*** 90 5.95-24.02
carapace width 0.0007 2.757 0.96*** 90 5.67-23.34
left chela length 0.0217 2.284 0.81*** 86 2.31-11.81
left chela width 0.3594 1.497 0.64*** 86 0.66-6.20
right chela length 0.0296 2.120 0.76*** 82 2.26-11.97
right chela width 0.3546 1.495 0.65*** 82 0.69-6.33
eye width 2.3543 2.780 0.70*** 90 0.35-1.40
eye length 0.9469 3.693 0.76*** 90 0.55-1.70
Mursia cristimanus 13.34 (0.75-37.93) carapace length 0.0009 2.843 0.85*** 101 9-50
carapace width 0.0010 2.730 0.92*** 101 8.8-54
left chela length 0.0079 2.296 0.84*** 101 7.5-50.0
left chela width 0.0590 1.994 0.78*** 101 4.4-28
right chela length 0.0087 2.279 0.88*** 97 5.6-40.0
right chela width 0.0667 1.957 0.74*** 95 4-25
eye width 2.2228 1.152 0.49*** 99 1-10
eye length               0.23n.s. 8 1.00-4.17
Plagusia chabrus 22.96 (0.55-82.20) carapace length 0.0015 2.759 0.99*** 38 7.0-49.1
carapace width 0.0014 2.718 0.98*** 37 9-58.4
left chela length 0.0024 3.082 0.95*** 36 6-34
left chela width 0.1018 2.755 0.90*** 36 2-15
right chela length 0.0033 2.971 0.90*** 35 5-35
right chela width 0.1198 2.658 0.89*** 37 2-15
eye width 5.7447 2.014 0.75*** 38 0.4-3.33
Thelxiope barbata 9.61 (2.06-17.79) carapace length 0.0064 2.131 0.54** 16 21-42
carapace width 0.1218 1.437 0.64*** 16 10-36
left chela length               0.10n.s. 14 15-29
left chela width               0.09n.s. 14 5-9
right chela length               0.19n.s. 12 12-31
right chela width 0.8759 1.245 0.37* 12 4-11
eye width 2.0795 1.548 0.21+ 16 1.88-3.75
Hermit crabs Diogenes brevirostris 0.74 (0.11-1.81) carapace length 0.0010 3.668 0.93*** 119 3.69-7.50
carapace width 0.0047 3.267 0.92*** 119 2.79-6.05
left chela length 0.0025 2.952 0.88*** 115 4.02-9.09
left chela width 0.0046 2.997 0.93*** 115 3.02-7.24
right chela length 0.0112 3.051 0.87*** 119 2.39-5.30
right chela width 0.1027 2.691 0.87*** 119 1.15-3.07
eye width 2.1560 4.450 0.76*** 119 0.55-0.95
eye length 0.8363 4.467 0.75*** 119 0.65-1.20
Table 1. Regression equations for 18 crustacean species from South Africa.
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Naga, The ICLARM Quarterly (Vol. 23, No. 2) April-June 2000 31
Parapagurus bouvieri 6.85 (1.97-15.62) carapace length 0.1993 1.136 0.25* 22 10-27
carapace width 0.4601 1.055 0.22* 21 6-15
left chela length 0.5013 1.039 0.22* 22 6-16
left chela width 1.9719 0.847 0.30** 22 2-6
right chela length 0.2151 1.099 0.32** 22 11-36
right chela width 0.4735 1.174 0.23* 22 5-13
eye width 1.2008 1.479 0.34** 22 1.88-4.38
Parapagurus dimorphus 6.89 (2.08-15.70) right chela length 0.4575 0.895 0.43*** 22 7.4-42.3
(with sponge shelter) right chela width 0.7854 1.003 0.43*** 22 3.1-12.0
eye width 4.6952 0.919 0.68*** 23 0.5-2.5
Parapagurus dimorphus 4.59 (1.96-12.61) carapace length 0.1647 1.196 0.50*** 70 10-24
(without sponge shelter) carapace width 0.8792 0.778 0.40*** 70 3-13
left chela length 0.9083 0.739 0.18*** 69 4-12
left chela width 2.5906 0.406 0.08* 69 2-6
right chela length 0.3753 0.892 0.28*** 70 9-26
right chela width 1.1432 0.683 0.44*** 70 3-19
eye width 1.8037 0.726 0.43*** 68 0.63-5.13
Mantis Pterygosquilla armata 5.30 (0.51-36.41) carapace length 0.0051 2.322 0.81*** 54 8.3-37
shrimps capensis carapace width 0.0545 1.730 0.70*** 54 4-28.6
left chela length 0.0023 2.428 0.82*** 53 11-42.7
left chela width 0.4126 1.709 0.71*** 52 1.3-10
right chela length 0.0038 2.266 0.75*** 54 10-43.8
right chela width 0.5008 1.528 0.65*** 54 1-10.4
eye width 0.6434 1.085 0.16** 54 2.5-8.75
Mud prawns Upogebia africana 4.83 (0.59-9.18) carapace length 0.0005 2.988 0.97*** 50 10.86-25.49
left chela length 0.0806 1.873 0.80*** 50 4.03-12.45
left chela width 1.2769 0.898 0.47*** 50 1.94-7.61
right chela length 0.0697 1.951 0.81*** 49 4.06-12.09
right chela width 1.2056 0.937 0.47*** 49 2.02-7.63
eye width 5.1263 2.854 0.69*** 51 0.6-1.3
Prawns Funchalia woodwardi 12.45 (7.40-17.07) carapace length 0.0011 2.343 0.46** 19 45.1-58.8
carapace width 0.0718 1.957 0.82*** 19 10.6-16.6
eye width                0.02n.s. 19 2.50-4.33
Paraeus sp. 0.21 (0.02-0.54) carapace length 0.0005 3.005 0.88*** 29 3.7-10.0
eye width 0.1156 2.146 0.45*** 29 0.83-1.67
eye length 0.0242 2.683 0.72*** 29 1.17-3.17
Parapenaeopsis 1.24 (0.17-4.94) carapace length 0.0016 2.025 0.76*** 83 12.2-36.0
acclivirostris carapace width 0.0414 1.922 0.70*** 83 3-10
left chela length 0.0028 1.736 0.26*** 51 22-49
left chela width 0.1277 1.304 0.32*** 51 3-9
eye width 0.5232 0.685 0.27*** 83 0.83-6.25
eye length 0.0272 3.432 0.83*** 32 1.83-4.00
Sergia potens 5.19 (2.53-13.27) carapace length 0.0063 2.034 0.85*** 30 22.1-45.8
carapace width 0.2667 1.436 0.69*** 30 5.7-14.2
eye width 2.6792 0.748 0.24** 30 1.5-4.17
Sand prawns Callianassa kraussi 1.96 (0.097-7.69) carapace length 0.0017 3.276 0.97*** 166 2.92-12.73
max chela length 0.0035 2.718 0.92*** 142 0.59-1.30
max chela width 0.0210 2.476 0.91*** 142 0.36-1.10
min chela length 0.0057 3.123 0.96*** 142 0.37-1.03
min chela width 0.0767 2.843 0.94*** 142 0.06-0.70
eye length                  214.7207 4.986 0.71*** 166 0.20-0.50
berry weight weight 0.2090 0.820 0.25* 22 2.03-4.10
Table continued....
Significance levels are:
n.s. = not significant.
+ p < 0.1. ** p < 0.01.
* p < 0.05. *** p < 0.001.
32 Naga, The ICLARM Quarterly (Vol. 23, No. 2) April-June 2000
is right-handed (see Table 1), while the
sand prawn Callianassa kraussi can
be either right or left-handed. As the
large and small chelae of this spe-
cies are easily distinguished mor-
phologically, we have grouped all
the large chelae together and all the
small chelae together. Crustacean
eyes were measured, although they
cannot generally be identified to
species. However, other fragments
in the stomach may provide a posi-
tive identification, in which case the
length-weight regression using eyes
becomes useful.
The wet weight (±0.01 g) of all
material was determined on a Sar-
torius balance, after the specimens
had been dried on blotting paper.
All length measurements were in
mm. Eye dimensions were mea-
sured at x20 magnification under a
dissecting microscope. All other
measurements were made using
electronic vernier calipers.
Relationships between wet
weight and the linear dimensions
of the different fragments were cal-
culated using least squares regres-
sion (Y = aXb), after log transformation
of both X and Y variables. Males and
females were not analyzed separately
because of the difficulty of assigning
a sex to fragments.
Results and
Discussion
The regression equations for the
18 crustacean species are summa-
rized in Table 1. In all instances,
wet weight is the dependent vari-
able and the dimensions of various
characteristic fragments are the in-
dependent variables. The propor-
tion of variance explained by each
relationship is also shown, as is the
sample size and the significance
level. Nonsignificant equations have
been identified to show which re-
lationships were assessed. The
mean mass of each species is also
included, because it provides an es-
timate of the mass of a specimen
when a fragment can be identified
to species level, but the regression
equation is nonsignificant. The range
of the various independent variables is
also shown (Table 1), as regression
relationships should be applied with
caution beyond the limits of the range
of the original data (Zar 1984). Two sets
of equations are given for Parapagurus
dimorphus, one set with and one
without its sponge shelter, because the
sponge could also provide nutrition for
a predator. The relationship between
berry weight and wet weight for
Callianassa kraussi is also given. This
relationship can be used to calculate the
mass of berry ingested by an animal if it
can be seen that some berry is present.
Overall, wet weight was strongly
correlated with carapace, chelae and
eye dimensions, and 92% (103 out
of the 112) of the relationships were
significant at the 5% level. Wet
weight was more strongly correlated
with carapace dimensions and chelae
length than with chelae width or eye
dimensions. This could reflect the
difficulty of measuring small body
parts accurately. Some of the scatter
of points in the relationships relat-
ing wet weight to chelae dimensions
could have arisen because individu-
als lose limbs and then regenerate a
new one that is smaller than the
original.
We have shown that the original
mass of many crustacean species can
be predicted from the size of their
indigestible fragments. This will
enable the relative contribution by
mass of some decapods and sto-
matopods to the diet of many ma-
rine animals to be estimated, rather
than just the relative contribution of
their prey fragments. This informa-
tion is essential for quantifying
trophic flows in marine food webs.
A knowledge of trophic flows is
needed in order to understand eco-
systems, so as to manage them
sustainably. The traditional ap-
proach of managing fisheries using
single-species models has had lim-
ited success (Bakun 1996). It has
been argued that to manage fisheries
resources sustainably, it is necessary
to use a multispecies approach
(Pauly et al. 1998). Trophic models such
as Ecopath have been developed for this
purpose (Christensen and Pauly 1992).
These tools not only require informa-
tion on the different prey items of a fish,
but information on the relative impor-
tance by mass of each prey item. It is
hoped that the relationships provided
in this paper will contribute toward this
end.
Acknowledgements
We would like to thank Michelle
van der Merwe and Liz Hoensen of
the South African Museum for their
generous assistance with the mu-
seum collections, and Z. Abrahams
and D. Thomas from the Zoology
Department at the University of the
Western Cape for their help in mea-
suring specimens. Mr. Ben Wessels
from Marine and Coastal Manage-
ment is thanked for his help in
searching for literature. Financial
support for CL and AJR was pro-
vided by the Benguela Ecology
Programme of the National Research
Foundation.
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Length-weight relationships for some important forage crustaceans from South Africa

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