Project Title
Predator-prey interactions on South African temperate reefs: Characterising the relationship
between elasmobranchs and octopus at baited camera stations.
Abstract
Predatory interactions between elasmobranchs and prey species, such as octopus in coastal
ecosystems can have profound ecological implications. This study utilized Baited Remote
Underwater Videos (BRUVs) in the Tsitsikamma National Park Marine Protected Area (MPA),
located with the South African temperate region to investigate these interactions. The
research aimed to understand how the presence and behaviours of predatory
elasmobranchs, primarily the pyjama sharks (Poroderma africanum), influenced the
behaviour of prey octopuses (Octopus vulgaris). A generalised linear model (GLM) (poisson
dispersion parameter = 1) showed that the total duration in FOV (p = 0.9) and MaxN (p =
0.7) of all elasmobranchs, as well as the total duration in FOV (p=0.8) and MaxN (p = 0.7) of
P. africanum had no effect on the duration O. vulgaris. Furthermore, GLM with a quasipoisson distribution (Poisson dispersion parameter > 10) showed that the total duration in
FOV (p = 0.16) and MaxN (p = 0.7) all elasmobranchs as well as the total duration in FOV
(p=0.8) and MaxN (p = 0.8) of P. africanum had no effect on the duration O. vulgaris was in
the FOV. Interestingly, the Predator presence had a significant effect on the duration that O.
vulgaris spent feeding (chi-squared = 16.7, df – 3, p = 0.0008). Pairwise comparisons
indicated that the effect of predator presence on the duration of O. vulgaris feeding was
restricted to P. africanum (p = 0.01). Overall, the abundance (MaxN) and presence of
elasmobranch species had no effect on MaxN and duration of O. vulgaris, as they continued
with behaviours such as feeding even in the presence of sharks which suggested a
competition over the bait, rather than a predator-prey relationship. The study highlights the
use of BRUVs as an effective tool to investigate predator-prey interactions in coastal
ecosystems. It sheds light on the dynamic behaviours exhibited by octopuses when faced
with predation risk from sharks. These findings contribute to a better understanding of
trophic interactions and the role of marine predators in shaping coastal ecosystems. Future
research in this field can build upon these findings to further explore the complexities of
predator-prey dynamics and their ecological consequences in South African coastal waters.
Introduction
Marine predators can play important roles in coastal ecosystems by exerting top–down
effects on prey through consumptive and non-consumptive interactions, i.e., predation and
fear-based effects. The exact nature of these roles varies based on several factors, including
how predator and prey directly interact. Studying predator-prey interactions and behaviours
can help in understanding and identifying crucial trophic linkages and the implications of
top-down effects of predation on aspects such as population dynamics and food webs
(Hunsicker et al. 2011, Sherman et al. 2020). Unfortunately, insights into predator-prey
interactions remain limited because of the difficulty of observing these interactions in the
wild. Baited Remote Underwater Videos (BRUVs) provide a unique opportunity to witness
and study predator-prey interactions (Harvey et al. 2013). In particular, BRUVs collected on
temperate reefs along the coast of South Africa have noted interactions between the
common octopus (Octopus vulgaris) and predatory elasmobranchs such as pyjama sharks
Poroderma africanum, short-tail stingrays (Bathytoshia brevicaudata), common smoothhound sharks (Mustelus mustelus), common eagle rays (Myliobatis aquila), leopard
catsharks (Poroderma pantherinum), Puffadder shysharks (Haploblepharus edwardsii), and
dark shysharks (Haploblepharus pictus).
Predation has been considered to have a significant influence on the behaviour patterns of
prey, as well as on other aspects such as abundance and movements (Sherman et al. 2020).
The impacts of predation on preys can be exerted through consumptive or fear-based
interactions (Sherman et al. 2020, Mitchell & Harborne 2020, Shea et al. 2020). An example
of a consumptive interaction is when the predator attacks and kills its prey for consumption
(Mitchell &Harborne 2020). On the other hand, fear-based effects can lead to a change in
habitat occupancy and the foraging ability of prey, which can ultimately slow the individual
growth and increase the prey's vulnerability to predation (Bax 1998, Sherman et al. 2020).
The influence of marine predators on prey behaviour has been observed across multiple
taxa and habitats (Bax 1998). For example, a study by Sherman et al. (2020) on the effects of
predator removal on rays in coral reef ecosystems revealed that rays were more likely to
appear when their predators, sharks, were not present. Furthermore, the behavioural
impact of the absence of predators was that rays spent more time in the video frame and
fed on bait (Sherman et al. 2020). A similar phenomenon of antipredator behaviour was
observed in an Australian mangrove-lined bay where juvenile rays avoided being detected
by predators by hiding in mangroves during high tide, and during low tide they hid by
submerging their bodies in shallow waters (Davy et al. 2015). BRUVs also revealed that the
feeding attempts of smooth dogfish (Mustelus canis) on bait off Cape Cod, Massachusetts,
were reduced in areas where white sharks (Carcharodom carcharias), one of their
predators, were abundant (Shea et al. 2013).
Apart from avoidance behaviour, prey can exhibit defence mechanisms when they are
attacked by their predators (Schmitt & Strand 1982). An instance of defence was observed
in the behaviour of the common Pacific coral reef octopus (Octopus cyanea) which was
studied using underwater videos, where the octopus was attacked by a small moray eel of
about 0.5 metres in length (Forsythe & Hanlon 1997). The octopus responded by violently
pulling away its arms, changing its body pattern, and excreting an ink substance in form of
black clouds, consequently causing the eel to fail to detect the octopus due to the ink which
interrupted its sight (Forsythe & Hanlon 1997). This instance reveals that a prey is capable of
escaping from their predators even if they are at high risk of being killed by using their
adaptations such as visual senses, body posture and chemical cues (Mitchell & Harborne
2020).
In some instances, prey can move in groups to defend themselves against predators
(Schmitt & Strand 1982), and predators also tend to respond by approaching the schooling
prey in a group (Motta & Huber 2012). Predatory elasmobranchs, which are known for
mostly solitary foraging, can also perform group foraging, such as the seven-gill sharks that
can attack their prey by forming a circle around them to prevent them from escaping (Motta
& Huber 2012). Another similar observational study was based on Santa Catalina Island,
where seven yellowtail fish (Seriola lalandei) attacked a large school of more than 2000 jack
mackerels (Trachurus symmetricus) in open waters splitting a smaller group from the main
group and chased the small group (Schmitt & Strand 1982).
While there are several studies investigating predator-prey interactions globally, there are
very limited examples from South African coastal waters. One such study investigated the
interaction between predatory elasmobranchs and chokka squid (Loligo vulgaris reynaudii)
using underwater video recordings (Smale et al. 2001). The study observed that the
nocturnal pyjama shark (Poroderma africanum) (Branch et al. 201), showed an unexpected
diurnal trait when it attacked a spawning female squid during daylight (Smale et al. 2001).
Pyjama catsharks rarely appeared out of their sheltering caves during daylight, more
especially when squids or fishing baits were not in their surroundings, hence this
observation came across as an ambush attack (Smale et al 2001). Another instance of
predation was observed off Aasvogel Point when three diamond rays (Gymunura
metalepsis) lay adjacent to the squid egg beds with their bodies partially covered by sand in
an attempt to attack the female squid that was moving downwards to spawn; however, the
squid managed to notice the rays and flee the area (Smale et al. 2001).
In a functioning ecosystem, species are constantly at risk of being attacked and eaten by
their predators (Bax 1998). This is because there are limitations when it comes to how long
the prey can successfully avoid predation (Bax 1998). The rationale for this limitation is that
prey species must continue to fulfil their biological ecological niches in order to survive
(Sherman et al. 2020). BRUVs are known to be cost-effective and robust instruments that
have been extensively used in coastal ecosystems (Harvey et al. 2013), and they consist of
bait to attract animals as well as an underwater camera held by frame to record what
species come to feed on the bait (Heupel 2019). BRUVs have historically been used to study
community structure and distribution, relative abundance, as well as sizes of marine species
(Harvey et al. 2013, Shea et al. 2020), however, studies looking at predator-prey interactions
are still quite limited. The robustness of BRUVs has allowed scientists to be able to noninvasively sample habitats that are not easily and safely reachable such as the sea bottom,
coral reefs, and rock crevices (Harvey et al. 2013). Furthermore, the use of bait has been
considered to be effective in eliciting the behaviour response of marine animals (Harvey et
al. 2013). Existing stereo-BRUVs were employed in this study to qualify and quantify the
interactions between predatory elasmobranchs and prey octopus at South African
temperate reefs, specifically the Tsitsikamma National Park Marine Protected Area (MPA).
Octopus vulgaris was chosen as the focal prey for the study because they are a known prey
item of the elasmobranchs, particularly the pyjama sharks (Poroderma africanum) (Branch
et al. 2010). The common octopus (O. vulgaris) is also an intelligent species that can evade
predators by hiding, fleeing, posturing, and camouflaging (Scheel et al. 2016; Miao 2016).
This study aimed to develop an understanding of how the presence and behaviours of
predatory elasmobranchs influenced the behaviour of prey octopuses (Octopus vulgaris).
The first objective of this study was to determine if the relative abundance of
elasmobranchs influenced the relative abundance of O. vulgaris. The second objective was
to investigate if the amount of time a potential predatory elasmobranch spent influenced
the amount of time spent by the focal prey species (O. vulgaris) in the frame. The third
objective was to determine if the co-occurrence of O. vulgaris with a potential predatory
elasmobranch influences the behaviours of O. vulgaris and if this effect was consistent
among potential predatory elasmobranchs. This study hypothesised that the relative
abundance of a potential predatory elasmobranch reduced the relative abundance of focal
prey species (O. vulgaris) in frame. The second hypothesis was that the amount of time
spent by the focal prey species (O. vulgaris) in the frame was less when potential predatory
elasmobranchs were present. The third hypothesis was that presence of potential predator
elasmobranch influenced in the behaviour of the focal prey species (O. vulgaris) and this
influence was consistent among the different elasmobranch species.
Methods
Study Area
Figure 2: The study area: Tsitsikamma National Park (TNP) Marine Protected Area (MPA)
(indicated by red arrow)
(MPA Forum South Africa 2023)
The Stereo-BRUVs' footages were collected inside the Tsitsikamma National Park Marine
Protected Area (MPA) (Figure 2). Tsitsikamma MPA is in the Eastern Cape Province of South
Africa, located along the temperate rocky Indian Ocean shoreline (MPA Forum South Africa
2023). The Tsitsikamma National Park MPA occupies 60 km of the coastline and is a ‘notake’ MPA which protects intertidal and subtidal habitats, as well as endemic overexploited
fish species (MPA Forum South Africa 2023).
Sampling method
Figure 3: Diagram showing Baited Remote Underwater Video system with underwater
housings for HD video cameras (A), underwater housing for blue LED lighting (B), basebar
(C),
the system’s frame (D), bait canister (E) and rope attachment for buoys during
deployment and retrieval (F).
(Drawing credit: Dr Elodie Heyns)
The videos were collected over a span of 3 days which were the 23rd, 24th, and 25th of July
2018, using BRUVs. Ethical permit to undertake this study was obtained from the South
African Institute for Aquatic Biodiversity (SAIAB). BRUVs are non-destructive sampling tools,
that are usually operated from a boat using the rope for deployment and retrieval (Figure
3). BRUVs consist of a frame, LED light, and a video camera in an underwater housing
(Heupel 2019) for sampling. They consist of bait which can be crushed fish put in the bait
canister to attract the animals (Harvey et al. 2013, Heupel 2019). During the collection of
footage samples, a BRUV was lowered over the side of the boat to settle on the bottom and
was left to record for a minimum of one hour. The data was collected from slightly different
locations and the BRUVs were deployed at different depths ranging between 5 to 60 m.
Depth was measured using a GPS-linked echo sounder that was attached to the boat, and
water temperate (°C) was recorded using the HOBO Onset temperature loggers placed on
the BRUVs frame (Mataboge 2021).
Video analysis
BORIS program (version 8.20.4) (Friard & Gamba 2016) was used to perform the stereoBRUVs observation. During the observations, all elasmobranch species and octopus present
in field of view were identified. For each BRUV sample, the time spent (from entering to
exiting the frame) by elasmobranchs and octopuses was recorded. The relative abundance
of each species using MaxN, which is the maximum number individuals of species available
in a single frame (Sherman et al. 2020) was also recorded. Additionally, the time spent in a
frame by individual predatory elasmobranchs on behaviours such as passive, feeding,
attacking were recorded (see Table 1.). Likewise, the time spent by octopus on behaviours
such as sheltering on reef, posturing, changing body colour, fleeing, feeding on bait, and
moving on reef were recorded on BORIS using an ethogram (see Table 2.).
Table 1: List of behaviours that were displayed by predatory elasmobranchs.
Category
Behaviour
Definition
Unit
of Data type
measurement
Aggressive
Feeding
bait
Attacking
on Elasmobranch was observed
Species
Time spent on
feeding on bait, aggressively
behaviour
taking consecutive bites.
(minutes)
Elasmobranch
Time spent on
in
physical
interaction
octopus
and
direct
biting
with Species
on
behaviour
(minutes)
octopus.
Relaxed
Passive
Elasmobranch was passing
Time spent on
around the bait or in the Species
behaviour
background.
(minutes)
Amount of time spent by
Time spent on
elasmobranch in field of Species
behaviour
view moving or searching
(minutes)
around the bait or in the
background (i.e., time exited
minus time entered)
Table 2: List of behaviours that were displayed by prey octopus.
Category
Behaviour
Definition
Unit
of Data type
measurement
Evasive
Fleeing
Octopus quickly moves away
Species
from the bait canister.
Time spent on
behaviour
(minutes)
Sheltering
on Octopus observed sheltering Species
reef
on the reef crevice or rock.
Time spent on
behaviour
(minutes)
Colour change
Octopus observed changing Species
Time spent on
body colour (e.g., colour
behaviour
change
(minutes)
to
dark,
camouflaging, flashing)
Posturing
Octopus observed recoiling Species
Time spent on
tentacles in response to the
behaviour
elasmobranch’s approach or
(minutes)
drawing itself upwards.
Moving
reef
on Octopus observed moving Species
around.
Time spent on
behaviour
(minutes)
Normal
Feeding
bait
on Octopus observed feeding
Species
Time spent on
on bait, body covering the
behaviour
bait canister and other times
(minutes)
competing over bait
Statistical analysis
All statistical analysis were performed in R version 4.2.2 (R Core Team 2018). To address the
first and second objectives of this study, a Generalised Linear Modelling (GLM) (Bodo 2013)
was used to analyse the data. Prior to modelling, detailed exploratory data analyses were
conducted following the protocol by Zuur et al. (2010). The purpose of the data exploration
was to understand the nature of the data. This protocol includes checking for collinearity
and interactions between covariates (see Table 3. for the full list), outliers in the data, zeroinflation, normality of the response variables, spatial patterns, and the balance of the data.
The error distribution (family) used in each model was based on the type of response
variable and over-dispersion parameters (i.e., Poisson for count data where the mean is
equal to the variance, quasi-poisson for count data where the variance is greater than the
mean. For each species, relative abundance (MaxN), was recorded per sample. For each
sample, multiple behavioural observations were made for O. vulgaris, and each
elasmobranch species observed. Behaviour was grouped at sample level to calculate total
time in FOV and total time in behaviour for O. vulgaris and each elasmobranch species. To
calculate total duration that all elasmobranchs spent in FOV, duration data was grouped for
all species present in sample. To calculate total MaxN for all elasmobranchs, MaxN of each
species was grouped per sample. For these data, GLMs were used to investigate the effect
of environmental variables, and the abundance and time spent in FOV by elasmobranchs on
the presence and behaviour of O. vulgaris.
Table 3: Covariates included in the best-fit generalised linear models (GLMs) applied to
investigate behaviour of O. vulgaris when elasmobranchs were in FOV.
Model
Type
Error
Best-fit
Excluded
distribution
structure
explanatory
variables
1
GLM
Poisson
Octopus MaxN Depth,
~
Habitat
+ Temperature,
Depth+
Habitat type
Elasmobranch
Maxn
+
P.
africanum
MaxN
+Elasmobranch
duration+P.
africanum
duration
2
GLM
Quasi-poisson
Octopus
duration
Depth,
~ Temperature,
Habitat+ Depth Habitat type
Elasmobranch
Maxn+P.
africanum
MaxN+
Elasmobranch
duration+P.
africanum
duration
Automated model selection was carried out with the R package MuMIn using the dredge
function (Barton, 2020) to identify the best-fit models. Here, a full model was fitted (i.e., a
model containing all the available covariates, see Table 3. above) and the Akaike
Information Criterion (AIC) score was calculated for all possible model formulations (Logan
2010). The model with the lowest AIC score was regarded as the best fit. The results of the
model selection process indicated that none of the explanatory variables influenced the
MaxN and duration of octopus. Therefore, the outcomes of the full model are presented in
the results.
To address the third objective, Kruskal-Wallis ANOVAs was used to test if the presence of a
potential predator influenced the behaviour of octopus when they were observed at the
same time in the videos. Where significant effects were observed, multiple pairwise
comparisons between groups were conducted using the pairwise Wilcox test. The first
ANOVA looked at the effect of the dominant elasmobranchs on the duration of O. vulgaris
feeding events. Data were filtered to only included observations when O. vulgaris was
present in the videos alone and when they occurred with the dominant elasmobranchs
observed in the samples, namely: P. africanum, M. mustelus and B. brevicaudata. A second
Kruskal-Wallis ANOVA was then used to determine which were the dominant behaviours
displayed by O. vulgaris when in co-occurring with P. africanum. Finally, an ANOVA was used
to determine the amount of time spent by octopus on different behaviours when there
were no predators present.
Results
A total of 24 videos were examined for this study, of which 12 showed interactions between
elasmobranchs and octopuses, 11 included elasmobranchs only, and one which had
octopuses. 1 489 behavioural observations in total, with 259 Octopus (including point and
periods observations) and 1230 elasmobranch observations (period observations) were
performed.
General patterns in octopus and elasmobranch community
There were 10 elasmobranch species that were recorded from the stereo-BRUVs
observations, with P. africanum, M. mustelus, B. brevicaudata being the most abundant
frequently observed species (see Table 4 and Figure 4a.). Overall, species that spent the
longest time in frame were P. africanum (mean = 1. 57; SD = 1.38), followed by O. vulgaris
(mean = 0.91; SD = 1.38), M. mustelus (mean = 0.83; SD = 1.19), and B. brevicaudata (mean
= 0.48; SD = 0.95) (Figure 4b.).
Table 4: Frequency table for all the elasmobranch species observed in this study.
Species
Frequency
Mean
Standard
Max
deviation
Acroteriobatus annulatus
1
0,3
0,9
3
Bathytoshia brevicaudata
4
0,6
1,2
4
Carcharias taurus
1
0,0
0,0
0
Haploblepharus
4
0,2
0,4
1
Haploblepharus pictus
2
0,0
0,0
0
Myliobatis aquila
1
0,0
0,0
0
edwardsii
Mustelus mustelus
4
1,1
1,5
4
Poroderma africanum
8
1,6
1,6
4
Carcharodon carcharias
1
0,1
0,3
1
Poroderma pantherinum
1
0,1
0,3
1
Figure 4: Bar graph showing the average abundance (MaxN, calculated across all samples)
with standard deviation (a) and a box and whisker plot showing the duration in the
field of view (FOV) when present (b) for Octopus vulgaris and all species of
elasmobranch seen in the baited remote underwater stereo-video systems.
Effect of elasmobranchs on octopuses behaviour
Overall, 217 period observations totalling 425.5 minutes were made with octopus and
elasmobranchs present at the same time from 12 stereo-BRUVs samples. Most cooccurrence observations were between O. vulgaris and the commonly recorded sharks,
P. africanum and M. mustelus (Table 5). Co-occurrence observation of O. vulgaris with other
shark species were rare (Table 5). On three occasions, O. vulgaris was observed with more
than one species of shark with two of these multi-species observations being with
P. africanum and M. mustelus and the last one with H. edwardsii and M. mustelus (Table 4).
Table 5: Summary of the frequency (n) and duration of co-occurrence observations between
Octopus vulgaris and the elasmobranchs present in the baited remote underwater
stereo-video system data. n = number of observations; SD = standard deviation.
Total
Species name
n
duration
Mean
SD
B. brevicaudata
2
49.3
24.6
NA
mustelus
1
7.7
NA
NA
M. mustelus
6
155.8
26
23.8
mustelus
2
10.8
5.4
NA
P. africanum
7
200.5
28.6
20.4
P. pantherinum
1
1.4
NA
NA
H.
P.
edwardsii
africanum
+
+
M.
M.
General patterns in octopus and elasmobranch behaviour
An overview of the common behaviours that were exhibited by all the species identified in
this study showed that O. vulgaris behaviour while in the presence of elasmobranchs was
dominated by feeding and fleeing from elasmobranchs (Table 2). Sheltering on the reef and
moving on the reef were the next most common behaviours (Table 2). In contrast, where O.
vulgaris were alone they were only observed feeding (total duration = 209.1 min, mean =
11, SD = 23.8) or moving on the reef (total duration 1.9 min, mean = 0.5, SD = 0.5) (Table 2
and Fig 8). The point behaviours which are the behaviours that did not have duration were
summarised in Table 2, and not included in Figure 5. P. africanum spent most of the time
feeding on the bait and on passive behaviours, and M. mustelus were rarely observed
displaying any other behaviours than passive (Figure 5). The B. brevicaudata spent most of
the time on passive behaviours, feeding on bait, and less time on attacking the octopus
(Figure 5). The other elasmobranch species such as P. pantherium, C. taurus, and H. pictus,
to name a few, were less frequently observed in field of view and spent most of the time on
passive behaviour and did not exhibit any attacks on octopus (Figure 5).
Figure 5: Duration of behaviours observed by the different species while in the field of view
(FOV) of the baited remote underwater stereo-video samples. Panels are arranged
based on the species frequency in the dataset. X and Y axis scales vary by species
for visualisation purposes.
Table 6: Summary of the occurrence and duration of behaviours displayed by Octopus
vulgaris while elasmobranchs were present. n = number of observations; Type =
observation type; SD = standard deviation.
Total
Behaviour type
n
Type
duration
Mean
SD
Colour change
9
Point
Colour change (camouflage) 2
Point
Colour change (flashing)
4
Point
Feeding
74
Period 341.4
4.6
9.2
Fleeing
68
Period 4.6
0.1
0.1
Moving on reef
30
Period 23.4
0.8
1.6
Posturing
13
Point
Sheltering on reef
37
Period 56.1
2.4
2.8
Correlation between elasmobranchs and octopus abundance and presence
A generalised linear model (GLM) with a Poisson error distribution (Poisson dispersion
parameter = 1) was run to test for the effect of predictor variables on the MaxN of O.
vulgaris from the sample set. The total duration in FOV (p = 0.9) and MaxN (p = 0.7) of all
elasmobranchs, as well as the total duration in FOV (p=0.8) and MaxN (p = 0.7) of P.
africanum had no effect on the duration O. vulgaris was in the FOV (Figure 6). Habitat (p =
0.12) and depth (p = 0.5) also had no effect on the time spent in the FOV by O. vulgaris.
Figure 6: Relationship between the MaxN of Octopus vulgaris and the abundance (a, c) and
time (min) spent in the field of view (b, d) by all elasmobranchs (a, b) and
Poroderma africanum (c, d). The line (black) is the predicted trend, and the error
shading (grey and green) is the 95 % confidence intervals estimated from the
generalised linear models.
A GLM with a Quasipoisson error distribution (Poisson dispersion parameter > 10) was run
to test for the effect of predictor variables on the duration that O. vulgaris spent in the field
of view (FOV). The total duration in FOV (p = 0.16) and MaxN (p = 0.7) all elasmobranchs as
well as the total duration in FOV (p=0.8) and MaxN (p = 0.8) of P. africanum had no effect on
the duration O. vulgaris was in the FOV (Figure 4). Habitat (p = 0.12) and depth (p = 0.5) also
had no effect on the time spent in the FOV by O. vulgaris (Figure 7).
Figure 7: Relationship between the time (min) spent in the field of view by Octopus vulgaris
and the abundance (a, c) and time (min) spent in the field of view (b, d) by all
elasmobranchs (a, b) and Poroderma africanum (c, d). The line (black) is the
predicted trend, and the error shading (grey and green) is the 95 % confidence
intervals estimated from the generalised linear models.
Effect of elasmobranchs on octopus behaviour
Overall, 217 period observations totalling 425.5 minutes were made with octopus and
elasmobranchs present at the same time from 12 stereo-BRUVs samples. Most cooccurrence observations were between O. vulgaris and the commonly recorded sharks,
P. africanum and M. mustelus (Table 5). Co-occurrence observation of O. vulgaris with other
shark species were rare (Table 5). On three occasions, O. vulgaris was observed with more
than one species of shark with two of these multi-species observations being with
P. africanum and M. mustelus and the last one with H. edwardsii and M. mustelus (Table 5).
Table 5: Summary of the frequency (n) and duration of co-occurrence observations between
Octopus vulgaris and the elasmobranchs present in the baited remote underwater
stereo-video system data. n = number of observations; SD = standard deviation.
Total
Species name
n
duration
Mean
SD
B. brevicaudata
2
49.3
24.6
NA
mustelus
1
7.7
NA
NA
M. mustelus
6
155.8
26
23.8
mustelus
2
10.8
5.4
NA
P. africanum
7
200.5
28.6
20.4
P. pantherinum
1
1.4
NA
NA
H.
P.
edwardsii
africanum
+
+
M.
M.
When alone, O. vulgaris spent significantly more time feeding than moving on the reef ((Chisquared = 4.79, df = 1, p = 0.028). No other behaviours were observed when O. vulgaris was
alone. The duration of O. vulgaris feeding on bait and moving on reef when there were no
sharks in FOV was mean = 660. 37; SD = 1430 and mean = 28.5; SD = 31.22, respectively
(Figure 8). Predator presence had a significant effect on the duration that O. vulgaris spent
feeding (chi-squared = 16.7, df – 3, p = 0.0008). Pairwise comparisons indicated that the
effect of predator presence on the duration of O. vulgaris feeding was restricted to P.
africanum (p = 0.01). The presence of M. mustelus (p = 0.34) and B. brevicaudata (p = 0.38)
had no effect on the duration that O. vulgaris spent feeding. There was a significant
difference in the amount of time O. vulgaris spent on different behaviours when P.
africanum were present (Chi-squared = 54.8, df = 3, p < 0.0001). There was no difference in
the time O. vulgaris spent feeding in comparison to sheltering on reef (p = 0.5) and moving
on reef (p = 0.08). With all the other pairwise comparisons there were significant differences
(p < 0.05 in all cases). When P. africanum was in field of view, O. vulgaris, spent time (mean =
152.60; SD = 259.79) sheltering and moving on reef, feeding, and fleeing (Table 6.). There
was less fleeing and most time spent (mean = 840.91; SD = 1148. 21) on feeding and moving
on reef when O. vulgaris were co-occurring with M. mustelus (Figure 8.). When B.
brevicaudata were in their, O. vulgaris spent time (mean = 307.22; SD = 440.57) feeding on
bait and moving on reef, also displayed evasive behaviours such as fleeing and sheltering on
reef (see Table 6 and Figure 8).
Table 6: Summary of the occurrence and duration of behaviours displayed by Octopus
vulgaris while elasmobranchs were present. n = number of observations; Type =
observation type; SD = standard deviation.
Total
Behaviour type
n
Type
Colour change
9
Point
Colour change (camouflage) 2
Point
Colour change (flashing)
4
Point
Feeding
74
Fleeing
duration
Mean
SD
Period 341.4
4.6
9.2
68
Period 4.6
0.1
0.1
Moving on reef
30
Period 23.4
0.8
1.6
Posturing
13
Point
Sheltering on reef
37
Period 56.1
2.4
2.8
Figure 8: Box and whisker plots showing the duration that Octopus vulgaris displayed
different high-level behaviours while alone and while co-occurring with Poroderma
africanum, Mustelus mustelus and Bathytoshia brevicaudata. The duration data are
log(x+1) transformed for visualisation purposes.
Discussion
Based on the observations, the abundance of elasmobranchs and duration in field of view
had no effect on the abundance and duration of O. vulgaris, which is contrary to the
hypothesis of this study. These findings suggested that the interactions may be more
competitive than predatory. Thes observations were also evidenced by the fact that O.
vulgaris spent most time feeding on bait even when its known predators, pyjama catsharks
(P. africanum) (Branch et al. 2020) were in the field of view. Moreover, in some instances,
they fought over the bait and pyjama catsharks would eventually leave the bait and O.
vulgaris continued feeding. Another interesting observation was the sheltering behaviour of
O. vulgaris which involved hanging around in the field of view (FOV) while waiting for the
sharks to move away from the bait so it could go back to feeding. These behavioural
observations suggested that the O. vulgaris were not completely scared of being killed by
the sharks as they continued to show their attraction to the bait. However, the presence of
P. africanum did disrupt the feeding behaviour of O. vulgaris as they also spent more time
fleeing and sheltering on reef compared to when other elasmobranch species were present.
When B. brevicaudata found the O. vulgaris feeding on the bait they chased them away so
they could feed on the bait. O. vulgaris displayed similar evasive behaviours by fleeing and
not leave the FOV but wait on the reef for short-tail stingrays to move away from the bait.
These observations supported the hypothesis that across various taxonomic groups,
increased time spent on actively avoiding predation reduces time spent on searching for
food (Wirsing & Ripple 2011; Johannesen 2013; Kiszka et al. 2015). A comparable
occurrence for these observations was seen in mud crabs (Panopeus herbstii), which
increased their usage of refuge when predatory blue crabs (Callinectes sapidus) and toadfish
(Opsanus tau) were in their vicinity (Belgrad & Griffen 2016).
Other evasive behaviours (point behaviours) such as change in body colour and pattern
(posturing) that were exhibited by O. vulgaris, have been associated with anti-predator
tactics (Scheel et al 2016; Miao 2016). In some observations, these behaviours have been
linked to the O. vulgaris resolution to entirely avoid predators or to defend themselves
during confrontations with predators (Scheel et al. 2016). Such behaviours in this study,
were observed in O. vulgaris, mostly when sharks were passing closely. Additionally, there
was rarely fleeing when shark species such as M. mustelus were present and the reason
might be because the sharks showed little interest in O. vulgaris which. Again, even during
presence of potential predators O. vulgaris did not stop feeding but used the opportunity to
remain feeding on the bait. A different scenario was observed in a study which revealed that
in avoidance of predation by tiger sharks; dugongs, and Indo-Pacific bottle-nose dolphins
moved from their productive feeding spot and foraged in deeper waters which were
relatively safe but had less food sources (Kiszka et al. 2015). Another example of reduced
feeding to avoid predation risk was observed in Yellowstone Ecosystem where elk moved
from open grasslands which were high abundant food source to coniferous forest cover to
avoid their predators, wolfs (Wirsing & Ripple 2011; Kiszka et al. 2015). Contradictory to the
last hypothesis of this study, the influence of potential predator elasmobranchs on the
behaviour of O. vulgaris was not consistent among the different elasmobranch species.
Environmental factors such as habitat type and depth have been observed to impact the
interactions between predators and prey (Belgrad and Griffen 2016), however, these
aspects had no effect on the MaxN and duration of O. vulgaris and elasmobranchs in FOV.
Nevertheless, other studies have shown that environmental factors influence predatorinteractions, and such case was observed in terrain features which increased elk
vulnerability to wolf predation by hindering their escape causing them to be killed (Ripple &
Beschta 2006). Based on the observations for this study, there were no mortalities, but
aggressive behaviours were common. This might possibly be because bait attracted larger O.
vulgaris which were able to compete with the sharks. Body size in predator-prey
interactions has been considered to have an influence, for instance, in a study by Hammill et
al. (2023), results showed that increased body width in Paramecium reduced the chances of
being eaten by their predator (Stenostomum). These observations may suggest that O.
vulgaris weighed up the cost benefits and if they are large, the potential benefits of feeding
outweighed the risk of mortality and if they were small, the potential benefits of risk
mortality outweighed the benefits of feeding on the bait (Mather & O’Dor 1991). With
regards to the prey size in predator-prey interactions, further research may be undertaken research
to address that aspect. This can also be interesting, as in some ecosystems where different
predator species had common prey, predators have been found to target specific
developmental phases of the same prey depending on factors such as their feeding
techniques and physical restrictions (De Roos et al. 2008).
Conclusion
This study aimed to understand how the presence and behaviours of predatory
elasmobranchs, primarily the pyjama sharks (Poroderma africanum), influenced the presence
and behaviour of prey octopuses (Octopus vulgaris). This study highlighted the use of
BRUVs as an effective tool to investigate predator-prey interactions in coastal ecosystems. It
sheds light on the dynamic behaviours exhibited by octopuses when faced with predation risk
from pyjama sharks. This study also noted the trade-off between octopus trying to feed on the
bait canister but avoided the predation risk from the pyjama sharks. These findings contribute
to a better understanding of trophic interactions and the role of marine predators in shaping
coastal ecosystems. Understanding these interactions is crucial for managing marine
protected areas and conserving the delicate balance of species in coastal environments. Future
research in this field can build upon these findings by incorporating octopus size and
potential predators as well as fish species to further explore the complexities of predator-prey
dynamics and their ecological consequences in South African temperate reefs.
Acknowledgements
I would like to express my special thanks to my supervisors, Dr Chantel Elston and Dr Anthony
Bernard at the South African Institute for Aquatic Biodiversity. Their support and guidance are
invaluable, and I could not have completed this honours project without them.
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