Introduction

The Food Preference Test is used to study food preference behaviors in octopuses. The test identifies an octopus’s food preferences and then analyzes whether it can locate its favorite food in five problem-solving tasks.

Animals rely on their sensory systems to perform numerous behaviors and activities essential for their survival, such as finding food, locating shelters, attracting mates, and avoiding predators. Octopuses mainly rely on chemoreception to determine habitat, feeding, defense, and mating choice, which can work alone or in combination with visual cues (Cosmo, Maselli, & Polese 2018). In the Food Preference Test, the different problem-solving tasks help analyze sensorial hierarchy in octopuses’ food choices. The Food Preference Test is performed in a fiberglass tank, in which the subject is presented with different types of food to assess its food preferences. In the problem-solving tasks, the food is placed in different jars that allow or limit chemical and visual cues. Transparent jars with pierced lids allow both chemical and visual cues to be presented. However, placing the food in blind (non-transparent) or sealed jars limits either visual or chemical cues. Furthermore, the subject’s problem-solving abilities can be challenged further in a confusion task in which the food is placed in blind jars with pierced lids, but a picture of a different type of food is placed outside each jar. Therefore, the Food Preference Test can help evaluate the priority given to chemical vs. visual cues in food choice but can also be used for other investigatory needs, such as investigating the effect of different environmental conditions on food choices. 

Other apparatuses used to study octopus behavior include the Octopus 3 Choice Maze and the Octopus Round Arena.

Apparatus and Equipment

The Food Preference Test is performed in a large fiberglass tank measuring 50 cm × 50 cm × 50 cm. The tank can be filled with enrichment objects such as rocks or amphora to serve as a den. Different jars are used during problem-solving tasks, such as transparent or blind jars, which allow or prevent visual cues. Moreover, the jars’ lids can either be sealed or perforated to allow or prevent chemical cues from being presented. 

Training Protocol

After each trial, change the tank’s water to avoid olfactory cues from previous trials interfering with task performance. Appropriately light the apparatus. A tracking and recording system such as the Noldus Ethovision XT can be used to assist with observations.

The following is a sample protocol used to assess chemical vs. visual preferences in food choices in octopuses.

Habituation 

Allow the subject to habituate in the tank for 15 days before starting experiments. Feed the subject with a different type of food from experimental trials during the habituation period. 

Food Preference Test Training 

Place three different food types (such as anchovy, clam, and mussel) within the subject’s visual field simultaneously and at the same distance. The first food consumed by the subject corresponds to its favorite choice. Conduct training once a day for seven consecutive days.

Food Preference Test Task

The Food Preference Test Task is performed to identify whether the subject can identify its favorite food in the five problem-solving tasks:

1. T1 (positive control)- place the three types of food in three separate transparent screw jars with pierced lids to provide both chemical and visual cues. 
2. T2 – place the different food in sealed jars without pierced lids to provide only visual cues.
3. T3 – place the food in non-transparent (blind) screw jars with pierced lids to provide only chemical cues. 
4. T4 (confusion tank) – place the food in blind screw jars with pierced lids but put a picture of a different type of food from the inside, outside each jar. 
5. T5 (negative control) – place the food in jars that are completely blind and sealed without pierced lids so that both chemical and visual cues are absent. 

 Conduct trials once a day for five consecutive days for each task.

Literature Review

Investigation of chemical and visual sensorial hierarchy in food choice in Octopus Vulgaris.

Maselli et al. (2020) investigated the priority given to chemical and visual perception to establish food choice sensorial hierarchy in octopuses using the Food Preference Test. The experiment was performed in a fiberglass tank. The subjects’ food preference was first tested using three different types of food (anchovy, Engraulis encrasicolus; clam, Ruditapes philippinarum; mussel, Mytilus edulis). The food was placed within the subject’s visual field, and the first food the subject ate out of the three was considered their favorite. Five problem-solving tasks were then performed to evaluate whether the subject could identity their favorite food. In the first task, the food was placed in transparent jars with pierced lids to provide both chemical and visual cues. In the second task, transparent jars without pierced lids were used. In the third task, non-transparent jars with pierced lids were used. The fourth task was a confusion task in which non-transparent jars with pierced lids were used, but a picture of a different type of food from the inside was placed outside each jar. In the last task, non-transparent and sealed jars were used so that both visual and chemical cues were absent. The results indicated that all the subjects performed behaviors such as touching, exploring, and opening the jars. A high significant variance in food recognition ability was observed in the subjects with the jar opening tasks in all problem-solving tasks. It was observed that the subjects could recognize their preferred food jar in all tasks except task 5. The subjects’ food choices were more dependent on chemical cues than visual ones. In task 4, although the subjects were cheated with a false picture, they still chose their preferred food jar in 100% of cases, indicating that they used their chemical sense. In task 2, when chemical cues were absent, they only chose the correct jar in 50% of cases. When chemical and visual cues were absent in task 5, the subjects selected a jar at random. 

Data Analysis

The following parameters can be observed using the Food Preference Test:

• The food the subject eats first during training trials.
• The jar the subject touches first during testing 
• The jar the subject opens during testing 
• The food the subject touches during testing 
• The food the subject eats during testing 
• The time taken for the subject to choose to open the jar, from the very first touch to the grab and wrap of the jar to open it.  

Strengths and Limitations

Strengths

The Food Preference Test is used to evaluate food preferences in octopus, and that preference is used to challenge the animal’s problem-solving abilities. The Food Preference Test utilizes different jars that either allow or prevent chemical or visual cues. Transparent jars with pierced lids allow both chemical and visual cues to be presented. However, using blind jars or sealed lids prevents the presentation of visual or chemical signals. Therefore, the Food Preference Test allows evaluating the priority given to chemical or visual cues in food choice. Moreover, the test can also be applied for other investigatory needs, such as investigating the effect of environmental conditions on food preferences.

Limitations

The species, gender, weight, and age of the subject can potentially affect task performance. The presence of unintentional stimuli, such as activities being performed in the experimental room, can affect task performance. All the different food types must be placed at the same distance simultaneously in front of the subject’s visual field. 

Summary

• The Food Preference Test is used to study food preference behaviors in octopuses. It consists of evaluating the octopus’s food preference, which is then used to test the subject’s problem-solving abilities. 
• The Food Preference Test is also used to observe the priority given to chemical vs. visual signals on food choice. 
• The Food Preference Test is performed in a fiberglass tank. During the problem-solving task, the food is placed in different jars that present or prevent chemical or visual cues. 
• The Food Preference Test can also be used for other investigatory needs, such as investigating the effect of environmental conditions on food preferences

References

1. Di Cosmo, A., Maselli, V., & Polese, G. (2018). Octopus vulgaris: An Alternative in Evolution. Results and problems in cell differentiation, 65, 585–598. https://doi.org/10.1007/978-3-319-92486-1_26
2. Maselli, V., Al-Soudy, A. S., Buglione, M., Aria, M., Polese, G., & Di Cosmo, A. (2020). Sensorial Hierarchy in Octopus vulgaris’s Food Choice: Chemical vs. Visual. Animals : an open access journal from MDPI, 10(3), 457. https://doi.org/10.3390/ani10030457