High diversity in copepods has allowed them to colonise many habitats and they are called the “insects of the sea”. They travel vast distances daily in the water column – termed diel vertical migration, and buoyancy control is very important. I observed that the morphology of the antennulae differ between copepod groups, for example length of antennulae which affects the drag of the copepod in water.  Comparison among copepod orders is extremely rare. I hypothesised that 1) sinking rate differs among copepod groups, 2) sinking rate increases when antennulae length decreases and 3) sinking rate of the copepod increases when density increases. Passive sinking experiments in freshwater and seawater were performed with preserved samples of five groups of copepods: family Calanidae (order Calanoida), Oithona sp. (order Cyclopoida), Oncaea sp. (order Poecilostomatoida), Farranula sp. (order Poecilostomatoida) and Macrosetella sp. (order Harpacticoida). Pictures of the antennulae length and size was measured in ImageJ software and density was computed from wet mass and size. Sinking rate was significantly different among groups in both freshwater (p=1.643x10^-05) and seawater (p=4.007x10^-07) which supports the first hypothesis. Interestingly, Calanidae had the longest antennulae, lowest density and fastest sinking rate. There was a significant positive correlation between antennulae length and sinking rate and a significant negative correlation between density and sinking rate, which is opposite to hypothesis 2 and 3. The rigid position of the antennulae may have affected the results. Further research can be done on the combined effects of antennae length, angle and lipid content. Understanding the ways to control buoyancy will help explain how copepods travel vast distances in the water column.

Copepods have colonised many habitats with different conditions (Huys and Boxshall, 1991: p9-12). In the ocean, they can be found in the water column, in sediments and in deep-sea vents. They also live in freshwater bodies, saline lakes, hot springs and even damp terrestrial environments (Huys and Boxshall, 1991: p9-12). There are many parasitic forms and they have parasitized species in most animal phyla (Huys and Boxshall, 1991: p11-12). It is interesting to look at the adaptations in copepods that have allowed them to occupy this large variety of habitats. Copepods are called “the insects of the sea” for their size, diversity and abundance (Huys and Boxshall, 1991: p9). There is a large diversity in planktonic copepods alone (some examples shown in figure 1).

Buoyancy plays a big role in marine environments. Many copepods exhibit diel vertical migration traversing 50-150m (Thorisson, 2006). After feeding in the mixed surface layer, satiated copepods stretch out their antennulae (see copepod body plan in figure 2) and slowly sink back into the aphotic zone where they are less conspicuous to predators (Thorisson, 2006). When they are hungry, they swim up to the surface to feed again (Thorisson, 2006). When Eucalanus crassus (Order Calanoida) feeds with its self-generated feeding current, the first antennae (antennulae) act as parachutes to slow down sinking and a slight negative buoyancy maintains the orientation for the flow field (Ruppert et al., 2004, p 672).  Morphology of the antennules between copepod groups may differ in terms of length, number of segments and organisation of setae (Huys and Boxshall, 1991, Boxshall and Halsey, 2004). I have observed that generally calanoids have long antennulae, while Corycaeus and Oncaea (Order Poecilostomatoida) have very short antennulae. As longer antennules increases the drag of copepods in water, it is reasonable to predict copepods with longer antennules sink slower.

Buoyancy can also be affected by the relative body density of copepods to surrounding seawater. If the density of a copepod is higher than density of seawater, it will sink. It has been proposed that lipid content in copepods may also be a means to control buoyancy, especially during diapause (Zarubin et al., 2014, Heath et al., 2004, Pond and Tarling, 2011). Lipids are stored in an oil sac in the prosome for energy-storage and reproduction (Zarubin et al., 2014) and they have lower density, higher compressibility and higher thermal expansion of esters compared to water (Pond and Tarling, 2011). Copepods found in deeper water was found to have higher lipid content and it has been suggested that they change their lipid content at different depths to adjust for buoyancy (Zarubin et al., 2014, Pond and Tarling, 2011).

This study determines to test if buoyancy differ among five copepod groups: order Calanoida, order Cyclopoida, Oncaea and Corycaeus (order Poecilostomatoida) and order Harpacticoida. I hypothesise that sinking rate differs among copepod groups. If this hypothesis is supported, I further hypothesise that sinking rate increases when antennule length decreases, and sinking rate of the copepod increases when density increases. Copepods have been well-studied at the single-species level, but there are hardly any studies of multiple orders. Comparison among copepod groups will help understand how different copepods use their respective habitat differently. 

Five copepod samples were obtained in the IMOS-SCIRO plankton Team in Brisbane, Australia: family Calanidae (order Calanoida), Oithona sp. (order Cyclopoida), Oncaea sp. (order Poecilostomatoida), Farrancula sp. (order Poecilostomatoida) and Macrostella sp. (order Harpacticoida) (figure 1). They were collected with a 100 µm mesh net not deeper than 100m in Australian waters and preserved in formalin.

Two vials (diameter 1cm, height 4.5cm) were filled up with 2cm³ of freshwater in one and unfiltered seawater in the other. Each copepods was rinsed with freshwater and individually placed below the meniscus in the freshwater vial. The time for the copepod to sink was timed until it lied flat on bottom. Three repetitions were made with each individual and the process was repeated in seawater.

Photographs were taken of the copepods under microscope so that their antennules were visible. Then they were placed on an absorbent glass-fibre filter to dry and were weighed to obtain wet mass.

In the software ImageJ, the mean antennae length of each copepod was measured. The copepod shape was simplified into two ellipsoids (volume of ellipsoid given by 4πabc /3) and the length of the equatorial (a) and polar (b) axes were assumed to be equal.  The radii (a and b) and length (c) of the prosome and urosome were measured (figure 3) and volume of each copepod was calculated. Density was calculated by dividing mass by volume.

A one-way ANOVA was conducted to test the difference passive sinking rate among copepod group in freshwater and seawater. Multiple regression was conducted to test the effect of density and antennae length on passive sinking rate. 

Sinking rate was significantly different among groups in both freshwater (p=1.643x10^-5, f=29.46, df=4) and seawater (p=4.007x10^-7, f=65.153, df=4) (Table 1). Calanidae had the fastest sinking rate in both freshwater (0.376 ± 0.007) cm/s and seawater (0.334 ± 0.023) cm/s. Farranula sp. had the slowest sinking rate in both freshwater (0.056 ± 0.001) cm/s and seawater (0.057 ±0.004) cm/s.

Generally sinking rate increased as antennae length increased (figure 4). Calanidae had the longest antennae length and the highest sinking rates. Oncaea sp. had the shortest antennae length but had the second highest sinking rates. The positive correlation between antennae length and sinking rate was significant in both freshwater (p=0.00464, F_{2, 12}=16.55) and seawater (p=1.78x10^-6, F_2,12=56.93).

Sinking rate decreased as copepod density increased (figure 5). Calanidae had the lowest density and the highest sinking rates. Macrosetella sp. had the lowest density and the second lowest sinking rates after Farranula sp. The negative correlation between density and sinking rate was significant in both freshwater (p=0.00303, F_{2, 12}=16.55) and seawater (p=0.00115, F_2,12=56.93).  

Table 1. Freshwater and seawater sinking rates for each copepod group. Data shows mean ± standard error.

Sinking rate was significantly different among copepod groups and the first hypothesis is supported. However, the positive correlation between antennae length and sinking rate and the negative correlation between density and sinking rate was opposite to what was predicted. Interestingly, calanidae had the longest antennules, lowest density and fastest sinking rate. Data of density was affected by the high percentage error in the mass of copepods as the mass was extremely small (for example Oncaea sp. weighed 0.0002 g).

Since the samples have been preserved, the antennules were rigid and the angle to which they were held against the body was fixed, which may affect the effect of antennules on buoyancy (Borazjani et al., 2010). The angle could not be measured as the calanoid, Macrosetella and Farranula could not balance to allow for photos of dorsal or ventral view. Further research can be done on the combined effects of antennae length, angle and lipid content (Zarubin et al., 2014).

Buoyancy control is a dynamic process that happens in copepod constantly. Understanding the ways to control buoyancy will help explain how copepods travel vast distances in the water column every day.

I would like to thank Julian-Uribe Palomino for his assistance in identifying samples, taking photos and for sharing his knowledge on copepods.  I also want to thank the IMOS-SCIRO plankton Team in Brisbane Australia for their support and supply of samples and equipment.

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