DIVE DRY WITH DR. BILL #847: THE KELP/EELGRASS ISOPOD
People are occasionally confused when asked about two groups of crustaceans, the amphipods and the isopods. Of course being the very experienced marine biologist that I am, I am not... but I do get confused about some other things. Now where did I leave my glasses? Amphipods are compressed laterally, side-to-side. Isopods look like someone has run over them with as truck and left them flattened top-to-bottom. The word isopod refers to their legs being extended on either side of the squashed body! See how easy it is to distinguish them?
One of the first critters I learned when I arrived on Catalina 50+ years ago was the kelp isopod, then known as Idotea resecata (also referred to as Idothea resecata. Later taxonomists would rename it is Pentidotea resecata. We initially found it on drifting kelp rafts that we sampled. It is found from Alaska to Mazatlan, Mexico. Up north it is usually found on eelgrass (Zostera marina) and they refer to it as the eelgrass isopod. In our waters it resides mostly on giant kelp (Macrocystis pyrifera).
Regardless of which one it is found on, they are well camouflaged due to their green or brown body color and therefore difficult to see. The color is derived from pigments in the alga or plant it is feeding on. The segmented body, up to 1.6" long, is quite obvious with pairs of legs extending on either side of the segment. The telson or tail section is indented making it easy to identify. There is a related species, Idotea rufescens, which is red in color. Some biologists think they are the same species.
Back in 1933 W. W. Lee claimed that they could change color from red to green to brown depending on which alga or plant they were living on and could change color when moved to another. I have not read his paper, so I don't know how quickly they could change. He also stated that the young individuals lived on red seaweed along the shore, while the older ones were found in eelgrass beds.
In our waters, this species grazes on the juicy young blades of kelp up near the canopy. In turn many species of fish including our senorita (Oxyjulis californica) munch on them. It is said that in the absence of such predators, the canopy of a kelp forest might be quickly decimated.
There is not much written about this interesting species. Fortunately the University of Oregon has posted information about their sex lives which I will relay here for my libidinous, licentious readers! Sexes are separate in this species, although some relatives can change sex. Copulation allows for the transfer of sperm to the female. Like other crustaceans, these isopods molt their exoskeleton... emerging with a soft body that hardens afterwards. To aid in copulation, the posterior end which contains the reproductive structures, molts first later followed by the forward or anterior section.
The developing eggs are brooded by the female. There is no real larval stage and the young emerge as miniatures of the adults. Therefore they do not disperse in the currents with the plankton and get to see the world before settling down. This is probably a good thing as they only live less than a year. I'm glad I have reached a ripe old age which allowed me to dive five of the continents (so far). However, they can disperse on drifting kelp rafts as verified by my early kelp rafting research in the 60s and 70s.
© 2020 Dr. Bill Bushing. For the entire archived set of nearly 850 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Images of the kelp isopod on kelp and two "cuddling" in a kelp curl (courtesy of Kevin Lee).
People are occasionally confused when asked about two groups of crustaceans, the amphipods and the isopods. Of course being the very experienced marine biologist that I am, I am not... but I do get confused about some other things. Now where did I leave my glasses? Amphipods are compressed laterally, side-to-side. Isopods look like someone has run over them with as truck and left them flattened top-to-bottom. The word isopod refers to their legs being extended on either side of the squashed body! See how easy it is to distinguish them?
One of the first critters I learned when I arrived on Catalina 50+ years ago was the kelp isopod, then known as Idotea resecata (also referred to as Idothea resecata. Later taxonomists would rename it is Pentidotea resecata. We initially found it on drifting kelp rafts that we sampled. It is found from Alaska to Mazatlan, Mexico. Up north it is usually found on eelgrass (Zostera marina) and they refer to it as the eelgrass isopod. In our waters it resides mostly on giant kelp (Macrocystis pyrifera).
Regardless of which one it is found on, they are well camouflaged due to their green or brown body color and therefore difficult to see. The color is derived from pigments in the alga or plant it is feeding on. The segmented body, up to 1.6" long, is quite obvious with pairs of legs extending on either side of the segment. The telson or tail section is indented making it easy to identify. There is a related species, Idotea rufescens, which is red in color. Some biologists think they are the same species.
Back in 1933 W. W. Lee claimed that they could change color from red to green to brown depending on which alga or plant they were living on and could change color when moved to another. I have not read his paper, so I don't know how quickly they could change. He also stated that the young individuals lived on red seaweed along the shore, while the older ones were found in eelgrass beds.
In our waters, this species grazes on the juicy young blades of kelp up near the canopy. In turn many species of fish including our senorita (Oxyjulis californica) munch on them. It is said that in the absence of such predators, the canopy of a kelp forest might be quickly decimated.
There is not much written about this interesting species. Fortunately the University of Oregon has posted information about their sex lives which I will relay here for my libidinous, licentious readers! Sexes are separate in this species, although some relatives can change sex. Copulation allows for the transfer of sperm to the female. Like other crustaceans, these isopods molt their exoskeleton... emerging with a soft body that hardens afterwards. To aid in copulation, the posterior end which contains the reproductive structures, molts first later followed by the forward or anterior section.
The developing eggs are brooded by the female. There is no real larval stage and the young emerge as miniatures of the adults. Therefore they do not disperse in the currents with the plankton and get to see the world before settling down. This is probably a good thing as they only live less than a year. I'm glad I have reached a ripe old age which allowed me to dive five of the continents (so far). However, they can disperse on drifting kelp rafts as verified by my early kelp rafting research in the 60s and 70s.
© 2020 Dr. Bill Bushing. For the entire archived set of nearly 850 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Images of the kelp isopod on kelp and two "cuddling" in a kelp curl (courtesy of Kevin Lee).
