Creature Feature: Paracerceis sculpta (Holmes, 1904)

Andrew Hosie's blog | Created 1 decade ago

The Rocky Horror Isopod Show:
The bizarre sexual antics of an alien marine slater

The marine slater, Paracerceis sculpta, is native to the Pacific Coast of North America, but has since been introduced to many parts of the world including Hawaii, Brazil, China, Europe and Australia. This species was first discovered in Townsville in 1975 and has since spread around southern Australia to Port Denison, WA. A relative to the slaters, pillbugs or woodlice in your backyard as well as the Giant Marine Slater of a previous post, this species reaches just over a centimetre in length and can be found in shallow waters living amongst seaweed, sea squirts, barnacles and sponges. Although this species is invasive there is no evidence to suggest that this species is having a negative impact on the ecosystems that it has invaded. Actually, the species is much more interesting for the strategies it employs during breeding season.

A male and female isopod shown next to one another
Figure 1. Alpha male (left) and mature female (right) Paracerceis sculpta, note the large ornate tail fan of the male.
Photo: A.Hosie, WAM

 

Love nests

When breeding season comes around, male Paracerceis sculpta set up territories in the cavities of sponges. The alpha males are easy to spot as they develop an ornamented tail fan that sets them apart from the rather plain females. This large tail fan is possibly used for grasping the female during copulation, courtship displays or fending off rival males. The females being attracted to large males with proportionally large tail fans don’t mind being a part of the alpha male’s harem, which can contain as many as 11 females. As relatively few males can effectively monopolise the available female, these alpha males need to defend their territory and harem from other males that haven’t been so lucky. Competition is high, so males have developed a couple of different methods of compromising the alpha male’s defences.

Diagram of major features of male and female isopods
Figure 2. Diagrams of the three male morphs compared to the female.
Image: Dr. Stephen Shuster, Northern Arizona University (http://www4.nau.edu/isopod/)

 

Priscilla Queen of the Sponge

Introducing beta and gamma males: these are two alternate male forms or morphs. The beta males are smaller than the alpha male and lack the large ornate tail fan and in fact look just like mature females, with one important difference—they possess a pair of appendix masculinae (the male sex organs). This cross-dressing tactic is termed inter-sexual mimicry where the male mimics the female of the same species in appearance and behaviours in order to avoid detection by the large alpha male. Gamma males on the other hand are even smaller and actually mimic juveniles. The males that use both of these strategies are not detected as a threat by the alpha male and are allowed entry to the sponge cavity, and sneakily mate with the females. These tactics aren’t unique to this isopod and have been adopted by some ant species, salmon and the appropriately named garter snake.

The One

The alpha males may have their work cut out for them, but for the females they only have one chance at finding Mr Right. Like all crustaceans, Paracerceis sculpta need to shed their exoskeleton (shell) in order to grow. The females will only mate while the new exoskeleton is still soft. While this in itself sounds odd it is a very common necessity in many crustacean groups. What makes the females of this species bizarre is that they also lose their mouthparts in the process. Once a female undergoes the moult to maturity she can no longer feed, and so spends the remainder of her life, in the sponge, brooding her eggs. The eggs are kept in a brood pouch where the embryos develop. Once they hatch the juveniles are released from the pouch and leave the sponge to feed. This strategy of undergoing a single reproductive event is called semelparity, and occurs in other animals like squid, spiders and salmon. Sometimes referred to as “big bang” reproduction as the female gathers her resources to reproduce and the result is usually fatal and large, either in terms of the number of offspring or their size.